* [PATCH v5 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
@ 2025-09-30 8:10 Lance Yang
2025-09-30 15:00 ` Zi Yan
` (3 more replies)
0 siblings, 4 replies; 6+ messages in thread
From: Lance Yang @ 2025-09-30 8:10 UTC (permalink / raw)
To: akpm, david, lorenzo.stoakes
Cc: peterx, ziy, baolin.wang, baohua, ryan.roberts, dev.jain, npache,
riel, Liam.Howlett, vbabka, harry.yoo, jannh, matthew.brost,
joshua.hahnjy, rakie.kim, byungchul, gourry, ying.huang, apopple,
usamaarif642, yuzhao, linux-kernel, linux-mm, ioworker0, stable,
Lance Yang
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>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Lance Yang <lance.yang@linux.dev>
---
v4 -> v5:
- Move ptep_get() call after the !pvmw.pte check, which handles PMD-mapped
THP migration entries.
- https://lore.kernel.org/linux-mm/20250930071053.36158-1-lance.yang@linux.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 | 15 ++++++++++-----
1 file changed, 10 insertions(+), 5 deletions(-)
diff --git a/mm/migrate.c b/mm/migrate.c
index ce83c2c3c287..e3065c9edb55 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));
@@ -364,13 +369,13 @@ static bool remove_migration_pte(struct folio *folio,
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))
--
2.49.0
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v5 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
2025-09-30 8:10 [PATCH v5 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage Lance Yang
@ 2025-09-30 15:00 ` Zi Yan
2025-10-02 1:14 ` Harry Yoo
` (2 subsequent siblings)
3 siblings, 0 replies; 6+ messages in thread
From: Zi Yan @ 2025-09-30 15:00 UTC (permalink / raw)
To: Lance Yang
Cc: akpm, david, lorenzo.stoakes, peterx, baolin.wang, baohua,
ryan.roberts, dev.jain, npache, riel, Liam.Howlett, vbabka,
harry.yoo, jannh, matthew.brost, joshua.hahnjy, rakie.kim,
byungchul, gourry, ying.huang, apopple, usamaarif642, yuzhao,
linux-kernel, linux-mm, ioworker0, stable
On 30 Sep 2025, at 4:10, 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>
> Reviewed-by: Dev Jain <dev.jain@arm.com>
> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> ---
> v4 -> v5:
> - Move ptep_get() call after the !pvmw.pte check, which handles PMD-mapped
> THP migration entries.
> - https://lore.kernel.org/linux-mm/20250930071053.36158-1-lance.yang@linux.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 | 15 ++++++++++-----
> 1 file changed, 10 insertions(+), 5 deletions(-)
>
Acked-by: Zi Yan <ziy@nvidia.com>
Best Regards,
Yan, Zi
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v5 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
2025-09-30 8:10 [PATCH v5 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage Lance Yang
2025-09-30 15:00 ` Zi Yan
@ 2025-10-02 1:14 ` Harry Yoo
2025-10-02 16:10 ` Liam R. Howlett
2025-10-14 11:19 ` Lorenzo Stoakes
3 siblings, 0 replies; 6+ messages in thread
From: Harry Yoo @ 2025-10-02 1:14 UTC (permalink / raw)
To: Lance Yang
Cc: akpm, david, lorenzo.stoakes, peterx, ziy, baolin.wang, baohua,
ryan.roberts, dev.jain, npache, riel, Liam.Howlett, vbabka,
jannh, matthew.brost, joshua.hahnjy, rakie.kim, byungchul,
gourry, ying.huang, apopple, usamaarif642, yuzhao, linux-kernel,
linux-mm, ioworker0, stable
On Tue, Sep 30, 2025 at 04:10:40PM +0800, 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>
> Reviewed-by: Dev Jain <dev.jain@arm.com>
> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> ---
Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
--
Cheers,
Harry / Hyeonggon
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v5 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
2025-09-30 8:10 [PATCH v5 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage Lance Yang
2025-09-30 15:00 ` Zi Yan
2025-10-02 1:14 ` Harry Yoo
@ 2025-10-02 16:10 ` Liam R. Howlett
2025-10-14 11:19 ` Lorenzo Stoakes
3 siblings, 0 replies; 6+ messages in thread
From: Liam R. Howlett @ 2025-10-02 16:10 UTC (permalink / raw)
To: Lance Yang
Cc: akpm, david, lorenzo.stoakes, peterx, ziy, baolin.wang, baohua,
ryan.roberts, dev.jain, npache, riel, vbabka, harry.yoo, jannh,
matthew.brost, joshua.hahnjy, rakie.kim, byungchul, gourry,
ying.huang, apopple, usamaarif642, yuzhao, linux-kernel,
linux-mm, ioworker0, stable
* Lance Yang <lance.yang@linux.dev> [250930 04:13]:
> 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>
> Reviewed-by: Dev Jain <dev.jain@arm.com>
> Signed-off-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
> ---
> v4 -> v5:
> - Move ptep_get() call after the !pvmw.pte check, which handles PMD-mapped
> THP migration entries.
> - https://lore.kernel.org/linux-mm/20250930071053.36158-1-lance.yang@linux.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 | 15 ++++++++++-----
> 1 file changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index ce83c2c3c287..e3065c9edb55 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));
> @@ -364,13 +369,13 @@ static bool remove_migration_pte(struct folio *folio,
> 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))
> --
> 2.49.0
>
>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v5 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
2025-09-30 8:10 [PATCH v5 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage Lance Yang
` (2 preceding siblings ...)
