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Mon, 29 Sep 2025 23:08:09 -0700 (PDT) From: Lance Yang To: akpm@linux-foundation.org, david@redhat.com, lorenzo.stoakes@oracle.com Cc: peterx@redhat.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, baohua@kernel.org, ryan.roberts@arm.com, dev.jain@arm.com, npache@redhat.com, riel@surriel.com, Liam.Howlett@oracle.com, vbabka@suse.cz, harry.yoo@oracle.com, jannh@google.com, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, usamaarif642@gmail.com, yuzhao@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ioworker0@gmail.com, stable@vger.kernel.org, Lance Yang Subject: [PATCH v3 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage Date: Tue, 30 Sep 2025 14:05:57 +0800 Message-ID: <20250930060557.85133-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.49.0 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 187AFC000A X-Stat-Signature: tiqf9zybcgkmm8tswwofnrn9uqpghs3r X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1759212490-203234 X-HE-Meta: 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 N+4eJBgT 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: From: Lance Yang When splitting an mTHP and replacing a zero-filled subpage with the shared zeropage, try_to_map_unused_to_zeropage() currently drops several important PTE bits. For userspace tools like CRIU, which rely on the soft-dirty mechanism for incremental snapshots, losing the soft-dirty bit means modified pages are missed, leading to inconsistent memory state after restore. As pointed out by David, the more critical uffd-wp bit is also dropped. This breaks the userfaultfd write-protection mechanism, causing writes to be silently missed by monitoring applications, which can lead to data corruption. Preserve both the soft-dirty and uffd-wp bits from the old PTE when creating the new zeropage mapping to ensure they are correctly tracked. Cc: Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp") Suggested-by: David Hildenbrand Suggested-by: Dev Jain Acked-by: David Hildenbrand Signed-off-by: Lance Yang --- v2 -> v3: - ptep_get() gets called only once per iteration (per Dev) - https://lore.kernel.org/linux-mm/20250930043351.34927-1-lance.yang@linux.dev/ v1 -> v2: - Avoid calling ptep_get() multiple times (per Dev) - Double-check the uffd-wp bit (per David) - Collect Acked-by from David - thanks! - https://lore.kernel.org/linux-mm/20250928044855.76359-1-lance.yang@linux.dev/ mm/migrate.c | 14 ++++++++++---- 1 file changed, 10 insertions(+), 4 deletions(-) diff --git a/mm/migrate.c b/mm/migrate.c index ce83c2c3c287..bafd8cb3bebe 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -297,6 +297,7 @@ bool isolate_folio_to_list(struct folio *folio, struct list_head *list) static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw, struct folio *folio, + pte_t old_pte, unsigned long idx) { struct page *page = folio_page(folio, idx); @@ -306,7 +307,7 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw, return false; VM_BUG_ON_PAGE(!PageAnon(page), page); VM_BUG_ON_PAGE(!PageLocked(page), page); - VM_BUG_ON_PAGE(pte_present(ptep_get(pvmw->pte)), page); + VM_BUG_ON_PAGE(pte_present(old_pte), page); if (folio_test_mlocked(folio) || (pvmw->vma->vm_flags & VM_LOCKED) || mm_forbids_zeropage(pvmw->vma->vm_mm)) @@ -322,6 +323,12 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw, newpte = pte_mkspecial(pfn_pte(my_zero_pfn(pvmw->address), pvmw->vma->vm_page_prot)); + + if (pte_swp_soft_dirty(old_pte)) + newpte = pte_mksoft_dirty(newpte); + if (pte_swp_uffd_wp(old_pte)) + newpte = pte_mkuffd_wp(newpte); + set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte); dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio)); @@ -344,7 +351,7 @@ static bool remove_migration_pte(struct folio *folio, while (page_vma_mapped_walk(&pvmw)) { rmap_t rmap_flags = RMAP_NONE; - pte_t old_pte; + pte_t old_pte = ptep_get(pvmw.pte); pte_t pte; swp_entry_t entry; struct page *new; @@ -365,12 +372,11 @@ static bool remove_migration_pte(struct folio *folio, } #endif if (rmap_walk_arg->map_unused_to_zeropage && - try_to_map_unused_to_zeropage(&pvmw, folio, idx)) + try_to_map_unused_to_zeropage(&pvmw, folio, old_pte, idx)) continue; folio_get(folio); pte = mk_pte(new, READ_ONCE(vma->vm_page_prot)); - old_pte = ptep_get(pvmw.pte); entry = pte_to_swp_entry(old_pte); if (!is_migration_entry_young(entry)) -- 2.49.0