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Tue, 30 Sep 2025 01:12:53 -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 v5 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 16:10:40 +0800 Message-ID: <20250930081040.80926-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.49.0 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 04ABD40004 X-Rspam-User: X-Rspamd-Server: rspam07 X-Stat-Signature: wbndzdx897ge4rwfb43pnphnh6ugqx1n X-HE-Tag: 1759219975-16823 X-HE-Meta: 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 4z/zzSPj 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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 Reviewed-by: Dev Jain Signed-off-by: Lance Yang --- v4 -> v5: - Move ptep_get() call after the !pvmw.pte check, which handles PMD-mapped THP migration entries. - https://lore.kernel.org/linux-mm/20250930071053.36158-1-lance.yang@linux.dev/ v3 -> v4: - Minor formatting tweak in try_to_map_unused_to_zeropage() function signature (per David and Dev) - Collect Reviewed-by from Dev - thanks! - https://lore.kernel.org/linux-mm/20250930060557.85133-1-lance.yang@linux.dev/ 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 | 15 ++++++++++----- 1 file changed, 10 insertions(+), 5 deletions(-) diff --git a/mm/migrate.c b/mm/migrate.c index ce83c2c3c287..e3065c9edb55 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -296,8 +296,7 @@ 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, - unsigned long idx) + struct folio *folio, pte_t old_pte, unsigned long idx) { struct page *page = folio_page(folio, idx); pte_t newpte; @@ -306,7 +305,7 @@ 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 +321,12 @@ 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)); @@ -364,13 +369,13 @@ static bool remove_migration_pte(struct folio *folio, continue; } #endif + old_pte = ptep_get(pvmw.pte); 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