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

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Message-ID: <20250928044855.76359-1-lance.yang@linux.dev>
Date: Sun, 28 Sep 2025 12:48:55 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: akpm@...ux-foundation.org,
	david@...hat.com,
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	Lance Yang <lance.yang@...ux.dev>
Subject: [PATCH 1/1] mm/rmap: fix soft-dirty bit loss when remapping zero-filled mTHP subpage to shared zeropage

From: Lance Yang <lance.yang@...ux.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@...r.kernel.org>
Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
Signed-off-by: Lance Yang <lance.yang@...ux.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