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This is preparation for adding the vmemmap_tails array to struct pglist_data, which requires MAX_FOLIO_ORDER to be available in mmzone.h. Signed-off-by: Kiryl Shutsemau Acked-by: David Hildenbrand (Red Hat) Acked-by: Muchun Song Acked-by: Zi Yan --- include/linux/mm.h | 31 ------------------------------- include/linux/mmzone.h | 31 +++++++++++++++++++++++++++++++ 2 files changed, 31 insertions(+), 31 deletions(-) diff --git a/include/linux/mm.h b/include/linux/mm.h index 7c79b3369b82..2c409f583569 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -26,7 +26,6 @@ #include #include #include -#include #include #include #include @@ -2074,36 +2073,6 @@ static inline unsigned long folio_nr_pages(const str= uct folio *folio) return folio_large_nr_pages(folio); } =20 -#if !defined(CONFIG_HAVE_GIGANTIC_FOLIOS) -/* - * We don't expect any folios that exceed buddy sizes (and consequently - * memory sections). - */ -#define MAX_FOLIO_ORDER MAX_PAGE_ORDER -#elif defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP) -/* - * Only pages within a single memory section are guaranteed to be - * contiguous. By limiting folios to a single memory section, all folio - * pages are guaranteed to be contiguous. - */ -#define MAX_FOLIO_ORDER PFN_SECTION_SHIFT -#elif defined(CONFIG_HUGETLB_PAGE) -/* - * There is no real limit on the folio size. We limit them to the maximum = we - * currently expect (see CONFIG_HAVE_GIGANTIC_FOLIOS): with hugetlb, we ex= pect - * no folios larger than 16 GiB on 64bit and 1 GiB on 32bit. - */ -#define MAX_FOLIO_ORDER get_order(IS_ENABLED(CONFIG_64BIT) ? SZ_16G : SZ_= 1G) -#else -/* - * Without hugetlb, gigantic folios that are bigger than a single PUD are - * currently impossible. - */ -#define MAX_FOLIO_ORDER PUD_ORDER -#endif - -#define MAX_FOLIO_NR_PAGES (1UL << MAX_FOLIO_ORDER) - /* * compound_nr() returns the number of pages in this potentially compound * page. compound_nr() can be called on a tail page, and is defined to diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h index 7fb7331c5725..6cfede39570a 100644 --- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -23,6 +23,7 @@ #include #include #include +#include #include =20 /* Free memory management - zoned buddy allocator. */ @@ -61,6 +62,36 @@ */ #define PAGE_ALLOC_COSTLY_ORDER 3 =20 +#if !defined(CONFIG_HAVE_GIGANTIC_FOLIOS) +/* + * We don't expect any folios that exceed buddy sizes (and consequently + * memory sections). + */ +#define MAX_FOLIO_ORDER MAX_PAGE_ORDER +#elif defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP) +/* + * Only pages within a single memory section are guaranteed to be + * contiguous. By limiting folios to a single memory section, all folio + * pages are guaranteed to be contiguous. + */ +#define MAX_FOLIO_ORDER PFN_SECTION_SHIFT +#elif defined(CONFIG_HUGETLB_PAGE) +/* + * There is no real limit on the folio size. We limit them to the maximum = we + * currently expect (see CONFIG_HAVE_GIGANTIC_FOLIOS): with hugetlb, we ex= pect + * no folios larger than 16 GiB on 64bit and 1 GiB on 32bit. + */ +#define MAX_FOLIO_ORDER get_order(IS_ENABLED(CONFIG_64BIT) ? SZ_16G : SZ_= 1G) +#else +/* + * Without hugetlb, gigantic folios that are bigger than a single PUD are + * currently impossible. + */ +#define MAX_FOLIO_ORDER PUD_ORDER +#endif + +#define MAX_FOLIO_NR_PAGES (1UL << MAX_FOLIO_ORDER) + enum migratetype { MIGRATE_UNMOVABLE, MIGRATE_MOVABLE, --=20 2.51.2 From nobody Sun Feb 8 15:59:22 2026 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E793539E171; Wed, 21 Jan 2026 16:23:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769012582; cv=none; b=kLjupfRl6RFEKGYDoGntp0GmE5pYR9U596OEtxsGPuNWsVpFhpqEOWGU7RIpPmpUoC619pllX6lYW3YkBJzld+sQIUgqhmFSt7OueR7mDfkfTtfmXwdGQmtuwDsYMlreow5VCJynrNthwfxB9JmaE4IOS/b+sbXijZstLRssAEU= ARC-Message-Signature: i=1; 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Wed, 21 Jan 2026 11:22:59 -0500 (EST) From: Kiryl Shutsemau To: Andrew Morton , Muchun Song , David Hildenbrand , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Kiryl Shutsemau Subject: [PATCHv4 02/14] mm: Change the interface of prep_compound_tail() Date: Wed, 21 Jan 2026 16:22:39 +0000 Message-ID: <20260121162253.2216580-3-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260121162253.2216580-1-kas@kernel.org> References: <20260121162253.2216580-1-kas@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Instead of passing down the head page and tail page index, pass the tail and head pages directly, as well as the order of the compound page. This is a preparation for changing how the head position is encoded in the tail page. Signed-off-by: Kiryl Shutsemau Reviewed-by: Muchun Song Reviewed-by: Zi Yan --- include/linux/page-flags.h | 4 +++- mm/hugetlb.c | 8 +++++--- mm/internal.h | 12 ++++++------ mm/mm_init.c | 2 +- mm/page_alloc.c | 2 +- 5 files changed, 16 insertions(+), 12 deletions(-) diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index 0091ad1986bf..d4952573a4af 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -865,7 +865,9 @@ static inline bool folio_test_large(const struct folio = *folio) return folio_test_head(folio); } =20 -static __always_inline void set_compound_head(struct page *page, struct pa= ge *head) +static __always_inline void set_compound_head(struct page *page, + const struct page *head, + unsigned int order) { WRITE_ONCE(page->compound_head, (unsigned long)head + 1); } diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 0455119716ec..a55d638975bd 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -3212,6 +3212,7 @@ int __alloc_bootmem_huge_page(struct hstate *h, int n= id) =20 /* Initialize [start_page:end_page_number] tail struct pages of a hugepage= */ static void __init hugetlb_folio_init_tail_vmemmap(struct folio *folio, + struct hstate *h, unsigned long start_page_number, unsigned long end_page_number) { @@ -3220,6 +3221,7 @@ static void __init hugetlb_folio_init_tail_vmemmap(st= ruct folio *folio, struct page *page =3D folio_page(folio, start_page_number); unsigned long head_pfn =3D folio_pfn(folio); unsigned long pfn, end_pfn =3D head_pfn + end_page_number; + unsigned int order =3D huge_page_order(h); =20 /* * As we marked all tail pages with memblock_reserved_mark_noinit(), @@ -3227,7 +3229,7 @@ static void __init hugetlb_folio_init_tail_vmemmap(st= ruct folio *folio, */ for (pfn =3D head_pfn + start_page_number; pfn < end_pfn; page++, pfn++) { __init_single_page(page, pfn, zone, nid); - prep_compound_tail((struct page *)folio, pfn - head_pfn); + prep_compound_tail(page, &folio->page, order); set_page_count(page, 0); } } @@ -3247,7 +3249,7 @@ static void __init hugetlb_folio_init_vmemmap(struct = folio *folio, __folio_set_head(folio); ret =3D folio_ref_freeze(folio, 1); VM_BUG_ON(!ret); - hugetlb_folio_init_tail_vmemmap(folio, 1, nr_pages); + hugetlb_folio_init_tail_vmemmap(folio, h, 1, nr_pages); prep_compound_head((struct page *)folio, huge_page_order(h)); } =20 @@ -3304,7 +3306,7 @@ static void __init prep_and_add_bootmem_folios(struct= hstate *h, * time as this is early in boot and there should * be no contention. */ - hugetlb_folio_init_tail_vmemmap(folio, + hugetlb_folio_init_tail_vmemmap(folio, h, HUGETLB_VMEMMAP_RESERVE_PAGES, pages_per_huge_page(h)); } diff --git a/mm/internal.h b/mm/internal.h index 1561fc2ff5b8..f385370256b9 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -810,13 +810,13 @@ static inline void prep_compound_head(struct page *pa= ge, unsigned int order) INIT_LIST_HEAD(&folio->_deferred_list); } =20 -static inline void prep_compound_tail(struct page *head, int tail_idx) +static inline void prep_compound_tail(struct page *tail, + const struct page *head, + unsigned int order) { - struct page *p =3D head + tail_idx; - - p->mapping =3D TAIL_MAPPING; - set_compound_head(p, head); - set_page_private(p, 0); + tail->mapping =3D TAIL_MAPPING; + set_compound_head(tail, head, order); + set_page_private(tail, 0); } =20 void post_alloc_hook(struct page *page, unsigned int order, gfp_t gfp_flag= s); diff --git a/mm/mm_init.c b/mm/mm_init.c index 7712d887b696..87d1e0277318 100644 --- a/mm/mm_init.c +++ b/mm/mm_init.c @@ -1102,7 +1102,7 @@ static void __ref memmap_init_compound(struct page *h= ead, struct page *page =3D pfn_to_page(pfn); 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Wed, 21 Jan 2026 11:23:01 -0500 (EST) From: Kiryl Shutsemau To: Andrew Morton , Muchun Song , David Hildenbrand , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Kiryl Shutsemau Subject: [PATCHv4 03/14] mm: Rename the 'compound_head' field in the 'struct page' to 'compound_info' Date: Wed, 21 Jan 2026 16:22:40 +0000 Message-ID: <20260121162253.2216580-4-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260121162253.2216580-1-kas@kernel.org> References: <20260121162253.2216580-1-kas@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" The 'compound_head' field in the 'struct page' encodes whether the page is a tail and where to locate the head page. Bit 0 is set if the page is a tail, and the remaining bits in the field point to the head page. As preparation for changing how the field encodes information about the head page, rename the field to 'compound_info'. Signed-off-by: Kiryl Shutsemau Reviewed-by: Muchun Song Reviewed-by: Zi Yan --- .../admin-guide/kdump/vmcoreinfo.rst | 2 +- Documentation/mm/vmemmap_dedup.rst | 6 +++--- include/linux/mm_types.h | 20 +++++++++---------- include/linux/page-flags.h | 18 ++++++++--------- include/linux/types.h | 2 +- kernel/vmcore_info.c | 2 +- mm/page_alloc.c | 2 +- mm/slab.h | 2 +- mm/util.c | 2 +- 9 files changed, 28 insertions(+), 28 deletions(-) diff --git a/Documentation/admin-guide/kdump/vmcoreinfo.rst b/Documentation= /admin-guide/kdump/vmcoreinfo.rst index 404a15f6782c..7663c610fe90 100644 --- a/Documentation/admin-guide/kdump/vmcoreinfo.rst +++ b/Documentation/admin-guide/kdump/vmcoreinfo.rst @@ -141,7 +141,7 @@ nodemask_t The size of a nodemask_t type. Used to compute the number of online nodes. =20 -(page, flags|_refcount|mapping|lru|_mapcount|private|compound_order|compou= nd_head) +(page, flags|_refcount|mapping|lru|_mapcount|private|compound_order|compou= nd_info) --------------------------------------------------------------------------= -------- =20 User-space tools compute their values based on the offset of these diff --git a/Documentation/mm/vmemmap_dedup.rst b/Documentation/mm/vmemmap_= dedup.rst index b4a55b6569fa..1863d88d2dcb 100644 --- a/Documentation/mm/vmemmap_dedup.rst +++ b/Documentation/mm/vmemmap_dedup.rst @@ -24,7 +24,7 @@ For each base page, there is a corresponding ``struct pag= e``. Within the HugeTLB subsystem, only the first 4 ``struct page`` are used to contain unique information about a HugeTLB page. ``__NR_USED_SUBPAGE`` pro= vides this upper limit. The only 'useful' information in the remaining ``struct = page`` -is the compound_head field, and this field is the same for all tail pages. +is the compound_info field, and this field is the same for all tail pages. =20 By removing redundant ``struct page`` for HugeTLB pages, memory can be ret= urned to the buddy allocator for other uses. @@ -124,10 +124,10 @@ Here is how things look before optimization:: | | +-----------+ =20 -The value of page->compound_head is the same for all tail pages. The first +The value of page->compound_info is the same for all tail pages. The first page of ``struct page`` (page 0) associated with the HugeTLB page contains= the 4 ``struct page`` necessary to describe the HugeTLB. The only use of the rem= aining -pages of ``struct page`` (page 1 to page 7) is to point to page->compound_= head. +pages of ``struct page`` (page 1 to page 7) is to point to page->compound_= info. Therefore, we can remap pages 1 to 7 to page 0. Only 1 page of ``struct pa= ge`` will be used for each HugeTLB page. This will allow us to free the remaini= ng 7 pages to the buddy allocator. diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 90e5790c318f..a94683272869 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -125,14 +125,14 @@ struct page { atomic_long_t pp_ref_count; }; struct { /* Tail pages of compound page */ - unsigned long compound_head; /* Bit zero is set */ + unsigned long compound_info; /* Bit zero is set */ }; struct { /* ZONE_DEVICE pages */ /* - * The first word is used for compound_head or folio + * The first word is used for compound_info or folio * pgmap */ - void *_unused_pgmap_compound_head; + void *_unused_pgmap_compound_info; void *zone_device_data; /* * ZONE_DEVICE private pages are counted as being @@ -383,7 +383,7 @@ struct folio { /* private: avoid cluttering the output */ /* For the Unevictable "LRU list" slot */ struct { - /* Avoid compound_head */ + /* Avoid compound_info */ void *__filler; /* public: */ unsigned int mlock_count; @@ -484,7 +484,7 @@ struct folio { FOLIO_MATCH(flags, flags); FOLIO_MATCH(lru, lru); FOLIO_MATCH(mapping, mapping); -FOLIO_MATCH(compound_head, lru); +FOLIO_MATCH(compound_info, lru); FOLIO_MATCH(__folio_index, index); FOLIO_MATCH(private, private); FOLIO_MATCH(_mapcount, _mapcount); @@ -503,7 +503,7 @@ FOLIO_MATCH(_last_cpupid, _last_cpupid); static_assert(offsetof(struct folio, fl) =3D=3D \ offsetof(struct page, pg) + sizeof(struct page)) FOLIO_MATCH(flags, _flags_1); -FOLIO_MATCH(compound_head, _head_1); +FOLIO_MATCH(compound_info, _head_1); FOLIO_MATCH(_mapcount, _mapcount_1); FOLIO_MATCH(_refcount, _refcount_1); #undef FOLIO_MATCH @@ -511,13 +511,13 @@ FOLIO_MATCH(_refcount, _refcount_1); static_assert(offsetof(struct folio, fl) =3D=3D \ offsetof(struct page, pg) + 2 * sizeof(struct page)) FOLIO_MATCH(flags, _flags_2); -FOLIO_MATCH(compound_head, _head_2); +FOLIO_MATCH(compound_info, _head_2); #undef FOLIO_MATCH #define FOLIO_MATCH(pg, fl) \ static_assert(offsetof(struct folio, fl) =3D=3D \ offsetof(struct page, pg) + 3 * sizeof(struct page)) FOLIO_MATCH(flags, _flags_3); -FOLIO_MATCH(compound_head, _head_3); +FOLIO_MATCH(compound_info, _head_3); #undef FOLIO_MATCH =20 /** @@ -583,8 +583,8 @@ struct ptdesc { #define TABLE_MATCH(pg, pt) \ static_assert(offsetof(struct page, pg) =3D=3D offsetof(struct ptdesc, pt= )) TABLE_MATCH(flags, pt_flags); -TABLE_MATCH(compound_head, pt_list); -TABLE_MATCH(compound_head, _pt_pad_1); +TABLE_MATCH(compound_info, pt_list); +TABLE_MATCH(compound_info, _pt_pad_1); TABLE_MATCH(mapping, __page_mapping); TABLE_MATCH(__folio_index, pt_index); TABLE_MATCH(rcu_head, pt_rcu_head); diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index d4952573a4af..72c933a43b6a 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -213,7 +213,7 @@ static __always_inline const struct page *page_fixed_fa= ke_head(const struct page /* * Only addresses aligned with PAGE_SIZE of struct page may be fake head * struct page. The alignment check aims to avoid access the fields ( - * e.g. compound_head) of the @page[1]. It can avoid touch a (possibly) + * e.g. compound_info) of the @page[1]. It can avoid touch a (possibly) * cold cacheline in some cases. */ if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) && @@ -223,7 +223,7 @@ static __always_inline const struct page *page_fixed_fa= ke_head(const struct page * because the @page is a compound page composed with at least * two contiguous pages. */ - unsigned long head =3D READ_ONCE(page[1].compound_head); + unsigned long head =3D READ_ONCE(page[1].compound_info); =20 if (likely(head & 1)) return (const struct page *)(head - 1); @@ -281,7 +281,7 @@ static __always_inline int page_is_fake_head(const stru= ct page *page) =20 static __always_inline unsigned long _compound_head(const struct page *pag= e) { - unsigned long head =3D READ_ONCE(page->compound_head); + unsigned long head =3D READ_ONCE(page->compound_info); =20 if (unlikely(head & 1)) return head - 1; @@ -320,13 +320,13 @@ static __always_inline unsigned long _compound_head(c= onst struct page *page) =20 static __always_inline int PageTail(const struct page *page) { - return READ_ONCE(page->compound_head) & 1 || page_is_fake_head(page); + return READ_ONCE(page->compound_info) & 1 || page_is_fake_head(page); } =20 static __always_inline int PageCompound(const struct page *page) { return test_bit(PG_head, &page->flags.f) || - READ_ONCE(page->compound_head) & 1; + READ_ONCE(page->compound_info) & 1; } =20 #define PAGE_POISON_PATTERN -1l @@ -348,7 +348,7 @@ static const unsigned long *const_folio_flags(const str= uct folio *folio, { const struct page *page =3D &folio->page; =20 - VM_BUG_ON_PGFLAGS(page->compound_head & 1, page); + VM_BUG_ON_PGFLAGS(page->compound_info & 1, page); VM_BUG_ON_PGFLAGS(n > 0 && !test_bit(PG_head, &page->flags.f), page); return &page[n].flags.f; } @@ -357,7 +357,7 @@ static unsigned long *folio_flags(struct folio *folio, = unsigned n) { struct page *page =3D &folio->page; =20 - VM_BUG_ON_PGFLAGS(page->compound_head & 1, page); + VM_BUG_ON_PGFLAGS(page->compound_info & 1, page); VM_BUG_ON_PGFLAGS(n > 0 && !test_bit(PG_head, &page->flags.f), page); return &page[n].flags.f; } @@ -869,12 +869,12 @@ static __always_inline void set_compound_head(struct = page *page, const struct page *head, unsigned int order) { - WRITE_ONCE(page->compound_head, (unsigned long)head + 1); + WRITE_ONCE(page->compound_info, (unsigned long)head + 1); } =20 static __always_inline void clear_compound_head(struct page *page) { - WRITE_ONCE(page->compound_head, 0); + WRITE_ONCE(page->compound_info, 0); } =20 #ifdef CONFIG_TRANSPARENT_HUGEPAGE diff --git a/include/linux/types.h b/include/linux/types.h index 6dfdb8e8e4c3..3a65f0ef4a73 100644 --- a/include/linux/types.h +++ b/include/linux/types.h @@ -234,7 +234,7 @@ struct ustat { * * This guarantee is important for few reasons: * - future call_rcu_lazy() will make use of lower bits in the pointer; - * - the structure shares storage space in struct page with @compound_hea= d, + * - the structure shares storage space in struct page with @compound_inf= o, * which encode PageTail() in bit 0. The guarantee is needed to avoid * false-positive PageTail(). */ diff --git a/kernel/vmcore_info.c b/kernel/vmcore_info.c index e066d31d08f8..782bc2050a40 100644 --- a/kernel/vmcore_info.c +++ b/kernel/vmcore_info.c @@ -175,7 +175,7 @@ static int __init crash_save_vmcoreinfo_init(void) VMCOREINFO_OFFSET(page, lru); VMCOREINFO_OFFSET(page, _mapcount); VMCOREINFO_OFFSET(page, private); - VMCOREINFO_OFFSET(page, compound_head); + VMCOREINFO_OFFSET(page, compound_info); VMCOREINFO_OFFSET(pglist_data, node_zones); VMCOREINFO_OFFSET(pglist_data, nr_zones); #ifdef CONFIG_FLATMEM diff --git a/mm/page_alloc.c b/mm/page_alloc.c index fe77c00c99df..cecd6d89ff60 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -704,7 +704,7 @@ static inline bool pcp_allowed_order(unsigned int order) * The first PAGE_SIZE page is called the "head page" and have PG_head set. * * The remaining PAGE_SIZE pages are called "tail pages". PageTail() is en= coded - * in bit 0 of page->compound_head. The rest of bits is pointer to head pa= ge. + * in bit 0 of page->compound_info. The rest of bits is pointer to head pa= ge. * * The first tail page's ->compound_order holds the order of