2025-10-02 16:10 ` Liam R. Howlett
@ 2025-10-14 11:19 ` Lorenzo Stoakes
2025-10-14 12:25 ` Lance Yang
3 siblings, 1 reply; 6+ messages in thread
From: Lorenzo Stoakes @ 2025-10-14 11:19 UTC (permalink / raw)
To: Lance Yang
Cc: akpm, david, peterx, ziy, baolin.wang, baohua, ryan.roberts,
dev.jain, npache, riel, Liam.Howlett, vbabka, harry.yoo, jannh,
matthew.brost, joshua.hahnjy, rakie.kim, byungchul, gourry,
ying.huang, apopple, usamaarif642, yuzhao, linux-kernel,
linux-mm, ioworker0, stable
Feels like the mTHP implementation is hitting up on the buffers of the THP code
being something of a mess... :)
On Tue, Sep 30, 2025 at 04:10:40PM +0800, 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
It's slightly by-the-by, but CRIU in my view - as it relies on kernel
implementation details that can always change to operate - is not actually
something we have to strictly keep working.
HOWEVER, if we can reasonably do so without causing issues for us in the kernel
we ought to do so.
> 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.
Again, uffd-wp is a total mess. We shouldn't be in a position where its state
being correctly retained relies on everybody always getting the subtle,
uncommented, open-coded details right everywhere all the time.
But this is again a general comment... :)
>
> 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>
> Reviewed-by: Dev Jain <dev.jain@arm.com>
> Signed-off-by: Lance Yang <lance.yang@linux.dev>
Overall LGTM, so:
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> ---
> v4 -> v5:
> - Move ptep_get() call after the !pvmw.pte check, which handles PMD-mapped
> THP migration entries.
> - https://lore.kernel.org/linux-mm/20250930071053.36158-1-lance.yang@linux.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 | 15 ++++++++++-----
> 1 file changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index ce83c2c3c287..e3065c9edb55 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);
Kinda ugly that we pass old_pte when it's avaiable via the shared state object,
but probably nothing to really concern ourselves about.
Guess you could argue it both ways :)
It'd be good to convert these VM_BUG_ON_*() to VM_WARN_ON_*() but I guess that's
somewhat out of the scope of the code here and would be inconsistent to change
it for just one condition.
>
> 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));
> @@ -364,13 +369,13 @@ static bool remove_migration_pte(struct folio *folio,
> 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))
> --
> 2.49.0
>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v5 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
2025-10-14 11:19 ` Lorenzo Stoakes
@ 2025-10-14 12:25 ` Lance Yang
0 siblings, 0 replies; 6+ messages in thread
From: Lance Yang @ 2025-10-14 12:25 UTC (permalink / raw)
To: Lorenzo Stoakes
Cc: akpm, david, peterx, ziy, baolin.wang, baohua, ryan.roberts,
dev.jain, npache, riel, Liam.Howlett, vbabka, harry.yoo, jannh,
matthew.brost, joshua.hahnjy, rakie.kim, byungchul, gourry,
ying.huang, apopple, usamaarif642, yuzhao, linux-kernel,
linux-mm, ioworker0, stable
Thanks for the super energetic review!
On 2025/10/14 19:19, Lorenzo Stoakes wrote:
> Feels like the mTHP implementation is hitting up on the buffers of the THP code
> being something of a mess... :)
Haha, yeah, it really feels that way sometimes ;)
>
> On Tue, Sep 30, 2025 at 04:10:40PM +0800, 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
>
> It's slightly by-the-by, but CRIU in my view - as it relies on kernel
> implementation details that can always change to operate - is not actually
> something we have to strictly keep working.
>
> HOWEVER, if we can reasonably do so without causing issues for us in the kernel
> we ought to do so.
>
>> 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.
>
> Again, uffd-wp is a total mess. We shouldn't be in a position where its state
> being correctly retained relies on everybody always getting the subtle,
> uncommented, open-coded details right everywhere all the time.
>
> But this is again a general comment... :)
:)
>
>>
>> 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>
>> Reviewed-by: Dev Jain <dev.jain@arm.com>
>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>
> Overall LGTM, so:
>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cheers!
>
>> ---
>> v4 -> v5:
>> - Move ptep_get() call after the !pvmw.pte check, which handles PMD-mapped
>> THP migration entries.
>> - https://lore.kernel.org/linux-mm/20250930071053.36158-1-lance.yang@linux.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 | 15 ++++++++++-----
>> 1 file changed, 10 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index ce83c2c3c287..e3065c9edb55 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);
>
> Kinda ugly that we pass old_pte when it's avaiable via the shared state object,
> but probably nothing to really concern ourselves about.
>
> Guess you could argue it both ways :)
>
> It'd be good to convert these VM_BUG_ON_*() to VM_WARN_ON_*() but I guess that's
I agree.
> somewhat out of the scope of the code here and would be inconsistent to change
> it for just one condition.
Since this fix already landed in the mainline, just leave it as is here :)
Thanks,
Lance
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2025-10-14 12:25 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2025-09-30 8:10 [PATCH v5 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage Lance Yang
2025-09-30 15:00 ` Zi Yan
2025-10-02 1:14 ` Harry Yoo
2025-10-02 16:10 ` Liam R. Howlett
2025-10-14 11:19 ` Lorenzo Stoakes
2025-10-14 12:25 ` Lance Yang
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox