mm-rmap-fix-soft-dirty-and-uffd-wp-bit-loss-when-remapping-zero-filled-mthp-subpage-to-shared-zeropage.patch removed from -mm tree

The quilt patch titled
Subject: mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
has been removed from the -mm tree. Its filename was
mm-rmap-fix-soft-dirty-and-uffd-wp-bit-loss-when-remapping-zero-filled-mthp-subpage-to-shared-zeropage.patch

This patch was dropped because it was merged into the mm-hotfixes-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: Lance Yang <[email protected]>
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.

---

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

@@ -296,8 +296,7 @@ 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;

@@ -306,7 +305,7 @@ 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))

@@ -322,6 +321,12 @@ 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));

@@ -364,13 +369,13 @@ 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 [email protected] are

hung_task-fix-warnings-caused-by-unaligned-lock-pointers.patch mm-khugepaged-abort-collapse-scan-on-non-swap-entries.patch