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

Lance Yang
2025-09-28T04:49:22Z

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 the soft-dirty
bit.

For userspace tools like CRIU, which rely on the soft-dirty mechanism for
incremental snapshots, losing this bit means modified pages are missed,
leading to inconsistent memory state after restore.

Preserve the soft-dirty bit from the old PTE when creating the zeropage
mapping to ensure modified pages are correctly tracked.

Cc: <stable@vger.kernel.org>
Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
Signed-off-by: Lance Yang <lance.yang@linux.dev>
---
mm/migrate.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/mm/migrate.c b/mm/migrate.c
index ce83c2c3c287..bf364ba07a3f 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -322,6 +322,10 @@ 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(ptep_get(pvmw->pte)))
+ newpte = pte_mksoft_dirty(newpte);
+
set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte);

dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio));
--
2.49.0