allocation. * This usage means that zero-order pages may not be compound. diff --git a/mm/slab.h b/mm/slab.h index 078daecc7cf5..b471877af296 100644 --- a/mm/slab.h +++ b/mm/slab.h @@ -104,7 +104,7 @@ struct slab { #define SLAB_MATCH(pg, sl) \ static_assert(offsetof(struct page, pg) =3D=3D offsetof(struct slab, sl)) SLAB_MATCH(flags, flags); -SLAB_MATCH(compound_head, slab_cache); /* Ensure bit 0 is clear */ +SLAB_MATCH(compound_info, slab_cache); /* Ensure bit 0 is clear */ SLAB_MATCH(_refcount, __page_refcount); #ifdef CONFIG_MEMCG SLAB_MATCH(memcg_data, obj_exts); diff --git a/mm/util.c b/mm/util.c index 8989d5767528..cbf93cf3223a 100644 --- a/mm/util.c +++ b/mm/util.c @@ -1244,7 +1244,7 @@ void snapshot_page(struct page_snapshot *ps, const st= ruct page *page) again: memset(&ps->folio_snapshot, 0, sizeof(struct folio)); memcpy(&ps->page_snapshot, page, sizeof(*page)); - head =3D ps->page_snapshot.compound_head; 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Wed, 21 Jan 2026 11:23:03 -0500 (EST) From: Kiryl Shutsemau To: Andrew Morton , Muchun Song , David Hildenbrand , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Kiryl Shutsemau Subject: [PATCHv4 04/14] mm: Move set/clear_compound_head() next to compound_head() Date: Wed, 21 Jan 2026 16:22:41 +0000 Message-ID: <20260121162253.2216580-5-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260121162253.2216580-1-kas@kernel.org> References: <20260121162253.2216580-1-kas@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Move set_compound_head() and clear_compound_head() to be adjacent to the compound_head() function in page-flags.h. These functions encode and decode the same compound_info field, so keeping them together makes it easier to verify their logic is consistent, especially when the encoding changes. Signed-off-by: Kiryl Shutsemau Reviewed-by: Muchun Song Reviewed-by: Zi Yan --- include/linux/page-flags.h | 24 ++++++++++++------------ 1 file changed, 12 insertions(+), 12 deletions(-) diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index 72c933a43b6a..0de7db7efb00 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -290,6 +290,18 @@ static __always_inline unsigned long _compound_head(co= nst struct page *page) =20 #define compound_head(page) ((typeof(page))_compound_head(page)) =20 +static __always_inline void set_compound_head(struct page *page, + const struct page *head, + unsigned int order) +{ + WRITE_ONCE(page->compound_info, (unsigned long)head + 1); +} + +static __always_inline void clear_compound_head(struct page *page) +{ + WRITE_ONCE(page->compound_info, 0); +} + /** * page_folio - Converts from page to folio. * @p: The page. @@ -865,18 +877,6 @@ static inline bool folio_test_large(const struct folio= *folio) return folio_test_head(folio); } =20 -static __always_inline void set_compound_head(struct page *page, - const struct page *head, - unsigned int order) -{ - WRITE_ONCE(page->compound_info, (unsigned long)head + 1); -} - -static __always_inline void clear_compound_head(struct page *page) -{ - WRITE_ONCE(page->compound_info, 0); -} - #ifdef CONFIG_TRANSPARENT_HUGEPAGE static inline void ClearPageCompound(struct page *page) { --=20 2.51.2 From nobody Sun Feb 8 15:59:22 2026 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 566583AA190 for ; Wed, 21 Jan 2026 16:23:08 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769012588; cv=none; b=Wv2psS43wXvKphJyZphC0CYmM3CoS8hujpeZu4dCT8uT3pMi1G3GuMa32JZeBPGCl1h0XXKYQ0n3yKetkCfyPZG4DvmZPvnGMBH6+LQNvKKUXcLHjJ1Cr3RClqSrU1h2PnvCGLXpuy60uPfdBJBifvLobpeRZ6xup1j36x9DHLE= ARC-Message-Signature: i=1; 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Wed, 21 Jan 2026 11:23:05 -0500 (EST) From: Kiryl Shutsemau To: Andrew Morton , Muchun Song , David Hildenbrand , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Kiryl Shutsemau Subject: [PATCHv4 05/14] mm: Rework compound_head() for power-of-2 sizeof(struct page) Date: Wed, 21 Jan 2026 16:22:42 +0000 Message-ID: <20260121162253.2216580-6-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260121162253.2216580-1-kas@kernel.org> References: <20260121162253.2216580-1-kas@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" For tail pages, the kernel uses the 'compound_info' field to get to the head page. The bit 0 of the field indicates whether the page is a tail page, and if set, the remaining bits represent a pointer to the head page. For cases when size of struct page is power-of-2, change the encoding of compound_info to store a mask that can be applied to the virtual address of the tail page in order to access the head page. It is possible because struct page of the head page is naturally aligned with regards to order of the page. The significant impact of this modification is that all tail pages of the same order will now have identical 'compound_info', regardless of the compound page they are associated with. This paves the way for eliminating fake heads. The HugeTLB Vmemmap Optimization (HVO) creates fake heads and it is only applied when the sizeof(struct page) is power-of-2. Having identical tail pages allows the same page to be mapped into the vmemmap of all pages, maintaining memory savings without fake heads. If sizeof(struct page) is not power-of-2, there is no functional changes. Limit mask usage to SPARSEMEM_VMEMMAP where it makes a difference because HVO. The approach with mask would work for any memory model, but it requires validating that struct pages are naturally aligned for all orders up to the MAX_FOLIO order, which can be tricky. Signed-off-by: Kiryl Shutsemau Reviewed-by: Muchun Song Reviewed-by: Zi Yan --- include/linux/page-flags.h | 81 ++++++++++++++++++++++++++++++++++---- mm/util.c | 16 ++++++-- 2 files changed, 85 insertions(+), 12 deletions(-) diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index 0de7db7efb00..e16a4bc82856 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -198,6 +198,29 @@ enum pageflags { =20 #ifndef __GENERATING_BOUNDS_H =20 +/* + * For tail pages, if the size of struct page is power-of-2 ->compound_info + * encodes the mask that converts the address of the tail page address to + * the head page address. + * + * Otherwise, ->compound_info has direct pointer to head pages. + */ +static __always_inline bool compound_info_has_mask(void) +{ + /* + * Limit mask usage to SPARSEMEM_VMEMMAP where it makes a difference + * because of the HugeTLB vmemmap optimization (HVO). + * + * The approach with mask would work for any memory model, but it + * requires validating that struct pages are naturally aligned for + * all orders up to the MAX_FOLIO order, which can be tricky. + */ + if (!IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP)) + return false; + + return is_power_of_2(sizeof(struct page)); +} + #ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP DECLARE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key); =20 @@ -210,6 +233,10 @@ static __always_inline const struct page *page_fixed_f= ake_head(const struct page if (!static_branch_unlikely(&hugetlb_optimize_vmemmap_key)) return page; =20 + /* Fake heads only exists if compound_info_has_mask() is true */ + if (!compound_info_has_mask()) + return page; + /* * Only addresses aligned with PAGE_SIZE of struct page may be fake head * struct page. The alignment check aims to avoid access the fields ( @@ -223,10 +250,14 @@ static __always_inline const struct page *page_fixed_= fake_head(const struct page * because the @page is a compound page composed with at least * two contiguous pages. */ - unsigned long head =3D READ_ONCE(page[1].compound_info); + unsigned long info =3D READ_ONCE(page[1].compound_info); =20 - if (likely(head & 1)) - return (const struct page *)(head - 1); + /* See set_compound_head() */ + if (likely(info & 1)) { + unsigned long p =3D (unsigned long)page; + + return (const struct page *)(p & info); + } } return page; } @@ -281,11 +312,26 @@ static __always_inline int page_is_fake_head(const st= ruct page *page) =20 static __always_inline unsigned long _compound_head(const struct page *pag= e) { - unsigned long head =3D READ_ONCE(page->compound_info); + unsigned long info =3D READ_ONCE(page->compound_info); =20 - if (unlikely(head & 1)) - return head - 1; - return (unsigned long)page_fixed_fake_head(page); + /* Bit 0 encodes PageTail() */ + if (!(info & 1)) + return (unsigned long)page_fixed_fake_head(page); + + /* + * If compound_info_has_mask() is false, the rest of compound_info is + * the pointer to the head page. + */ + if (!compound_info_has_mask()) + return info - 1; + + /* + * If compoun_info_has_mask() is true the rest of the info encodes + * the mask that converts the address of the tail page to the head page. + * + * No need to clear bit 0 in the mask as 'page' always has it clear. + */ + return (unsigned long)page & info; } =20 #define compound_head(page) ((typeof(page))_compound_head(page)) @@ -294,7 +340,26 @@ static __always_inline void set_compound_head(struct p= age *page, const struct page *head, unsigned int order) { - WRITE_ONCE(page->compound_info, (unsigned long)head + 1); + unsigned int shift; + unsigned long mask; + + if (!compound_info_has_mask()) { + WRITE_ONCE(page->compound_info, (unsigned long)head | 1); + return; + } + + /* + * If the size of struct page is power-of-2, bits [shift:0] of the + * virtual address of compound head are zero. + * + * Calculate mask that can be applied to the virtual address of + * the tail page to get address of the head page. + */ + shift =3D order + order_base_2(sizeof(struct page)); + mask =3D GENMASK(BITS_PER_LONG - 1, shift); + + /* Bit 0 encodes PageTail() */ + WRITE_ONCE(page->compound_info, mask | 1); } =20 static __always_inline void clear_compound_head(struct page *page) diff --git a/mm/util.c b/mm/util.c index cbf93cf3223a..f01a9655067f 100644 --- a/mm/util.c +++ b/mm/util.c @@ -1234,7 +1234,7 @@ static void set_ps_flags(struct page_snapshot *ps, co= nst struct folio *folio, */ void snapshot_page(struct page_snapshot *ps, const struct page *page) { - unsigned long head, nr_pages =3D 1; + unsigned long info, nr_pages =3D 1; struct folio *foliop; int loops =3D 5; =20 @@ -1244,8 +1244,8 @@ void snapshot_page(struct page_snapshot *ps, const st= ruct page *page) again: memset(&ps->folio_snapshot, 0, sizeof(struct folio)); memcpy(&ps->page_snapshot, page, sizeof(*page)); - head =3D ps->page_snapshot.compound_info; - if ((head & 1) =3D=3D 0) { + info =3D ps->page_snapshot.compound_info; + if ((info & 1) =3D=3D 0) { ps->idx =3D 0; foliop =3D (struct folio *)&ps->page_snapshot; if (!folio_test_large(foliop)) { @@ -1256,7 +1256,15 @@ void snapshot_page(struct page_snapshot *ps, const s= truct page *page) } foliop =3D (struct folio *)page; } else { - foliop =3D (struct folio *)(head - 1); + /* See compound_head() */ + if (compound_info_has_mask()) { + unsigned long p =3D (unsigned long)page; + + foliop =3D (struct folio *)(p & info); + } else { + foliop =3D (struct folio *)(info - 1); + } + ps->idx =3D folio_page_idx(foliop, page); } =20 --=20 2.51.2 From nobody Sun Feb 8 15:59:22 2026 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id BD4AB31B812 for ; Wed, 21 Jan 2026 16:23:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769012589; cv=none; 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Wed, 21 Jan 2026 11:23:07 -0500 (EST) From: Kiryl Shutsemau To: Andrew Morton , Muchun Song , David Hildenbrand , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Kiryl Shutsemau Subject: [PATCHv4 06/14] mm: Make page_zonenum() use head page Date: Wed, 21 Jan 2026 16:22:43 +0000 Message-ID: <20260121162253.2216580-7-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260121162253.2216580-1-kas@kernel.org> References: <20260121162253.2216580-1-kas@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" With the upcoming changes to HVO, a single page of tail struct pages will be shared across all huge pages of the same order on a node. Since huge pages on the same node may belong to different zones, the zone information stored in shared tail page flags would be incorrect. Always fetch zone information from the head page, which has unique and correct zone flags for each compound page. Signed-off-by: Kiryl Shutsemau Acked-by: Zi Yan --- include/linux/mmzone.h | 1 + 1 file changed, 1 insertion(+) diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h index 6cfede39570a..390ce11b3765 100644 --- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -1223,6 +1223,7 @@ static inline enum zone_type memdesc_zonenum(memdesc_= flags_t flags) =20 static inline enum zone_type page_zonenum(const struct page *page) { + page =3D compound_head(page); return memdesc_zonenum(page->flags); } =20 --=20 2.51.2 From nobody Sun Feb 8 15:59:22 2026 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 03FBE3B961B; Wed, 21 Jan 2026 16:23:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; 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Wed, 21 Jan 2026 11:23:10 -0500 (EST) From: Kiryl Shutsemau To: Andrew Morton , Muchun Song , David Hildenbrand , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Kiryl Shutsemau Subject: [PATCHv4 07/14] mm/sparse: Check memmap alignment for compound_info_has_mask() Date: Wed, 21 Jan 2026 16:22:44 +0000 Message-ID: <20260121162253.2216580-8-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260121162253.2216580-1-kas@kernel.org> References: <20260121162253.2216580-1-kas@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" If page->compound_info encodes a mask, it is expected that memmap to be naturally aligned to the maximum folio size. Add a warning if it is not. A warning is sufficient as MAX_FOLIO_ORDER is very rarely used, so the kernel is still likely to be functional if this strict check fails. Signed-off-by: Kiryl Shutsemau Acked-by: Zi Yan --- include/linux/mmzone.h | 1 + mm/sparse.c | 5 +++++ 2 files changed, 6 insertions(+) diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h index 390ce11b3765..7e4f69b9d760 100644 --- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -91,6 +91,7 @@ #endif =20 #define MAX_FOLIO_NR_PAGES (1UL << MAX_FOLIO_ORDER) +#define MAX_FOLIO_SIZE (PAGE_SIZE << MAX_FOLIO_ORDER) =20 enum migratetype { MIGRATE_UNMOVABLE, diff --git a/mm/sparse.c b/mm/sparse.c index 17c50a6415c2..5f41a3edcc24 100644 --- a/mm/sparse.c +++ b/mm/sparse.c @@ -600,6 +600,11 @@ void __init sparse_init(void) BUILD_BUG_ON(!is_power_of_2(sizeof(struct mem_section))); memblocks_present(); =20 + if (compound_info_has_mask()) { + WARN_ON(!IS_ALIGNED((unsigned long)pfn_to_page(0), + MAX_FOLIO_SIZE / sizeof(struct page))); + } + pnum_begin =3D first_present_section_nr(); nid_begin =3D sparse_early_nid(__nr_to_section(pnum_begin)); =20 --=20 2.51.2 From nobody Sun Feb 8 15:59:22 2026 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id BA5993D7D88; 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Wed, 21 Jan 2026 11:23:12 -0500 (EST) From: Kiryl Shutsemau To: Andrew Morton , Muchun Song , David Hildenbrand , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Kiryl Shutsemau Subject: [PATCHv4 08/14] mm/hugetlb: Refactor code around vmemmap_walk Date: Wed, 21 Jan 2026 16:22:45 +0000 Message-ID: <20260121162253.2216580-9-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260121162253.2216580-1-kas@kernel.org> References: <20260121162253.2216580-1-kas@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" To prepare for removing fake head pages, the vmemmap_walk code is being rew= orked. The reuse_page and reuse_addr variables are being eliminated. There will no longer be an expectation regarding the reuse address in relation to the operated range. Instead, the caller will provide head and tail vmemmap pages, along with the vmemmap_start address where the head page is located. Currently, vmemmap_head and vmemmap_tail are set to the same page, but this will change in the future. The only functional change is that __hugetlb_vmemmap_optimize_folio() will abandon optimization if memory allocation fails. Signed-off-by: Kiryl Shutsemau --- mm/hugetlb_vmemmap.c | 200 ++++++++++++++++++------------------------- 1 file changed, 84 insertions(+), 116 deletions(-) diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c index ba0fb1b6a5a8..a51c0e293175 100644 --- a/mm/hugetlb_vmemmap.c +++ b/mm/hugetlb_vmemmap.c @@ -24,8 +24,9 @@ * * @remap_pte: called for each lowest-level entry (PTE). * @nr_walked: the number of walked pte. - * @reuse_page: the page which is reused for the tail vmemmap pages. - * @reuse_addr: the virtual address of the @reuse_page page. + * @vmemmap_start: the start of vmemmap range, where head page is located + * @vmemmap_head: the page to be installed as first in the vmemmap range + * @vmemmap_tail: the page to be installed as non-first in the vmemmap ran= ge * @vmemmap_pages: the list head of the vmemmap pages that can be freed * or is mapped from. * @flags: used to modify behavior in vmemmap page table walking @@ -34,11 +35,14 @@ struct vmemmap_remap_walk { void (*remap_pte)(pte_t *pte, unsigned long addr, struct vmemmap_remap_walk *walk); + unsigned long nr_walked; - struct page *reuse_page; - unsigned long reuse_addr; + unsigned long vmemmap_start; + struct page *vmemmap_head; + struct page *vmemmap_tail; struct list_head *vmemmap_pages; =20 + /* Skip the TLB flush when we split the PMD */ #define VMEMMAP_SPLIT_NO_TLB_FLUSH BIT(0) /* Skip the TLB flush when we remap the PTE */ @@ -140,14 +144,7 @@ static int vmemmap_pte_entry(pte_t *pte, unsigned long= addr, { struct vmemmap_remap_walk *vmemmap_walk =3D walk->private; =20 - /* - * The reuse_page is found 'first' in page table walking before - * starting remapping. - */ - if (!vmemmap_walk->reuse_page) - vmemmap_walk->reuse_page =3D pte_page(ptep_get(pte)); - else - vmemmap_walk->remap_pte(pte, addr, vmemmap_walk); + vmemmap_walk->remap_pte(pte, addr, vmemmap_walk); vmemmap_walk->nr_walked++; =20 return 0; @@ -207,18 +204,12 @@ static void free_vmemmap_page_list(struct list_head *= list) static void vmemmap_remap_pte(pte_t *pte, unsigned long addr, struct vmemmap_remap_walk *walk) { - /* - * Remap the tail pages as read-only to catch illegal write operation - * to the tail pages. - */ - pgprot_t pgprot =3D PAGE_KERNEL_RO; struct page *page =3D pte_page(ptep_get(pte)); pte_t entry; =20 /* Remapping the head page requires r/w */ - if (unlikely(addr =3D=3D walk->reuse_addr)) { - pgprot =3D PAGE_KERNEL; - list_del(&walk->reuse_page->lru); + if (unlikely(addr =3D=3D walk->vmemmap_start)) { + list_del(&walk->vmemmap_head->lru); =20 /* * Makes sure that preceding stores to the page contents from @@ -226,9 +217,16 @@ static void vmemmap_remap_pte(pte_t *pte, unsigned lon= g addr, * write. */ smp_wmb(); + + entry =3D mk_pte(walk->vmemmap_head, PAGE_KERNEL); + } else { + /* + * Remap the tail pages as read-only to catch illegal write + * operation to the tail pages. + */ + entry =3D mk_pte(walk->vmemmap_tail, PAGE_KERNEL_RO); } =20 - entry =3D mk_pte(walk->reuse_page, pgprot); list_add(&page->lru, walk->vmemmap_pages); set_pte_at(&init_mm, addr, pte, entry); } @@ -255,16 +253,13 @@ static inline void reset_struct_pages(struct page *st= art) static void vmemmap_restore_pte(pte_t *pte, unsigned long addr, struct vmemmap_remap_walk *walk) { - pgprot_t pgprot =3D PAGE_KERNEL; struct page *page; void *to; =20 - BUG_ON(pte_page(ptep_get(pte)) !