[PATCH 1/1] mm/rmap: fix soft-dirty 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 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));
On 28/09/25 10:18 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 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));
I think this should work.
You can pass old_pte = ptep_get(pvmw->pte) to this function to avoid calling ptep_get() multiple times.
On 2025/9/29 12:44, Dev Jain wrote:
On 28/09/25 10:18 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 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));
I think this should work.
You can pass old_pte = ptep_get(pvmw->pte) to this function to avoid calling ptep_get() multiple times.
Good catch! Will do in v2, thanks.
On 28.09.25 06:48, 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 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));
It's interesting that there isn't a single occurrence of the stof-dirty flag in khugepaged code. I guess it all works because we do the
_pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
and the pmd_mkdirty() will imply marking it soft-dirty.
Now to the problem at hand: I don't think this is particularly problematic in the common case: if the page is zero, it likely was never written to (that's what the unerused shrinker is targeted at), so the soft-dirty setting on the PMD is actually just an over-indication for this page.
For example, when we just install the shared zeropage directly in do_anonymous_page(), we obviously also don't set it dirty/soft-dirty.
Now, one could argue that if the content was changed from non-zero to zero, it ould actually be soft-dirty.
Long-story short: I don't think this matters much in practice, but it's an easy fix.
As said by dev, please avoid double ptep_get() if possible.
Acked-by: David Hildenbrand david@redhat.com
@Lance, can you double-check that the uffd-wp bit is handled correctly? I strongly assume we lose that as well here.
On 2025/9/29 15:25, David Hildenbrand wrote:
On 28.09.25 06:48, 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 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));
It's interesting that there isn't a single occurrence of the stof-dirty flag in khugepaged code. I guess it all works because we do the
_pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
and the pmd_mkdirty() will imply marking it soft-dirty.
Now to the problem at hand: I don't think this is particularly problematic in the common case: if the page is zero, it likely was never written to (that's what the unerused shrinker is targeted at), so the soft-dirty setting on the PMD is actually just an over-indication for this page.
Cool. Thanks for the insight! Good to know that ;)
For example, when we just install the shared zeropage directly in do_anonymous_page(), we obviously also don't set it dirty/soft-dirty.
Now, one could argue that if the content was changed from non-zero to zero, it ould actually be soft-dirty.
Exactly. A false negative could be a problem for the userspace tools, IMO.
Long-story short: I don't think this matters much in practice, but it's an easy fix.
As said by dev, please avoid double ptep_get() if possible.
Sure, will do. I'll refactor it in the next version.
Acked-by: David Hildenbrand david@redhat.com
Thanks!
@Lance, can you double-check that the uffd-wp bit is handled correctly? I strongly assume we lose that as well here.
Certainly, I'll check the uffd-wp bit as well and get back to you soon.
Cheers, Lance
On 2025/9/29 18:29, Lance Yang wrote:
On 2025/9/29 15:25, David Hildenbrand wrote:
On 28.09.25 06:48, 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 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));
It's interesting that there isn't a single occurrence of the stof- dirty flag in khugepaged code. I guess it all works because we do the
_pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
and the pmd_mkdirty() will imply marking it soft-dirty.
Now to the problem at hand: I don't think this is particularly problematic in the common case: if the page is zero, it likely was never written to (that's what the unerused shrinker is targeted at), so the soft-dirty setting on the PMD is actually just an over- indication for this page.
Cool. Thanks for the insight! Good to know that ;)
For example, when we just install the shared zeropage directly in do_anonymous_page(), we obviously also don't set it dirty/soft-dirty.
Now, one could argue that if the content was changed from non-zero to zero, it ould actually be soft-dirty.
Exactly. A false negative could be a problem for the userspace tools, IMO.
Long-story short: I don't think this matters much in practice, but it's an easy fix.
As said by dev, please avoid double ptep_get() if possible.
Sure, will do. I'll refactor it in the next version.
Acked-by: David Hildenbrand david@redhat.com
Thanks!
@Lance, can you double-check that the uffd-wp bit is handled correctly? I strongly assume we lose that as well here.
Yes, the uffd-wp bit was indeed being dropped, but ...
The shared zeropage is read-only, which triggers a fault. IIUC, The kernel then falls back to checking the VM_UFFD_WP flag on the VMA and correctly generates a uffd-wp event, masking the fact that the uffd-wp bit on the PTE was lost.
IMHO, explicitly preserving the uffd-wp bit on the PTE is still necessary, since we're not sure if losing that bit is safe in all cases :)
Certainly, I'll check the uffd-wp bit as well and get back to you soon.
Cheers, Lance
On 29.09.25 13:29, Lance Yang wrote:
On 2025/9/29 18:29, Lance Yang wrote:
On 2025/9/29 15:25, David Hildenbrand wrote:
On 28.09.25 06:48, 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 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));
It's interesting that there isn't a single occurrence of the stof- dirty flag in khugepaged code. I guess it all works because we do the
_pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
and the pmd_mkdirty() will imply marking it soft-dirty.
Now to the problem at hand: I don't think this is particularly problematic in the common case: if the page is zero, it likely was never written to (that's what the unerused shrinker is targeted at), so the soft-dirty setting on the PMD is actually just an over- indication for this page.
Cool. Thanks for the insight! Good to know that ;)
For example, when we just install the shared zeropage directly in do_anonymous_page(), we obviously also don't set it dirty/soft-dirty.
Now, one could argue that if the content was changed from non-zero to zero, it ould actually be soft-dirty.
Exactly. A false negative could be a problem for the userspace tools, IMO.
Long-story short: I don't think this matters much in practice, but it's an easy fix.
As said by dev, please avoid double ptep_get() if possible.
Sure, will do. I'll refactor it in the next version.
