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

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Message-ID: <20250930071053.36158-1-lance.yang@linux.dev>
Date: Tue, 30 Sep 2025 15:10:53 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: akpm@...ux-foundation.org,
	david@...hat.com,
	lorenzo.stoakes@...cle.com
Cc: peterx@...hat.com,
	ziy@...dia.com,
	baolin.wang@...ux.alibaba.com,
	baohua@...nel.org,
	ryan.roberts@....com,
	dev.jain@....com,
	npache@...hat.com,
	riel@...riel.com,
	Liam.Howlett@...cle.com,
	vbabka@...e.cz,
	harry.yoo@...cle.com,
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	matthew.brost@...el.com,
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	rakie.kim@...com,
	byungchul@...com,
	gourry@...rry.net,
	ying.huang@...ux.alibaba.com,
	apopple@...dia.com,
	usamaarif642@...il.com,
	yuzhao@...gle.com,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	ioworker0@...il.com,
	stable@...r.kernel.org,
	Lance Yang <lance.yang@...ux.dev>
Subject: [PATCH v4 1/1] mm/rmap: fix soft-dirty and uffd-wp 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 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@...r.kernel.org>
Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
Suggested-by: David Hildenbrand <david@...hat.com>
Suggested-by: Dev Jain <dev.jain@....com>
Acked-by: David Hildenbrand <david@...hat.com>
Reviewed-by: Dev Jain <dev.jain@....com>
Signed-off-by: Lance Yang <lance.yang@...ux.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