=3D walk->reuse_page); - page =3D list_first_entry(walk->vmemmap_pages, struct page, lru); list_del(&page->lru); to =3D page_to_virt(page); - copy_page(to, (void *)walk->reuse_addr); + copy_page(to, (void *)walk->vmemmap_start); reset_struct_pages(to); =20 /* @@ -272,7 +267,7 @@ static void vmemmap_restore_pte(pte_t *pte, unsigned lo= ng addr, * before the set_pte_at() write. */ smp_wmb(); - set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot)); + set_pte_at(&init_mm, addr, pte, mk_pte(page, PAGE_KERNEL)); } =20 /** @@ -282,33 +277,29 @@ static void vmemmap_restore_pte(pte_t *pte, unsigned = long addr, * to remap. * @end: end address of the vmemmap virtual address range that we wa= nt to * remap. - * @reuse: reuse address. - * * Return: %0 on success, negative error code otherwise. */ -static int vmemmap_remap_split(unsigned long start, unsigned long end, - unsigned long reuse) +static int vmemmap_remap_split(unsigned long start, unsigned long end) { struct vmemmap_remap_walk walk =3D { .remap_pte =3D NULL, + .vmemmap_start =3D start, .flags =3D VMEMMAP_SPLIT_NO_TLB_FLUSH, }; =20 - /* See the comment in the vmemmap_remap_free(). */ - BUG_ON(start - reuse !=3D PAGE_SIZE); - - return vmemmap_remap_range(reuse, end, &walk); + return vmemmap_remap_range(start, end, &walk); } =20 /** * vmemmap_remap_free - remap the vmemmap virtual address range [@start, @= end) - * to the page which @reuse is mapped to, then free vmemmap - * which the range are mapped to. + * to use @vmemmap_head/tail, then free vmemmap which + * the range are mapped to. * @start: start address of the vmemmap virtual address range that we want * to remap. * @end: end address of the vmemmap virtual address range that we want to * remap. - * @reuse: reuse address. + * @vmemmap_head: the page to be installed as first in the vmemmap range + * @vmemmap_tail: the page to be installed as non-first in the vmemmap ran= ge * @vmemmap_pages: list to deposit vmemmap pages to be freed. It is calle= rs * responsibility to free pages. * @flags: modifications to vmemmap_remap_walk flags @@ -316,69 +307,40 @@ static int vmemmap_remap_split(unsigned long start, u= nsigned long end, * Return: %0 on success, negative error code otherwise. */ static int vmemmap_remap_free(unsigned long start, unsigned long end, - unsigned long reuse, + struct page *vmemmap_head, + struct page *vmemmap_tail, struct list_head *vmemmap_pages, unsigned long flags) { int ret; struct vmemmap_remap_walk walk =3D { .remap_pte =3D vmemmap_remap_pte, - .reuse_addr =3D reuse, + .vmemmap_start =3D start, + .vmemmap_head =3D vmemmap_head, + .vmemmap_tail =3D vmemmap_tail, .vmemmap_pages =3D vmemmap_pages, .flags =3D flags, }; - int nid =3D page_to_nid((struct page *)reuse); - gfp_t gfp_mask =3D GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN; + + ret =3D vmemmap_remap_range(start, end, &walk); + if (!ret || !walk.nr_walked) + return ret; + + end =3D start + walk.nr_walked * PAGE_SIZE; =20 /* - * Allocate a new head vmemmap page to avoid breaking a contiguous - * block of struct page memory when freeing it back to page allocator - * in free_vmemmap_page_list(). This will allow the likely contiguous - * struct page backing memory to be kept contiguous and allowing for - * more allocations of hugepages. Fallback to the currently - * mapped head page in case should it fail to allocate. + * vmemmap_pages contains pages from the previous vmemmap_remap_range() + * call which failed. These are pages which were removed from + * the vmemmap. They will be restored in the following call. */ - walk.reuse_page =3D alloc_pages_node(nid, gfp_mask, 0); - if (walk.reuse_page) { - copy_page(page_to_virt(walk.reuse_page), - (void *)walk.reuse_addr); - list_add(&walk.reuse_page->lru, vmemmap_pages); - memmap_pages_add(1); - } + walk =3D (struct vmemmap_remap_walk) { + .remap_pte =3D vmemmap_restore_pte, + .vmemmap_start =3D start, + .vmemmap_pages =3D vmemmap_pages, + .flags =3D 0, + }; =20 - /* - * In order to make remapping routine most efficient for the huge pages, - * the routine of vmemmap page table walking has the following rules - * (see more details from the vmemmap_pte_range()): - * - * - The range [@start, @end) and the range [@reuse, @reuse + PAGE_SIZE) - * should be continuous. - * - The @reuse address is part of the range [@reuse, @end) that we are - * walking which is passed to vmemmap_remap_range(). - * - The @reuse address is the first in the complete range. - * - * So we need to make sure that @start and @reuse meet the above rules. - */ - BUG_ON(start - reuse !=3D PAGE_SIZE); - - ret =3D vmemmap_remap_range(reuse, end, &walk); - if (ret && walk.nr_walked) { - end =3D reuse + walk.nr_walked * PAGE_SIZE; - /* - * vmemmap_pages contains pages from the previous - * vmemmap_remap_range call which failed. These - * are pages which were removed from the vmemmap. - * They will be restored in the following call. - */ - walk =3D (struct vmemmap_remap_walk) { - .remap_pte =3D vmemmap_restore_pte, - .reuse_addr =3D reuse, - .vmemmap_pages =3D vmemmap_pages, - .flags =3D 0, - }; - - vmemmap_remap_range(reuse, end, &walk); - } + vmemmap_remap_range(start, end, &walk); =20 return ret; } @@ -415,29 +377,25 @@ static int alloc_vmemmap_page_list(unsigned long star= t, unsigned long end, * to remap. * @end: end address of the vmemmap virtual address range that we want to * remap. - * @reuse: reuse address. * @flags: modifications to vmemmap_remap_walk flags * * Return: %0 on success, negative error code otherwise. */ static int vmemmap_remap_alloc(unsigned long start, unsigned long end, - unsigned long reuse, unsigned long flags) + unsigned long flags) { LIST_HEAD(vmemmap_pages); struct vmemmap_remap_walk walk =3D { .remap_pte =3D vmemmap_restore_pte, - .reuse_addr =3D reuse, + .vmemmap_start =3D start, .vmemmap_pages =3D &vmemmap_pages, .flags =3D flags, }; =20 - /* See the comment in the vmemmap_remap_free(). */ - BUG_ON(start - reuse !=3D PAGE_SIZE); - if (alloc_vmemmap_page_list(start, end, &vmemmap_pages)) return -ENOMEM; =20 - return vmemmap_remap_range(reuse, end, &walk); + return vmemmap_remap_range(start, end, &walk); } =20 DEFINE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key); @@ -454,8 +412,7 @@ static int __hugetlb_vmemmap_restore_folio(const struct= hstate *h, struct folio *folio, unsigned long flags) { int ret; - unsigned long vmemmap_start =3D (unsigned long)&folio->page, vmemmap_end; - unsigned long vmemmap_reuse; + unsigned long vmemmap_start, vmemmap_end; =20 VM_WARN_ON_ONCE_FOLIO(!folio_test_hugetlb(folio), folio); VM_WARN_ON_ONCE_FOLIO(folio_ref_count(folio), folio); @@ -466,18 +423,18 @@ static int __hugetlb_vmemmap_restore_folio(const stru= ct hstate *h, if (flags & VMEMMAP_SYNCHRONIZE_RCU) synchronize_rcu(); =20 + vmemmap_start =3D (unsigned long)&folio->page; vmemmap_end =3D vmemmap_start + hugetlb_vmemmap_size(h); - vmemmap_reuse =3D vmemmap_start; - vmemmap_start +=3D HUGETLB_VMEMMAP_RESERVE_SIZE; + + vmemmap_start +=3D HUGETLB_VMEMMAP_RESERVE_SIZE; =20 /* * The pages which the vmemmap virtual address range [@vmemmap_start, - * @vmemmap_end) are mapped to are freed to the buddy allocator, and - * the range is mapped to the page which @vmemmap_reuse is mapped to. + * @vmemmap_end) are mapped to are freed to the buddy allocator. * When a HugeTLB page is freed to the buddy allocator, previously * discarded vmemmap pages must be allocated and remapping. */ - ret =3D vmemmap_remap_alloc(vmemmap_start, vmemmap_end, vmemmap_reuse, fl= ags); + ret =3D vmemmap_remap_alloc(vmemmap_start, vmemmap_end, flags); if (!ret) { folio_clear_hugetlb_vmemmap_optimized(folio); static_branch_dec(&hugetlb_optimize_vmemmap_key); @@ -565,9 +522,9 @@ static int __hugetlb_vmemmap_optimize_folio(const struc= t hstate *h, struct list_head *vmemmap_pages, unsigned long flags) { - int ret =3D 0; - unsigned long vmemmap_start =3D (unsigned long)&folio->page, vmemmap_end; - unsigned long vmemmap_reuse; + unsigned long vmemmap_start, vmemmap_end; + struct page *vmemmap_head, *vmemmap_tail; + int nid, ret =3D 0; =20 VM_WARN_ON_ONCE_FOLIO(!folio_test_hugetlb(folio), folio); VM_WARN_ON_ONCE_FOLIO(folio_ref_count(folio), folio); @@ -592,18 +549,31 @@ static int __hugetlb_vmemmap_optimize_folio(const str= uct hstate *h, */ folio_set_hugetlb_vmemmap_optimized(folio); =20 + nid =3D folio_nid(folio); + vmemmap_head =3D alloc_pages_node(nid, GFP_KERNEL, 0); + + if (!vmemmap_head) { + ret =3D -ENOMEM; + goto out; + } + + copy_page(page_to_virt(vmemmap_head), folio); + list_add(&vmemmap_head->lru, vmemmap_pages); + memmap_pages_add(1); + + vmemmap_tail =3D vmemmap_head; + vmemmap_start =3D (unsigned long)&folio->page; vmemmap_end =3D vmemmap_start + hugetlb_vmemmap_size(h); - vmemmap_reuse =3D vmemmap_start; - vmemmap_start +=3D HUGETLB_VMEMMAP_RESERVE_SIZE; =20 /* - * Remap the vmemmap virtual address range [@vmemmap_start, @vmemmap_end) - * to the page which @vmemmap_reuse is mapped to. Add pages previously - * mapping the range to vmemmap_pages list so that they can be freed by - * the caller. + * Remap the vmemmap virtual address range [@vmemmap_start, @vmemmap_end). + * Add pages previously mapping the range to vmemmap_pages list so that + * they can be freed by the caller. */ - ret =3D vmemmap_remap_free(vmemmap_start, vmemmap_end, vmemmap_reuse, + ret =3D vmemmap_remap_free(vmemmap_start, vmemmap_end, + vmemmap_head, vmemmap_tail, vmemmap_pages, flags); +out: