Re: [PATCH v4 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage

On 2025/9/30 15:10, Lance Yang wrote:

From: Lance Yang lance.yang@linux.dev

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: stable@vger.kernel.org Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp") 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 Signed-off-by: Lance Yang 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 | 16 ++++++++++------ 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c index ce83c2c3c287..21a2a1bf89f7 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)); @@ -344,7 +349,7 @@ 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);
  

Oops, I just found a NULL pointer dereference bug in my changes to remove_migration_pte() when we encounter a PMD-mapped THP migration entry.

#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION /* PMD-mapped THP migration entry */ if (!pvmw.pte) { VM_BUG_ON_FOLIO(folio_test_hugetlb(folio) || !folio_test_pmd_mappable(folio), folio); remove_migration_pmd(&pvmw, new); continue; } #endif

ptep_get() is called too early... before the !pvmw.pte check for PMD-mapped entries.

The initialization of old_pte must be moved to after that if block.

Sorry for the churn :( Lance

pte_t pte;
swp_entry_t entry;
struct page *new;

@@ -365,12 +370,11 @@ 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))