From nobody Sun Dec 14 19:29:43 2025 Received: from mout-p-202.mailbox.org (mout-p-202.mailbox.org [80.241.56.172]) (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 C7FE62874FA; Thu, 22 May 2025 09:03:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=80.241.56.172 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1747904589; cv=none; b=Uv3c/J/+J1T4RNk0F2qSW36ilmn6vavVF3xj4GgXoZPNiZbx7Fzhskd2bEKEwIONIoyGncXJlFvXE03v8YBXhoEFSFPq+mn2IZW62mpjrPQXZw6+dGqY8XpiBL9dcPVXnyokrhUsUBPIFN679RQixrGL9GhkExqm+7r8NWO8unY= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1747904589; c=relaxed/simple; bh=N8AnvZ6bkNUoFEjqD/FmJwkOEekfKsIRvChCjR7Bvh4=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=j5l0IyWn+zyPtBeiOY8oaYnU4huEVkTFc7zRBmmkE+He5//4MLz5y1FqQZy5WIxOpzDngGbSoByTKrTZF4+SGj1obv5ZjPaYKn+ma9WRNYRABU0eVi2JSiq45IiC+TIuK5WyNFOqiwdkVRqPQm+ewH/yzu/hBXNRuPdwcUnLRow= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=fail (p=none dis=none) header.from=samsung.com; spf=pass smtp.mailfrom=pankajraghav.com; arc=none smtp.client-ip=80.241.56.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=fail (p=none dis=none) header.from=samsung.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=pankajraghav.com Received: from smtp2.mailbox.org (smtp2.mailbox.org [10.196.197.2]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by mout-p-202.mailbox.org (Postfix) with ESMTPS id 4b32Pf4dHZz9scV; Thu, 22 May 2025 11:03:02 +0200 (CEST) From: Pankaj Raghav To: Suren Baghdasaryan , Vlastimil Babka , Ryan Roberts , Mike Rapoport , Michal Hocko , Thomas Gleixner , Nico Pache , Dev Jain , Baolin Wang , Borislav Petkov , Ingo Molnar , "H . Peter Anvin" , Zi Yan , Dave Hansen , David Hildenbrand , Lorenzo Stoakes , Andrew Morton , "Liam R . Howlett" , Jens Axboe Cc: linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, "Darrick J . Wong" , gost.dev@samsung.com, kernel@pankajraghav.com, hch@lst.de, linux-kernel@vger.kernel.org, linux-mm@kvack.org, willy@infradead.org, x86@kernel.org, mcgrof@kernel.org, Pankaj Raghav Subject: [RFC v2 1/2] mm: add THP_HUGE_ZERO_PAGE_ALWAYS config option Date: Thu, 22 May 2025 11:02:42 +0200 Message-ID: <20250522090243.758943-2-p.raghav@samsung.com> In-Reply-To: <20250522090243.758943-1-p.raghav@samsung.com> References: <20250522090243.758943-1-p.raghav@samsung.com> 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" There are many places in the kernel where we need to zeroout larger chunks but the maximum segment we can zeroout at a time by ZERO_PAGE is limited by PAGE_SIZE. This is especially annoying in block devices and filesystems where we attach multiple ZERO_PAGEs to the bio in different bvecs. With multipage bvec support in block layer, it is much more efficient to send out larger zero pages as a part of single bvec. This concern was raised during the review of adding LBS support to XFS[1][2]. Usually huge_zero_folio is allocated on demand, and it will be deallocated by the shrinker if there are no users of it left. Add a config option THP_HUGE_ZERO_PAGE_ALWAYS that will always allocate the huge_zero_folio, and it will never be freed. This makes using the huge_zero_folio without having to pass any mm struct and call put_folio in the destructor. We can enable it by default for x86_64 where the PMD size is 2M. It is good compromise between the memory and efficiency. As a THP zero page might be wasteful for architectures with bigger page sizes, let's not enable it for them. [1] https://lore.kernel.org/linux-xfs/20231027051847.GA7885@lst.de/ [2] https://lore.kernel.org/linux-xfs/ZitIK5OnR7ZNY0IG@infradead.org/ Suggested-by: David Hildenbrand Signed-off-by: Pankaj Raghav --- arch/x86/Kconfig | 1 + mm/Kconfig | 12 +++++++++ mm/huge_memory.c | 63 ++++++++++++++++++++++++++++++++++++++---------- 3 files changed, 63 insertions(+), 13 deletions(-) diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index 055204dc211d..2e1527580746 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -152,6 +152,7 @@ config X86 select ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP if X86_64 select ARCH_WANT_HUGETLB_VMEMMAP_PREINIT if X86_64 select ARCH_WANTS_THP_SWAP if X86_64 + select ARCH_WANTS_THP_ZERO_PAGE_ALWAYS if X86_64 select ARCH_HAS_PARANOID_L1D_FLUSH select BUILDTIME_TABLE_SORT select CLKEVT_I8253 diff --git a/mm/Kconfig b/mm/Kconfig index bd08e151fa1b..a2994e7d55ba 100644 --- a/mm/Kconfig +++ b/mm/Kconfig @@ -823,6 +823,9 @@ config ARCH_WANT_GENERAL_HUGETLB