if (ret) { static_branch_dec(&hugetlb_optimize_vmemmap_key); folio_clear_hugetlb_vmemmap_optimized(folio); @@ -632,21 +602,19 @@ void hugetlb_vmemmap_optimize_folio(const struct hsta= te *h, struct folio *folio) =20 static int hugetlb_vmemmap_split_folio(const struct hstate *h, struct foli= o *folio) { - unsigned long vmemmap_start =3D (unsigned long)&folio->page, vmemmap_end; - unsigned long vmemmap_reuse; + unsigned long vmemmap_start, vmemmap_end; =20 if (!vmemmap_should_optimize_folio(h, folio)) return 0; 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Wed, 21 Jan 2026 11:23:14 -0500 (EST) From: Kiryl Shutsemau To: Andrew Morton , Muchun Song , David Hildenbrand , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Kiryl Shutsemau Subject: [PATCHv4 09/14] mm/hugetlb: Remove fake head pages Date: Wed, 21 Jan 2026 16:22:46 +0000 Message-ID: <20260121162253.2216580-10-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260121162253.2216580-1-kas@kernel.org> References: <20260121162253.2216580-1-kas@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" HugeTLB Vmemmap Optimization (HVO) reduces memory usage by freeing most vmemmap pages for huge pages and remapping the freed range to a single page containing the struct page metadata. With the new mask-based compound_info encoding (for power-of-2 struct page sizes), all tail pages of the same order are now identical regardless of which compound page they belong to. This means the tail pages can be truly shared without fake heads. Allocate a single page of initialized tail struct pages per NUMA node per order in the vmemmap_tails[] array in pglist_data. All huge pages of that order on the node share this tail page, mapped read-only into their vmemmap. The head page remains unique per huge page. Redefine MAX_FOLIO_ORDER using ilog2(). The define has to produce a compile-constant as it is used to specify vmemmap_tail array size. For some reason, compiler is not able to solve get_order() at compile-time, but ilog2() works. Avoid PUD_ORDER to define MAX_FOLIO_ORDER as it adds dependency to which generates hard-to-break include loop. This eliminates fake heads while maintaining the same memory savings, and simplifies compound_head() by removing fake head detection. Signed-off-by: Kiryl Shutsemau --- include/linux/mmzone.h | 18 ++++++++-- mm/hugetlb_vmemmap.c | 80 ++++++++++++++++++++++++++++-------------- mm/sparse-vmemmap.c | 44 ++++++++++++++++++----- 3 files changed, 106 insertions(+), 36 deletions(-) diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h index 7e4f69b9d760..7e6beeca4d40 100644 --- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -81,13 +81,17 @@ * currently expect (see CONFIG_HAVE_GIGANTIC_FOLIOS): with hugetlb, we ex= pect * no folios larger than 16 GiB on 64bit and 1 GiB on 32bit. */ -#define MAX_FOLIO_ORDER get_order(IS_ENABLED(CONFIG_64BIT) ? SZ_16G : SZ_= 1G) +#ifdef CONFIG_64BIT +#define MAX_FOLIO_ORDER (ilog2(SZ_16G) - PAGE_SHIFT) +#else +#define MAX_FOLIO_ORDER (ilog2(SZ_1G) - PAGE_SHIFT) +#endif #else /* * Without hugetlb, gigantic folios that are bigger than a single PUD are * currently impossible. */ -#define MAX_FOLIO_ORDER PUD_ORDER +#define MAX_FOLIO_ORDER (PUD_SHIFT - PAGE_SHIFT) #endif =20 #define MAX_FOLIO_NR_PAGES (1UL << MAX_FOLIO_ORDER) @@ -1407,6 +1411,13 @@ struct memory_failure_stats { }; #endif =20 +/* + * vmemmap optimization (like HVO) is only possible for page orders that f= ill + * two or more pages with struct pages. + */ +#define VMEMMAP_TAIL_MIN_ORDER (ilog2(2 * PAGE_SIZE / sizeof(struct page))) +#define NR_VMEMMAP_TAILS (MAX_FOLIO_ORDER - VMEMMAP_TAIL_MIN_ORDER + 1) + /* * On NUMA machines, each NUMA node would have a pg_data_t to describe * it's memory layout. On UMA machines there is a single pglist_data which @@ -1555,6 +1566,9 @@ typedef struct pglist_data { #ifdef CONFIG_MEMORY_FAILURE struct memory_failure_stats mf_stats; #endif +#ifdef CONFIG_SPARSEMEM_VMEMMAP + unsigned long vmemmap_tails[NR_VMEMMAP_TAILS]; +#endif } pg_data_t; =20 #define node_present_pages(nid) (NODE_DATA(nid)->node_present_pages) diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c index a51c0e293175..51bb6c73db92 100644 --- a/mm/hugetlb_vmemmap.c +++ b/mm/hugetlb_vmemmap.c @@ -18,6 +18,7 @@ #include #include #include "hugetlb_vmemmap.h" +#include "internal.h" =20 /** * struct vmemmap_remap_walk - walk vmemmap page table @@ -231,36 +232,25 @@ static void vmemmap_remap_pte(pte_t *pte, unsigned lo= ng addr, set_pte_at(&init_mm, addr, pte, entry); } =20 -/* - * How many struct page structs need to be reset. When we reuse the head - * struct page, the special metadata (e.g. page->flags or page->mapping) - * cannot copy to the tail struct page structs. The invalid value will be - * checked in the free_tail_page_prepare(). In order to avoid the message - * of "corrupted mapping in tail page". We need to reset at least 4 (one - * head struct page struct and three tail struct page structs) struct page - * structs. - */ -#define NR_RESET_STRUCT_PAGE 4 - -static inline void reset_struct_pages(struct page *start) -{ - struct page *from =3D start + NR_RESET_STRUCT_PAGE; - - BUILD_BUG_ON(NR_RESET_STRUCT_PAGE * 2 > PAGE_SIZE / sizeof(struct page)); - memcpy(start, from, sizeof(*from) * NR_RESET_STRUCT_PAGE); -} - static void vmemmap_restore_pte(pte_t *pte, unsigned long addr, struct vmemmap_remap_walk *walk) { struct page *page; - void *to; + struct page *from, *to; =20 page =3D list_first_entry(walk->vmemmap_pages, struct page, lru); list_del(&page->lru); + + /* + * Initialize all tail pages with the value of the first non-special + * tail pages. The first 4 tail pages of the hugetlb folio contain + * special metadata. + */ + from =3D compound_head((struct page *)addr) + 4; to =3D page_to_virt(page); - copy_page(to, (void *)walk->vmemmap_start); - reset_struct_pages(to); + for (int i =3D 0; i < PAGE_SIZE / sizeof(struct page); i++, to++) { + *to =3D *from; + } =20 /* * Makes sure that preceding stores to the page contents become visible @@ -425,8 +415,7 @@ static int __hugetlb_vmemmap_restore_folio(const struct= hstate *h, =20 vmemmap_start =3D (unsigned long)&folio->page; vmemmap_end =3D vmemmap_start + hugetlb_vmemmap_size(h); - - vmemmap_start +=3D HUGETLB_VMEMMAP_RESERVE_SIZE; + vmemmap_start +=3D HUGETLB_VMEMMAP_RESERVE_SIZE; =20 /* * The pages which the vmemmap virtual address range [@vmemmap_start, @@ -517,6 +506,41 @@ static bool vmemmap_should_optimize_folio(const struct= hstate *h, struct folio * return true; } =20 +static struct page *vmemmap_get_tail(unsigned int order, int node) +{ + unsigned long pfn; + unsigned int idx; + struct page *tail, *p; + + idx =3D order - VMEMMAP_TAIL_MIN_ORDER; + pfn =3D NODE_DATA(node)->vmemmap_tails[idx]; + if (pfn) + return pfn_to_page(pfn); + + tail =3D alloc_pages_node(node, GFP_KERNEL | __GFP_ZERO, 0); + if (!tail) + return NULL; + + p =3D page_to_virt(tail); + for (int i =3D 0; i < PAGE_SIZE / sizeof(struct page); i++) + prep_compound_tail(p + i, NULL, order); + + spin_lock(&hugetlb_lock); + if (!NODE_DATA(node)->vmemmap_tails[idx]) { + pfn =3D PHYS_PFN(virt_to_phys(p)); + NODE_DATA(node)->vmemmap_tails[idx] =3D pfn; + tail =3D NULL; + } else { + pfn =3D NODE_DATA(node)->vmemmap_tails[idx]; + } + spin_unlock(&hugetlb_lock); + + if (tail) + __free_page(tail); + + return pfn_to_page(pfn); +} + static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h, struct folio *folio, struct list_head *vmemmap_pages, @@ -532,6 +556,12 @@ static int __hugetlb_vmemmap_optimize_folio(const stru= ct hstate *h, if (!vmemmap_should_optimize_folio(h, folio)) return ret; =20 + nid =3D folio_nid(folio); + + vmemmap_tail =3D vmemmap_get_tail(h->order, nid); + if (!vmemmap_tail) + return -ENOMEM; + static_branch_inc(&hugetlb_optimize_vmemmap_key); =20 if (flags & VMEMMAP_SYNCHRONIZE_RCU) @@ -549,7 +579,6 @@ static int __hugetlb_vmemmap_optimize_folio(const struc= t hstate *h, */ folio_set_hugetlb_vmemmap_optimized(folio); =20 - nid =3D folio_nid(folio); vmemmap_head =3D alloc_pages_node(nid, GFP_KERNEL, 0); =20 if (!vmemmap_head) { @@ -561,7 +590,6 @@ static int __hugetlb_vmemmap_optimize_folio(const struc= t hstate *h, list_add(&vmemmap_head->lru, vmemmap_pages); memmap_pages_add(1); =20 - vmemmap_tail =3D vmemmap_head; vmemmap_start =3D (unsigned long)&folio->page; vmemmap_end =3D vmemmap_start + hugetlb_vmemmap_size(h); =20 diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c index dbd8daccade2..94b4e90fa00f 100644 --- a/mm/sparse-vmemmap.c +++ b/mm/sparse-vmemmap.c @@ -378,16 +378,45 @@ void vmemmap_wrprotect_hvo(unsigned long addr, unsign= ed long end, } } =20 -/* - * Populate vmemmap pages HVO-style. The first page contains the head - * page and needed tail pages, the other ones are mirrors of the first - * page. - */ +static __meminit unsigned long vmemmap_get_tail(unsigned int order, int no= de) +{ + unsigned long pfn; + unsigned int idx; + struct page *p; + + BUG_ON(order < VMEMMAP_TAIL_MIN_ORDER); + BUG_ON(order > MAX_FOLIO_ORDER); + + idx =3D order - VMEMMAP_TAIL_MIN_ORDER; + pfn =3D NODE_DATA(node)->vmemmap_tails[idx]; + if (pfn) + return pfn; + + p =3D vmemmap_alloc_block_zero(PAGE_SIZE, node); + if (!p) + return 0; + + for (int i =3D 0; i < PAGE_SIZE / sizeof(struct page); i++) + prep_compound_tail(p + i, NULL, order); + + pfn =3D PHYS_PFN(virt_to_phys(p)); + NODE_DATA(node)->vmemmap_tails[idx] =3D pfn; + + return pfn; +} + int __meminit vmemmap_populate_hvo(unsigned long addr, unsigned long end, int node, unsigned long headsize) { + unsigned long