[PATCH 1/3] mm: don't PTE-batch a swap-in over a hardware-poisoned subpage

[PATCH 1/3] mm: don't PTE-batch a swap-in over a hardware-poisoned subpage

[Usama Arif]

do_swap_page() checks PageHWPoison() on the faulting page, but the
large-folio batching path then maps the rest of the folio without looking
at the other subpages. A poisoned subpage that is not the one being
faulted gets mapped anyway, and the fault that would have reported it
never happens.

Skip batching for such a folio and let each page take its own fault,
where the existing check catches it.

Scan the subpages rather than testing the folio-level PG_has_hwpoisoned:
memory_failure() sets PageHWPoison() on the subpage first and only sets
the folio flag much later, and the bail-outs in between - for example
get_hwpoison_page() losing a page lifecycle race - return with the
subpage marked and the folio flag never set.

Fixes: 508758960b8d ("mm: swap: entirely map large folios found in swapcache")
Cc: stable@vger.kernel.org
Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 mm/internal.h | 15 +++++++++++++++
 mm/memory.c   |  4 ++++
 2 files changed, 19 insertions(+)

diff --git a/mm/internal.h b/mm/internal.h
index cd3675c044e0a..e61c02f4019b0 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -28,6 +28,21 @@ struct folio_batch;
 int __copy_remote_mm_str(struct mm_struct *mm, unsigned long addr,
 		void *buf, int len, unsigned int gup_flags);
 
+/*
+ * Check every page of the folio for HWPoison, rather than relying on the
+ * folio-level PG_has_hwpoisoned, which memory_failure() can leave clear.
+ */
+static inline bool folio_has_any_hwpoisoned_page(const struct folio *folio)
+{
+	long nr = folio_nr_pages(folio);
+	long i;
+
+	for (i = 0; i < nr; i++)
+		if (PageHWPoison(folio_page(folio, i)))
+			return true;
+	return false;
+}
+
 /* mm/workingset.c */
 bool workingset_test_recent(void *shadow, bool file, bool *workingset,
 			    bool flush);
diff --git a/mm/memory.c b/mm/memory.c
index d477a4f986bc8..4b7d456332200 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5158,6 +5158,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 		    swap_pte_batch(folio_ptep, nr, folio_pte) != nr)
 			goto check_folio;
 
+		/* Let a poisoned subpage take its own fault. */
+		if (folio_has_any_hwpoisoned_page(folio))
+			goto check_folio;
+
 		page_idx = idx;
 		address = folio_start;
 		ptep = folio_ptep;
-- 
2.53.0-Meta

Re: [PATCH 1/3] mm: don't PTE-batch a swap-in over a hardware-poisoned subpage

[Lorenzo Stoakes (ARM)]

On Wed, Nov 11, 2026 at 03:38:29AM -0800, Usama Arif wrote:
> do_swap_page() checks PageHWPoison() on the faulting page, but the
> large-folio batching path then maps the rest of the folio without looking
> at the other subpages. A poisoned subpage that is not the one being
> faulted gets mapped anyway, and the fault that would have reported it
> never happens.
>
> Skip batching for such a folio and let each page take its own fault,
> where the existing check catches it.
>
> Scan the subpages rather than testing the folio-level PG_has_hwpoisoned:
> memory_failure() sets PageHWPoison() on the subpage first and only sets
> the folio flag much later, and the bail-outs in between - for example
> get_hwpoison_page() losing a page lifecycle race - return with the
> subpage marked and the folio flag never set.
>
> Fixes: 508758960b8d ("mm: swap: entirely map large folios found in swapcache")
> Cc: stable@vger.kernel.org
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---
>  mm/internal.h | 15 +++++++++++++++
>  mm/memory.c   |  4 ++++
>  2 files changed, 19 insertions(+)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index cd3675c044e0a..e61c02f4019b0 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -28,6 +28,21 @@ struct folio_batch;
>  int __copy_remote_mm_str(struct mm_struct *mm, unsigned long addr,
>  		void *buf, int len, unsigned int gup_flags);
>
> +/*
> + * Check every page of the folio for HWPoison, rather than relying on the
> + * folio-level PG_has_hwpoisoned, which memory_failure() can leave clear.
> + */
> +static inline bool folio_has_any_hwpoisoned_page(const struct folio *folio)
> +{
> +	long nr = folio_nr_pages(folio);
> +	long i;
> +
> +	for (i = 0; i < nr; i++)
> +		if (PageHWPoison(folio_page(folio, i)))
> +			return true;
> +	return false;
> +}
> +
>  /* mm/workingset.c */
>  bool workingset_test_recent(void *shadow, bool file, bool *workingset,
>  			    bool flush);
> diff --git a/mm/memory.c b/mm/memory.c
> index d477a4f986bc8..4b7d456332200 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5158,6 +5158,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  		    swap_pte_batch(folio_ptep, nr, folio_pte) != nr)
>  			goto check_folio;
>
> +		/* Let a poisoned subpage take its own fault. */
> +		if (folio_has_any_hwpoisoned_page(folio))
> +			goto check_folio;
> +

Hm, isn't scanning every single page in the folio potentially quite costly
here?  I thought the point of batching was to improve performance overall?

It seems broken to have stuff potentially interact with hwpoison but then
to only set the folio-level flag much later?