Acked-by: David Hildenbrand david@redhat.com
Thanks!
@Lance, can you double-check that the uffd-wp bit is handled correctly? I strongly assume we lose that as well here.
Yes, the uffd-wp bit was indeed being dropped, but ...
The shared zeropage is read-only, which triggers a fault. IIUC, The kernel then falls back to checking the VM_UFFD_WP flag on the VMA and correctly generates a uffd-wp event, masking the fact that the uffd-wp bit on the PTE was lost.
That's not how VM_UFFD_WP works :)
On 2025/9/29 20:08, David Hildenbrand wrote:
On 29.09.25 13:29, Lance Yang wrote:
On 2025/9/29 18:29, Lance Yang wrote:
On 2025/9/29 15:25, David Hildenbrand wrote:
On 28.09.25 06:48, 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 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));
It's interesting that there isn't a single occurrence of the stof- dirty flag in khugepaged code. I guess it all works because we do the
_pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
and the pmd_mkdirty() will imply marking it soft-dirty.
Now to the problem at hand: I don't think this is particularly problematic in the common case: if the page is zero, it likely was never written to (that's what the unerused shrinker is targeted at), so the soft-dirty setting on the PMD is actually just an over- indication for this page.
Cool. Thanks for the insight! Good to know that ;)
For example, when we just install the shared zeropage directly in do_anonymous_page(), we obviously also don't set it dirty/soft-dirty.
Now, one could argue that if the content was changed from non-zero to zero, it ould actually be soft-dirty.
Exactly. A false negative could be a problem for the userspace tools, IMO.
Long-story short: I don't think this matters much in practice, but it's an easy fix.
As said by dev, please avoid double ptep_get() if possible.
Sure, will do. I'll refactor it in the next version.
Acked-by: David Hildenbrand david@redhat.com
Thanks!
@Lance, can you double-check that the uffd-wp bit is handled correctly? I strongly assume we lose that as well here.
Yes, the uffd-wp bit was indeed being dropped, but ...
The shared zeropage is read-only, which triggers a fault. IIUC, The kernel then falls back to checking the VM_UFFD_WP flag on the VMA and correctly generates a uffd-wp event, masking the fact that the uffd-wp bit on the PTE was lost.
That's not how VM_UFFD_WP works :)
My bad! Please accept my apologies for the earlier confusion :(
I messed up my test environment (forgot to enable mTHP), which led me to a completely wrong conclusion...
You're spot on. With mTHP enabled, the WP fault was not caught on the shared zeropage after it replaced a zero-filled subpage during an mTHP split.
This is because do_wp_page() requires userfaultfd_pte_wp() to be true, which in turn needs both userfaultfd_wp(vma) and pte_uffd_wp(pte).
static inline bool userfaultfd_pte_wp(struct vm_area_struct *vma, pte_t pte) { return userfaultfd_wp(vma) && pte_uffd_wp(pte); }
userfaultfd_pte_wp() fails as we lose the uffd-wp bit on the PTE ...
Please correct me if I missed something important!
On 29.09.25 15:22, Lance Yang wrote:
On 2025/9/29 20:08, David Hildenbrand wrote:
On 29.09.25 13:29, Lance Yang wrote:
On 2025/9/29 18:29, Lance Yang wrote:
On 2025/9/29 15:25, David Hildenbrand wrote:
On 28.09.25 06:48, 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 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));
It's interesting that there isn't a single occurrence of the stof- dirty flag in khugepaged code. I guess it all works because we do the
_pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
and the pmd_mkdirty() will imply marking it soft-dirty.
Now to the problem at hand: I don't think this is particularly problematic in the common case: if the page is zero, it likely was never written to (that's what the unerused shrinker is targeted at), so the soft-dirty setting on the PMD is actually just an over- indication for this page.
Cool. Thanks for the insight! Good to know that ;)
For example, when we just install the shared zeropage directly in do_anonymous_page(), we obviously also don't set it dirty/soft-dirty.
Now, one could argue that if the content was changed from non-zero to zero, it ould actually be soft-dirty.
Exactly. A false negative could be a problem for the userspace tools, IMO.
Long-story short: I don't think this matters much in practice, but it's an easy fix.
As said by dev, please avoid double ptep_get() if possible.
Sure, will do. I'll refactor it in the next version.
Acked-by: David Hildenbrand david@redhat.com
Thanks!
@Lance, can you double-check that the uffd-wp bit is handled correctly? I strongly assume we lose that as well here.
Yes, the uffd-wp bit was indeed being dropped, but ...
The shared zeropage is read-only, which triggers a fault. IIUC, The kernel then falls back to checking the VM_UFFD_WP flag on the VMA and correctly generates a uffd-wp event, masking the fact that the uffd-wp bit on the PTE was lost.
That's not how VM_UFFD_WP works :)
My bad! Please accept my apologies for the earlier confusion :(
I messed up my test environment (forgot to enable mTHP), which led me to a completely wrong conclusion...
You're spot on. With mTHP enabled, the WP fault was not caught on the shared zeropage after it replaced a zero-filled subpage during an mTHP split.
This is because do_wp_page() requires userfaultfd_pte_wp() to be true, which in turn needs both userfaultfd_wp(vma) and pte_uffd_wp(pte).
static inline bool userfaultfd_pte_wp(struct vm_area_struct *vma, pte_t pte) { return userfaultfd_wp(vma) && pte_uffd_wp(pte); }
userfaultfd_pte_wp() fails as we lose the uffd-wp bit on the PTE ...
That's my understanding. And FWIW, that's a much more important fix. (in contrast to soft-dirty, uffd-wp actually is precise)
Can you test+send a fix ... please? :)
linux-stable-mirror@lists.linaro.org
-
David Hildenbrand -
Dev Jain -
Lance Yang