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

[Andrew Morton <akpm@linux-foundation.org>]

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 <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>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: Harry Yoo <harry.yoo@oracle.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: 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
@@ -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 lance.yang@linux.dev are

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