maddr, len, tail_pfn; + unsigned int order; pte_t *pte; - unsigned long maddr; + + len =3D end - addr; + order =3D ilog2(len * sizeof(struct page) / PAGE_SIZE); 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Wed, 21 Jan 2026 11:23:16 -0500 (EST) From: Kiryl Shutsemau To: Andrew Morton , Muchun Song , David Hildenbrand , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Kiryl Shutsemau Subject: [PATCHv4 10/14] mm: Drop fake head checks Date: Wed, 21 Jan 2026 16:22:47 +0000 Message-ID: <20260121162253.2216580-11-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260121162253.2216580-1-kas@kernel.org> References: <20260121162253.2216580-1-kas@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" With fake head pages eliminated in the previous commit, remove the supporting infrastructure: - page_fixed_fake_head(): no longer needed to detect fake heads; - page_is_fake_head(): no longer needed; - page_count_writable(): no longer needed for RCU protection; - RCU read_lock in page_ref_add_unless(): no longer needed; This substantially simplifies compound_head() and page_ref_add_unless(), removing both branches and RCU overhead from these hot paths. Signed-off-by: Kiryl Shutsemau Reviewed-by: Muchun Song --- include/linux/page-flags.h | 93 ++------------------------------------ include/linux/page_ref.h | 8 +--- 2 files changed, 4 insertions(+), 97 deletions(-) diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index e16a4bc82856..660f9154a211 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -221,102 +221,15 @@ static __always_inline bool compound_info_has_mask(v= oid) return is_power_of_2(sizeof(struct page)); } =20 -#ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP DECLARE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key); =20 -/* - * Return the real head page struct iff the @page is a fake head page, oth= erwise - * return the @page itself. See Documentation/mm/vmemmap_dedup.rst. - */ -static __always_inline const struct page *page_fixed_fake_head(const struc= t page *page) -{ - if (!static_branch_unlikely(&hugetlb_optimize_vmemmap_key)) - return page; - - /* Fake heads only exists if compound_info_has_mask() is true */ - if (!compound_info_has_mask()) - return page; - - /* - * Only addresses aligned with PAGE_SIZE of struct page may be fake head - * struct page. The alignment check aims to avoid access the fields ( - * e.g. compound_info) of the @page[1]. It can avoid touch a (possibly) - * cold cacheline in some cases. - */ - if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) && - test_bit(PG_head, &page->flags.f)) { - /* - * We can safely access the field of the @page[1] with PG_head - * because the @page is a compound page composed with at least - * two contiguous pages. - */ - unsigned long info =3D READ_ONCE(page[1].compound_info); - - /* See set_compound_head() */ - if (likely(info & 1)) { - unsigned long p =3D (unsigned long)page; - - return (const struct page *)(p & info); - } - } - return page; -} - -static __always_inline bool page_count_writable(const struct page *page, i= nt u) -{ - if (!static_branch_unlikely(&hugetlb_optimize_vmemmap_key)) - return true; - - /* - * The refcount check is ordered before the fake-head check to prevent - * the following race: - * CPU 1 (HVO) CPU 2 (speculative PFN walker) - * - * page_ref_freeze() - * synchronize_rcu() - * rcu_read_lock() - * page_is_fake_head() is false - * vmemmap_remap_pte() - * XXX: struct page[] becomes r/o - * - * page_ref_unfreeze() - * page_ref_count() is not zero - * - * atomic_add_unless(&page->_refcount) - * XXX: try to modify r/o struct page[] - * - * The refcount check also prevents modification attempts to other (r/o) - * tail pages that are not fake heads. - */ - if (atomic_read_acquire(&page->_refcount) =3D=3D u) - return false; - - return page_fixed_fake_head(page) =3D=3D page; -} -#else -static inline const struct page *page_fixed_fake_head(const struct page *p= age) -{ - return page; -} - -static inline bool page_count_writable(const struct page *page, int u) -{ - return true; -} -#endif - -static __always_inline int page_is_fake_head(const struct page *page) -{ - return page_fixed_fake_head(page) !=3D page; -} - static __always_inline unsigned long _compound_head(const struct page *pag= e) { unsigned long info =3D READ_ONCE(page->compound_info); =20 /* Bit 0 encodes PageTail() */ if (!(info & 1)) - return (unsigned long)page_fixed_fake_head(page); + return (unsigned long)page; =20 /* * If compound_info_has_mask() is false, the rest of compound_info is @@ -397,7 +310,7 @@ static __always_inline void clear_compound_head(struct = page *page) =20 static __always_inline int PageTail(const struct page *page) { - return READ_ONCE(page->compound_info) & 1 || page_is_fake_head(page); + return READ_ONCE(page->compound_info) & 1; } =20 static __always_inline int PageCompound(const struct page *page) @@ -924,7 +837,7 @@ static __always_inline bool folio_test_head(const struc= t folio *folio) static __always_inline int PageHead(const struct page *page) { PF_POISONED_CHECK(page); - return test_bit(PG_head, &page->flags.f) && !page_is_fake_head(page); + return test_bit(PG_head, &page->flags.f); } =20 __SETPAGEFLAG(Head, head, PF_ANY) diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h index 544150d1d5fd..490d0ad6e56d 100644 --- a/include/linux/page_ref.h +++ b/include/linux/page_ref.h @@ -230,13 +230,7 @@ static inline int folio_ref_dec_return(struct folio *f= olio) =20 static inline bool page_ref_add_unless(struct page *page, int nr, int u) { - bool ret =3D false; - - rcu_read_lock(); - /* avoid writing to the vmemmap area being remapped */ - if (page_count_writable(page, u)) - ret =3D atomic_add_unless(&page->_refcount, nr, u); - rcu_read_unlock(); + bool ret =3D atomic_add_unless(&page->_refcount, nr, u); 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Wed, 21 Jan 2026 11:23:18 -0500 (EST) From: Kiryl Shutsemau To: Andrew Morton , Muchun Song , David Hildenbrand , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Kiryl Shutsemau Subject: [PATCHv4 11/14] hugetlb: Remove VMEMMAP_SYNCHRONIZE_RCU Date: Wed, 21 Jan 2026 16:22:48 +0000 Message-ID: <20260121162253.2216580-12-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260121162253.2216580-1-kas@kernel.org> References: <20260121162253.2216580-1-kas@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" The VMEMMAP_SYNCHRONIZE_RCU flag triggered synchronize_rcu() calls to prevent a race between HVO remapping and page_ref_add_unless(). The race could occur when a speculative PFN walker tried to modify the refcount on a struct page that was in the process of being remapped to a fake head. With fake heads eliminated, page_ref_add_unless() no longer needs RCU protection. Remove the flag and synchronize_rcu() calls. Signed-off-by: Kiryl Shutsemau Reviewed-by: Muchun Song --- mm/hugetlb_vmemmap.c | 20 ++++---------------- 1 file changed, 4 insertions(+), 16 deletions(-) diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c index 51bb6c73db92..f2a3340c8e22 100644 --- a/mm/hugetlb_vmemmap.c +++ b/mm/hugetlb_vmemmap.c @@ -48,8 +48,6 @@ struct vmemmap_remap_walk { #define VMEMMAP_SPLIT_NO_TLB_FLUSH BIT(0) /* Skip the TLB flush when we remap the PTE */ #define VMEMMAP_REMAP_NO_TLB_FLUSH BIT(1) -/* synchronize_rcu() to avoid writes from page_ref_add_unless() */ -#define VMEMMAP_SYNCHRONIZE_RCU BIT(2) unsigned long flags; }; =20 @@ -410,9 +408,6 @@ static int __hugetlb_vmemmap_restore_folio(const struct= hstate *h, if (!folio_test_hugetlb_vmemmap_optimized(folio)) return 0; =20 - if (flags & VMEMMAP_SYNCHRONIZE_RCU) - synchronize_rcu(); - vmemmap_start =3D (unsigned long)&folio->page; vmemmap_end =3D vmemmap_start + hugetlb_vmemmap_size(h); vmemmap_start +=3D HUGETLB_VMEMMAP_RESERVE_SIZE; @@ -444,7 +439,7 @@ static int __hugetlb_vmemmap_restore_folio(const struct= hstate *h, */ int hugetlb_vmemmap_restore_folio(const struct hstate *h, struct folio *fo= lio) { - return __hugetlb_vmemmap_restore_folio(h, folio, VMEMMAP_SYNCHRONIZE_RCU); + return __hugetlb_vmemmap_restore_folio(h, folio, 0); } =20 /** @@ -467,14 +462,11 @@ long hugetlb_vmemmap_restore_folios(const struct hsta= te *h, struct folio *folio, *t_folio; long restored =3D 0; long ret =3D 0; - unsigned long flags =3D VMEMMAP_REMAP_NO_TLB_FLUSH | VMEMMAP_SYNCHRONIZE_= RCU; + unsigned long flags =3D VMEMMAP_REMAP_NO_TLB_FLUSH; =20 list_for_each_entry_safe(folio, t_folio, folio_list, lru) { if (folio_test_hugetlb_vmemmap_optimized(folio)) { ret =3D __hugetlb_vmemmap_restore_folio(h, folio, flags); - /* only need to synchronize_rcu() once for each batch */ - flags &=3D ~VMEMMAP_SYNCHRONIZE_RCU; - if (ret) break; restored++; @@ -564,8 +556,6 @@ static int __hugetlb_vmemmap_optimize_folio(const struc= t hstate *h, =20 static_branch_inc(&hugetlb_optimize_vmemmap_key); =20 - if (flags & VMEMMAP_SYNCHRONIZE_RCU) - synchronize_rcu(); /* * Very Subtle * If VMEMMAP_REMAP_NO_TLB_FLUSH is set, TLB flushing is not performed @@ -624,7 +614,7 @@ void hugetlb_vmemmap_optimize_folio(const struct hstate= *h, struct folio *folio) { LIST_HEAD(vmemmap_pages); =20 - __hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, VMEMMAP_SYNCHR= ONIZE_RCU); + __hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, 0); free_vmemmap_page_list(&vmemmap_pages); } =20 @@ -652,7 +642,7 @@ static void __hugetlb_vmemmap_optimize_folios(struct hs= tate *h, struct folio *folio; int nr_to_optimize; LIST_HEAD(vmemmap_pages); - unsigned long flags =3D VMEMMAP_REMAP_NO_TLB_FLUSH | VMEMMAP_SYNCHRONIZE_= RCU; + unsigned long flags =3D VMEMMAP_REMAP_NO_TLB_FLUSH; =20 nr_to_optimize =3D 0; list_for_each_entry(folio, folio_list, lru) { @@ -705,8 +695,6 @@ static void __hugetlb_vmemmap_optimize_folios(struct hs= tate *h, int ret; =20 ret =3D __hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, flags= ); - /* only need to synchronize_rcu() once for each batch */ - flags &=3D ~VMEMMAP_SYNCHRONIZE_RCU; =20 /* * Pages to be freed may have been accumulated. 