The patch titled Subject: mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-rmap-fix-soft-dirty-and-uffd-wp-bit-loss-when-remapping-zero-filled-mthp-subpage-to-shared-zeropage.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Lance Yang lance.yang@linux.dev Subject: 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

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.

Link: https://lkml.kernel.org/r/20250930081040.80926-1-lance.yang@linux.dev Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp") Signed-off-by: Lance Yang lance.yang@linux.dev Suggested-by: David Hildenbrand david@redhat.com Suggested-by: Dev Jain dev.jain@arm.com Acked-by: David Hildenbrand david@redhat.com Reviewed-by: Dev Jain dev.jain@arm.com Acked-by: Zi Yan ziy@nvidia.com Cc: Alistair Popple apopple@nvidia.com Cc: Baolin Wang baolin.wang@linux.alibaba.com Cc: Barry Song baohua@kernel.org Cc: Byungchul Park byungchul@sk.com Cc: Gregory Price gourry@gourry.net Cc: "Huang, Ying" ying.huang@linux.alibaba.com Cc: Jann Horn jannh@google.com Cc: Joshua Hahn joshua.hahnjy@gmail.com Cc: Liam Howlett liam.howlett@oracle.com Cc: Lorenzo Stoakes lorenzo.stoakes@oracle.com Cc: Mariano Pache npache@redhat.com Cc: Mathew Brost matthew.brost@intel.com Cc: Peter Xu peterx@redhat.com Cc: Rakie Kim rakie.kim@sk.com Cc: Rik van Riel riel@surriel.com Cc: Ryan Roberts ryan.roberts@arm.com Cc: Usama Arif usamaarif642@gmail.com Cc: Vlastimil Babka vbabka@suse.cz Cc: Yu Zhao yuzhao@google.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/migrate.c | 15 ++++++++++----- 1 file changed, 10 insertions(+), 5 deletions(-)

--- a/mm/migrate.c~mm-rmap-fix-soft-dirty-and-uffd-wp-bit-loss-when-remapping-zero-filled-mthp-subpage-to-shared-zeropage +++ a/mm/migrate.c @@ -297,8 +297,7 @@ bool isolate_folio_to_list(struct folio }

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; @@ -307,7 +306,7 @@ static bool try_to_map_unused_to_zeropag 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)) @@ -323,6 +322,12 @@ static bool try_to_map_unused_to_zeropag

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)); @@ -365,13 +370,13 @@ static bool remove_migration_pte(struct 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)) _

Patches currently in -mm which might be from lance.yang@linux.dev are

hung_task-fix-warnings-caused-by-unaligned-lock-pointers.patch mm-thp-fix-mte-tag-mismatch-when-replacing-zero-filled-subpages.patch mm-rmap-fix-soft-dirty-and-uffd-wp-bit-loss-when-remapping-zero-filled-mthp-subpage-to-shared-zeropage.patch mm-clean-up-is_guard_pte_marker.patch