[PATCH v2 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@linux.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@vger.kernel.org Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp") Suggested-by: David Hildenbrand david@redhat.com Suggested-by: Dev Jain dev.jain@arm.com Acked-by: David Hildenbrand david@redhat.com Signed-off-by: Lance Yang 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 | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/mm/migrate.c b/mm/migrate.c index ce83c2c3c287..50aa91d9ab4e 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -300,13 +300,14 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw, unsigned long idx) { struct page *page = folio_page(folio, idx); + pte_t oldpte = ptep_get(pvmw->pte); pte_t newpte;
if (PageCompound(page)) 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(oldpte), page);
if (folio_test_mlocked(folio) || (pvmw->vma->vm_flags & VM_LOCKED) || mm_forbids_zeropage(pvmw->vma->vm_mm)) @@ -322,6 +323,12 @@ 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(oldpte)) + newpte = pte_mksoft_dirty(newpte); + if (pte_swp_uffd_wp(oldpte)) + 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));
On 30/09/25 10:03 am, Lance Yang wrote:
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 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@vger.kernel.org Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp") Suggested-by: David Hildenbrand david@redhat.com Suggested-by: Dev Jain dev.jain@arm.com Acked-by: David Hildenbrand david@redhat.com Signed-off-by: Lance Yang 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 | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/mm/migrate.c b/mm/migrate.c index ce83c2c3c287..50aa91d9ab4e 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -300,13 +300,14 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw, unsigned long idx) { struct page *page = folio_page(folio, idx);
- pte_t oldpte = ptep_get(pvmw->pte);
What I meant to say was, you can pass oldpte from remove_migration_pte to this function. Basically define old_pte = ptep_get(pvmw.pte) in the declarations of the start of the while block in remove_migration_pte and remove the existing one. That will ensure ptep_get() gets called only once per iteration.
pte_t newpte; if (PageCompound(page)) 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(oldpte), page);
if (folio_test_mlocked(folio) || (pvmw->vma->vm_flags & VM_LOCKED) || mm_forbids_zeropage(pvmw->vma->vm_mm)) @@ -322,6 +323,12 @@ 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(oldpte))
newpte = pte_mksoft_dirty(newpte);- if (pte_swp_uffd_wp(oldpte))
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));
On 2025/9/30 12:50, Dev Jain wrote:
On 30/09/25 10:03 am, Lance Yang wrote:
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 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@vger.kernel.org Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp") Suggested-by: David Hildenbrand david@redhat.com Suggested-by: Dev Jain dev.jain@arm.com Acked-by: David Hildenbrand david@redhat.com Signed-off-by: Lance Yang 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 | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/mm/migrate.c b/mm/migrate.c index ce83c2c3c287..50aa91d9ab4e 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -300,13 +300,14 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw, unsigned long idx) { struct page *page = folio_page(folio, idx); + pte_t oldpte = ptep_get(pvmw->pte);
What I meant to say was, you can pass oldpte from remove_migration_pte to this function. Basically define old_pte = ptep_get(pvmw.pte) in the declarations of the start of the while block in remove_migration_pte and remove the existing one. That will ensure ptep_get() gets called only once per iteration.
Ah, got it. Thanks for the clarification!
IIUC, you mean something like this:
``` diff --git a/mm/migrate.c b/mm/migrate.c index ce83c2c3c287..bafd8cb3bebe 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -297,6 +297,7 @@ 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, + pte_t old_pte, unsigned long idx) { struct page *page = folio_page(folio, idx); @@ -306,7 +307,7 @@ 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 +323,12 @@ 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 +351,7 @@ 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 +372,11 @@ 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)) ```
ptep_get() gets called only once per iteration, right?
pte_t newpte; if (PageCompound(page)) 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(oldpte), page); if (folio_test_mlocked(folio) || (pvmw->vma->vm_flags & VM_LOCKED) || mm_forbids_zeropage(pvmw->vma->vm_mm)) @@ -322,6 +323,12 @@ 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(oldpte)) + newpte = pte_mksoft_dirty(newpte); + if (pte_swp_uffd_wp(oldpte)) + 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));
On 30/09/25 10:52 am, Lance Yang wrote:
On 2025/9/30 12:50, Dev Jain wrote:
On 30/09/25 10:03 am, Lance Yang wrote:
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 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@vger.kernel.org Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp") Suggested-by: David Hildenbrand david@redhat.com Suggested-by: Dev Jain dev.jain@arm.com Acked-by: David Hildenbrand david@redhat.com Signed-off-by: Lance Yang 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 | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/mm/migrate.c b/mm/migrate.c index ce83c2c3c287..50aa91d9ab4e 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -300,13 +300,14 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw, unsigned long idx) { struct page *page = folio_page(folio, idx); + pte_t oldpte = ptep_get(pvmw->pte);
What I meant to say was, you can pass oldpte from remove_migration_pte to this function. Basically define old_pte = ptep_get(pvmw.pte) in the declarations of the start of the while block in remove_migration_pte and remove the existing one. That will ensure ptep_get() gets called only once per iteration.
Ah, got it. Thanks for the clarification!
IIUC, you mean something like this:
diff --git a/mm/migrate.c b/mm/migrate.c index ce83c2c3c287..bafd8cb3bebe 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -297,6 +297,7 @@ 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, + pte_t old_pte, unsigned long idx) { struct page *page = folio_page(folio, idx); @@ -306,7 +307,7 @@ 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 +323,12 @@ 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 +351,7 @@ 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 +372,11 @@ 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))ptep_get() gets called only once per iteration, right?
Yup.
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