New subject: [syzbot ci] Re: mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage

30 Sep 2025

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);
    	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))
-- 
2.49.0



    

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