config ARCH_WANTS_THP_SWAP def_bool n =20 +config ARCH_WANTS_THP_ZERO_PAGE_ALWAYS + def_bool n + config MM_ID def_bool n =20 @@ -895,6 +898,15 @@ config READ_ONLY_THP_FOR_FS support of file THPs will be developed in the next few release cycles. =20 +config THP_ZERO_PAGE_ALWAYS + def_bool y + depends on TRANSPARENT_HUGEPAGE && ARCH_WANTS_THP_ZERO_PAGE_ALWAYS + help + Typically huge_zero_folio, which is a THP of zeroes, is allocated + on demand and deallocated when not in use. This option will always + allocate huge_zero_folio for zeroing and it is never deallocated. + Not suitable for memory constrained systems. + config NO_PAGE_MAPCOUNT bool "No per-page mapcount (EXPERIMENTAL)" help diff --git a/mm/huge_memory.c b/mm/huge_memory.c index d3e66136e41a..1a0556ca3839 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -247,9 +247,16 @@ static void put_huge_zero_page(void) BUG_ON(atomic_dec_and_test(&huge_zero_refcount)); } =20 +/* + * If THP_ZERO_PAGE_ALWAYS is enabled, @mm can be NULL, i.e, the huge_zero= _folio + * is not associated with any mm_struct. + */ struct folio *mm_get_huge_zero_folio(struct mm_struct *mm) { - if (test_bit(MMF_HUGE_ZERO_PAGE, &mm->flags)) + if (!IS_ENABLED(CONFIG_THP_ZERO_PAGE_ALWAYS) && !mm) + return NULL; + + if (IS_ENABLED(CONFIG_THP_ZERO_PAGE_ALWAYS) || test_bit(MMF_HUGE_ZERO_PAG= E, &mm->flags)) return READ_ONCE(huge_zero_folio); =20 if (!get_huge_zero_page()) @@ -263,6 +270,9 @@ struct folio *mm_get_huge_zero_folio(struct mm_struct *= mm) =20 void mm_put_huge_zero_folio(struct mm_struct *mm) { + if (IS_ENABLED(CONFIG_THP_ZERO_PAGE_ALWAYS)) + return; + if (test_bit(MMF_HUGE_ZERO_PAGE, &mm->flags)) put_huge_zero_page(); } @@ -274,14 +284,21 @@ static unsigned long shrink_huge_zero_page_count(stru= ct shrinker *shrink, return atomic_read(&huge_zero_refcount) =3D=3D 1 ? HPAGE_PMD_NR : 0; } =20 +static void _put_huge_zero_folio(void) +{ + struct folio *zero_folio; + + zero_folio =3D xchg(&huge_zero_folio, NULL); + BUG_ON(zero_folio =3D=3D NULL); + WRITE_ONCE(huge_zero_pfn, ~0UL); + folio_put(zero_folio); +} + static unsigned long shrink_huge_zero_page_scan(struct shrinker *shrink, struct shrink_control *sc) { if (atomic_cmpxchg(&huge_zero_refcount, 1, 0) =3D=3D 1) { - struct folio *zero_folio =3D xchg(&huge_zero_folio, NULL); - BUG_ON(zero_folio =3D=3D NULL); - WRITE_ONCE(huge_zero_pfn, ~0UL); - folio_put(zero_folio); + _put_huge_zero_folio(); return HPAGE_PMD_NR; } =20 @@ -850,10 +867,6 @@ static inline void hugepage_exit_sysfs(struct kobject = *hugepage_kobj) =20 static int __init thp_shrinker_init(void) { - huge_zero_page_shrinker =3D shrinker_alloc(0, "thp-zero"); - if (!huge_zero_page_shrinker) - return -ENOMEM; - deferred_split_shrinker =3D shrinker_alloc(SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE | SHRINKER_NONSLAB, @@ -863,14 +876,21 @@ static int __init thp_shrinker_init(void) return -ENOMEM; } =20 - huge_zero_page_shrinker->count_objects =3D shrink_huge_zero_page_count; - huge_zero_page_shrinker->scan_objects =3D shrink_huge_zero_page_scan; - shrinker_register(huge_zero_page_shrinker); - deferred_split_shrinker->count_objects =3D deferred_split_count; deferred_split_shrinker->scan_objects =3D deferred_split_scan; shrinker_register(deferred_split_shrinker); =20 + if (IS_ENABLED(CONFIG_THP_ZERO_PAGE_ALWAYS)) + return 0; + + huge_zero_page_shrinker =3D shrinker_alloc(0, "thp-zero"); + if (!huge_zero_page_shrinker) + return -ENOMEM; + + huge_zero_page_shrinker->count_objects =3D shrink_huge_zero_page_count; + huge_zero_page_shrinker->scan_objects =3D shrink_huge_zero_page_scan; + shrinker_register(huge_zero_page_shrinker); + return 0; } =20 @@ -880,6 +900,17 @@ static void __init thp_shrinker_exit(void) shrinker_free(deferred_split_shrinker); } =20 +static int __init huge_zero_page_init(void) { + + if (!IS_ENABLED(CONFIG_THP_ZERO_PAGE_ALWAYS)) + return 0; + + if (!get_huge_zero_page()) { + return -ENOMEM; + } + return 0; +} + static int __init hugepage_init(void) { int err; @@ -903,6 +934,10 @@ static int __init hugepage_init(void) if (err) goto err_slab; =20 + err =3D huge_zero_page_init(); + if (err) + goto err_huge_zero_page; + err =3D thp_shrinker_init(); if (err) goto err_shrinker; @@ -925,6 +960,8 @@ static int __init hugepage_init(void) err_khugepaged: thp_shrinker_exit(); err_shrinker: + _put_huge_zero_folio(); +err_huge_zero_page: khugepaged_destroy(); err_slab: hugepage_exit_sysfs(hugepage_kobj); --=20 2.47.2