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Wed, 20 May 2026 08:16:57 -0700 (PDT) Received: from localhost.localdomain ([240e:390:6ed:f081:3ad9:490a:885c:dca6]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2bd5b568cefsm224364315ad.0.2026.05.20.08.16.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 20 May 2026 08:16:57 -0700 (PDT) From: yizhang089@gmail.com To: linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, david@redhat.com, william.kucharski@linux.dev, karol.wachowski@linux.intel.com, yi.zhang@huawei.com, yi.zhang@huaweicloud.com, yizhang089@gmail.com, liuyongqiang13@huawei.com, wangkefeng.wang@huawei.com, yangerkun@huawei.com Subject: [PATCH v3] mm: do not install PMD mappings when handling a COW fault Date: Wed, 20 May 2026 11:16:24 -0400 Message-ID: <20260520151624.78370-1-yizhang089@gmail.com> X-Mailer: git-send-email 2.54.0 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" From: Zhang Yi When pinning a page with FOLL_LONGTERM in a CoW VMA and a PMD-aligned (2MB on x86) large folio follow_page_mask() failed to obtain a valid anonymous page, resulting in an infinite loop issue. The specific triggering process is as follows: 1. User call mmap with a 2MB size in MAP_PRIVATE mode for a file that has a 2MB large folio installed in the page cache. addr =3D mmap(NULL, 2*1024*1024, PROT_READ, MAP_PRIVATE, file_fd, 0); 2. The kernel driver pass this mapped address to pin_user_pages_fast() in FOLL_LONGTERM mode. pin_user_pages_fast(addr, 512, FOLL_LONGTERM, pages); -> pin_user_pages_fast() | gup_fast_fallback() | __gup_longterm_locked() | __get_user_pages_locked() | __get_user_pages() | follow_page_mask() | follow_p4d_mask() | follow_pud_mask() | follow_pmd_mask() //pmd_leaf(pmdval) is true because the | //huge PMD is installed. This is normal | //in the first round, but it shouldn't | //happen in the second round. | follow_huge_pmd() //require an anonymous page | return -EMLINK; | faultin_page() | handle_mm_fault() | wp_huge_pmd() //remove PMD and fall back to PTE | handle_pte_fault() | do_pte_missing() | do_fault() | do_read_fault() //FAULT_FLAG_WRITE is not set | finish_fault() | do_set_pmd() //install a huge PMD again, this is wrong!!! | do_wp_page() //create private anonymous pages <- goto retry; Due to an incorrectly large PMD set in do_read_fault(), follow_pmd_mask() always returns -EMLINK, causing an infinite loop. David pointed out that we can preallocate a page table and remap the PMD to be mapped by a PTE table in wp_huge_pmd() in the future. But now we can avoid this issue by not installing PMD mappings when handling a COW and unshare fault in do_set_pmd(). Fixes: a7f226604170 ("mm/gup: trigger FAULT_FLAG_UNSHARE when R/O-pinning a= possibly shared anonymous page") Reported-by: Karol Wachowski Closes: https://lore.kernel.org/linux-ext4/844e5cd4-462e-4b88-b3b5-816465a3= b7e3@linux.intel.com/ Suggested-by: David Hildenbrand Signed-off-by: Zhang Yi Acked-by: David Hildenbrand Reviewed-by: Oscar Salvador (SUSE) Reviewed-by: William Kucharski --- v2->v3: - Update comments to clarify why we shouldn't install PMD mappings while doing CoW. mm/memory.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/mm/memory.c b/mm/memory.c index ea6568571131..b1aed4f08224 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -5520,6 +5520,17 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct f= olio *folio, struct page *pa if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER)) return ret; =20 + /* + * We're about to trigger a write or unshare fault on a CoW + * mapping, breaking the shared folio into private anonymous + * copies at PTE granularity. A PMD mapping would bind an + * entire PMD-sized range to the shared folio, defeating CoW. + * Fall back to direct PTE mapping. + */ + if (is_cow_mapping(vma->vm_flags) && + (vmf->flags & (FAULT_FLAG_WRITE | FAULT_FLAG_UNSHARE))) + return ret; + if (!is_pmd_order(folio_order(folio))) return ret; page =3D &folio->page; --=20 2.52.0