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It allowed skipping the fake head checks entirely when HVO was not in use. With fake heads eliminated and the detection code removed, the static key serves no purpose. Remove its definition and all increment/decrement calls. Signed-off-by: Kiryl Shutsemau Reviewed-by: Muchun Song --- include/linux/page-flags.h | 2 -- mm/hugetlb_vmemmap.c | 14 ++------------ 2 files changed, 2 insertions(+), 14 deletions(-) diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index 660f9154a211..f89702e101e8 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -221,8 +221,6 @@ static __always_inline bool compound_info_has_mask(void) return is_power_of_2(sizeof(struct page)); } =20 -DECLARE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key); - static __always_inline unsigned long _compound_head(const struct page *pag= e) { unsigned long info =3D READ_ONCE(page->compound_info); diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c index f2a3340c8e22..379a130cf1d7 100644 --- a/mm/hugetlb_vmemmap.c +++ b/mm/hugetlb_vmemmap.c @@ -386,9 +386,6 @@ static int vmemmap_remap_alloc(unsigned long start, uns= igned long end, return vmemmap_remap_range(start, end, &walk); } =20 -DEFINE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key); -EXPORT_SYMBOL(hugetlb_optimize_vmemmap_key); - static bool vmemmap_optimize_enabled =3D IS_ENABLED(CONFIG_HUGETLB_PAGE_OP= TIMIZE_VMEMMAP_DEFAULT_ON); static int __init hugetlb_vmemmap_optimize_param(char *buf) { @@ -419,10 +416,8 @@ static int __hugetlb_vmemmap_restore_folio(const struc= t hstate *h, * discarded vmemmap pages must be allocated and remapping. */ ret =3D vmemmap_remap_alloc(vmemmap_start, vmemmap_end, flags); - if (!ret) { + if (!ret) folio_clear_hugetlb_vmemmap_optimized(folio); - static_branch_dec(&hugetlb_optimize_vmemmap_key); - } =20 return ret; } @@ -554,8 +549,6 @@ static int __hugetlb_vmemmap_optimize_folio(const struc= t hstate *h, if (!vmemmap_tail) return -ENOMEM; =20 - static_branch_inc(&hugetlb_optimize_vmemmap_key); - /* * Very Subtle * If VMEMMAP_REMAP_NO_TLB_FLUSH is set, TLB flushing is not performed @@ -592,10 +585,8 @@ static int __hugetlb_vmemmap_optimize_folio(const stru= ct hstate *h, vmemmap_head, vmemmap_tail, vmemmap_pages, flags); out: - if (ret) { - static_branch_dec(&hugetlb_optimize_vmemmap_key); 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Wed, 21 Jan 2026 11:23:22 -0500 (EST) From: Kiryl Shutsemau To: Andrew Morton , Muchun Song , David Hildenbrand , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Kiryl Shutsemau Subject: [PATCHv4 13/14] mm: Remove the branch from compound_head() Date: Wed, 21 Jan 2026 16:22:50 +0000 Message-ID: <20260121162253.2216580-14-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260121162253.2216580-1-kas@kernel.org> References: <20260121162253.2216580-1-kas@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" The compound_head() function is a hot path. For example, the zap path calls it for every leaf page table entry. Rewrite the helper function in a branchless manner to eliminate the risk of CPU branch misprediction. Signed-off-by: Kiryl Shutsemau Reviewed-by: Muchun Song Reviewed-by: Zi Yan --- include/linux/page-flags.h | 27 +++++++++++++++++---------- 1 file changed, 17 insertions(+), 10 deletions(-) diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index f89702e101e8..95ac963b78af 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -224,25 +224,32 @@ static __always_inline bool compound_info_has_mask(vo= id) static __always_inline unsigned long _compound_head(const struct page *pag= e) { unsigned long info =3D READ_ONCE(page->compound_info); + unsigned long mask; + + if (!compound_info_has_mask()) { + /* Bit 0 encodes PageTail() */ + if (info & 1) + return info - 1; =20 - /* Bit 0 encodes PageTail() */ - if (!(info & 1)) return (unsigned long)page; - - /* - * If compound_info_has_mask() is false, the rest of compound_info is - * the pointer to the head page. - */ - if (!compound_info_has_mask()) - return info - 1; + } =20 /* * If compoun_info_has_mask() is true the rest of the info encodes * the mask that converts the address of the tail page to the head page. * * No need to clear bit 0 in the mask as 'page' always has it clear. + * + * Let's do it in a branchless manner. */ - return (unsigned long)page & info; 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Wed, 21 Jan 2026 11:23:24 -0500 (EST) From: Kiryl Shutsemau To: Andrew Morton , Muchun Song , David Hildenbrand , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Kiryl Shutsemau Subject: [PATCHv4 14/14] hugetlb: Update vmemmap_dedup.rst Date: Wed, 21 Jan 2026 16:22:51 +0000 Message-ID: <20260121162253.2216580-15-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260121162253.2216580-1-kas@kernel.org> References: <20260121162253.2216580-1-kas@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Update the documentation regarding vmemmap optimization for hugetlb to reflect the changes in how the kernel maps the tail pages. Fake heads no longer exist. Remove their description. Signed-off-by: Kiryl Shutsemau Reviewed-by: Muchun Song --- Documentation/mm/vmemmap_dedup.rst | 60 +++++++++++++----------------- 1 file changed, 26 insertions(+), 34 deletions(-) diff --git a/Documentation/mm/vmemmap_dedup.rst b/Documentation/mm/vmemmap_= dedup.rst index 1863d88d2dcb..fca9d0ce282a 100644 --- a/Documentation/mm/vmemmap_dedup.rst +++ b/Documentation/mm/vmemmap_dedup.rst @@ -124,33 +124,35 @@ Here is how things look before optimization:: | | +-----------+ =20 -The value of page->compound_info is the same for all tail pages. The first -page of ``struct page`` (page 0) associated with the HugeTLB page contains= the 4 -``struct page`` necessary to describe the HugeTLB. The only use of the rem= aining -pages of ``struct page`` (page 1 to page 7) is to point to page->compound_= info. -Therefore, we can remap pages 1 to 7 to page 0. Only 1 page of ``struct pa= ge`` -will be used for each HugeTLB page. This will allow us to free the remaini= ng -7 pages to the buddy allocator. +The first page of ``struct page`` (page 0) associated with the HugeTLB page +contains the 4 ``struct page`` necessary to describe the HugeTLB. The rema= ining +pages of ``struct page`` (page 1 to page 7) are tail pages. + +The optimization is only applied when the size of the struct page is a pow= er-of-2 +In this case, all tail pages of the same order are identical. See +compound_head(). This allows us to remap the tail pages of the vmemmap to a +shared, read-only page. The head page is also remapped to a new page. This +allows the original vmemmap pages to be freed. =20 Here is how things look after remapping:: =20 - HugeTLB struct pages(8 pages) page frame(8 pa= ges) - +-----------+ ---virt_to_page---> +-----------+ mapping to +---------= --+ - | | | 0 | -------------> | 0 = | - | | +-----------+ +---------= --+ - | | | 1 | ---------------^ ^ ^ ^ ^ = ^ ^ - | | +-----------+ | | | | = | | - | | | 2 | -----------------+ | | | = | | - | | +-----------+ | | | = | | - | | | 3 | -------------------+ | | = | | - | | +-----------+ | | = | | - | | | 4 | ---------------------+ | = | | - | PMD | +-----------+ | = | | - | level | | 5 | -----------------------+ = | | - | mapping | +-----------+ = | | - | | | 6 | -------------------------= + | - | | +-----------+ = | - | | | 7 | -------------------------= --+ + HugeTLB struct pages(8 pages) page fr= ame + +-----------+ ---virt_to_page---> +-----------+ mapping to +---------= -------+ + | | | 0 | -------------> | 0 = | + | | +-----------+ +---------= -------+ + | | | 1 | ------=E2=94=90 + | | +-----------+ | + | | | 2 | ------=E2=94=BC +-= ---------------------------+ + | | +-----------+ | | A single= , per-node page | + | | | 3 | ------=E2=94=BC------> | = frame shared among all | + | | +-----------+ | | hugepage= s of the same size | + | | | 4 | ------=E2=94=BC +-= ---------------------------+ + | | +-----------+ | + | | | 5 | ------=E2=94=BC + | PMD | +-----------+ | + | level | | 6 | ------=E2=94=BC + | mapping | +-----------+ | + | | | 7 | ------=E2=94=98 | | +-----------+ | | | | @@ -172,16 +174,6 @@ The contiguous bit is used to increase the mapping siz= e at the pmd and pte (last) level. So this type of HugeTLB page can be optimized only when its size of the ``struct page`` structs is greater than **1** page. =20 -Notice: The head vmemmap page is not freed to the buddy allocator and all -tail vmemmap pages are mapped to the head vmemmap page frame. So we can see -more than one ``struct page`` struct with ``PG_head`` (e.g. 8 per 2 MB Hug= eTLB -page) associated with each HugeTLB page. The ``compound_head()`` can handle -this correctly. There is only **one** head ``struct page``, the tail -``struct page`` with ``PG_head`` are fake head ``struct page``. We need an -approach to distinguish between those two different types of ``struct page= `` so -that ``compound_head()`` can return the real head ``struct page`` when the -parameter is the tail ``struct page`` but with ``PG_head``. - Device DAX =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D =20 --=20 2.51.2