Also why are we swapping out folios containing hwpoison pages? Shouldn't we
migrate away from the hwpoison? :) (I'm not really familiar with the hw
poison stuff)


>  		page_idx = idx;
>  		address = folio_start;
>  		ptep = folio_ptep;
> --
> 2.53.0-Meta
>

--
Cheers, Lorenzo

Re: [PATCH 1/3] mm: don't PTE-batch a swap-in over a hardware-poisoned subpage

[Usama Arif]

On Wed, 11 Nov 2026 13:18:36 +0000 "Lorenzo Stoakes (ARM)" <ljs@kernel.org> wrote:

> On Wed, Nov 11, 2026 at 03:38:29AM -0800, Usama Arif wrote:
> > do_swap_page() checks PageHWPoison() on the faulting page, but the
> > large-folio batching path then maps the rest of the folio without looking
> > at the other subpages. A poisoned subpage that is not the one being
> > faulted gets mapped anyway, and the fault that would have reported it
> > never happens.
> >
> > Skip batching for such a folio and let each page take its own fault,
> > where the existing check catches it.
> >
> > Scan the subpages rather than testing the folio-level PG_has_hwpoisoned:
> > memory_failure() sets PageHWPoison() on the subpage first and only sets
> > the folio flag much later, and the bail-outs in between - for example
> > get_hwpoison_page() losing a page lifecycle race - return with the
> > subpage marked and the folio flag never set.
> >
> > Fixes: 508758960b8d ("mm: swap: entirely map large folios found in swapcache")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Usama Arif <usama.arif@linux.dev>
> > ---
> >  mm/internal.h | 15 +++++++++++++++
> >  mm/memory.c   |  4 ++++
> >  2 files changed, 19 insertions(+)
> >
> > diff --git a/mm/internal.h b/mm/internal.h
> > index cd3675c044e0a..e61c02f4019b0 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -28,6 +28,21 @@ struct folio_batch;
> >  int __copy_remote_mm_str(struct mm_struct *mm, unsigned long addr,
> >  		void *buf, int len, unsigned int gup_flags);
> >
> > +/*
> > + * Check every page of the folio for HWPoison, rather than relying on the
> > + * folio-level PG_has_hwpoisoned, which memory_failure() can leave clear.
> > + */
> > +static inline bool folio_has_any_hwpoisoned_page(const struct folio *folio)
> > +{
> > +	long nr = folio_nr_pages(folio);
> > +	long i;
> > +
> > +	for (i = 0; i < nr; i++)
> > +		if (PageHWPoison(folio_page(folio, i)))
> > +			return true;
> > +	return false;
> > +}
> > +
> >  /* mm/workingset.c */
> >  bool workingset_test_recent(void *shadow, bool file, bool *workingset,
> >  			    bool flush);
> > diff --git a/mm/memory.c b/mm/memory.c
> > index d477a4f986bc8..4b7d456332200 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -5158,6 +5158,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >  		    swap_pte_batch(folio_ptep, nr, folio_pte) != nr)
> >  			goto check_folio;
> >
> > +		/* Let a poisoned subpage take its own fault. */
> > +		if (folio_has_any_hwpoisoned_page(folio))
> > +			goto check_folio;
> > +

Hello Lorenzo,

Thanks for the reviews!

> 
> Hm, isn't scanning every single page in the folio potentially quite costly
> here?  I thought the point of batching was to improve performance overall?

Swapping in requires checking every subpage, so there would be a cost.
I guess trading it off for correctness :(

> 
> It seems broken to have stuff potentially interact with hwpoison but then
> to only set the folio-level flag much later?
> 

PG_has_hwpoisoned lives in the second page, so memory_failure() only
sets it once it holds a folio reference, otherwise it could race with a
THP split. If get_hwpoison_page() can't get a reference,
memory_failure() gives up with only the subpage marked. The comment
above folio_set_has_hwpoisoned() in memory_failure() says that is fine:

  Don't need care about the above error handling paths for
  get_hwpoison_page() since they handle either free page
  or unhandlable page.

But a large folio that is briefly off the LRU, e.g. isolated by
reclaim, takes those paths too and can still be mapped afterwards.

> Also why are we swapping out folios containing hwpoison pages? Shouldn't we
> migrate away from the hwpoison? :) (I'm not really familiar with the hw
> poison stuff)
> 
> 

The error hits a folio that is already in the swap
cache, e.g. while reclaim has it off the LRU to write it out, and
get_hwpoison_page() fails for any folio that is off the LRU. Normally
memory_failure() would split the folio and isolate the bad page (for
an uncorrected error there is nothing to migrate; that is soft
offline). Here it does neither, and the folio stays in the swap cache
with only the subpage marked.

While looking at this I also noticed that the fallback drops a freshly
read (not yet anon) folio from the swap cache and frees it with the
poisoned page in it. We should actually actually also check folio_test_anon()
before checking folio_has_any_hwpoisoned_page().

> >  		page_idx = idx;
> >  		address = folio_start;
> >  		ptep = folio_ptep;
> > --
> > 2.53.0-Meta
> >
> 
> --
> Cheers, Lorenzo
> 

Re: [PATCH 1/3] mm: don't PTE-batch a swap-in over a hardware-poisoned subpage

[Lorenzo Stoakes (ARM)]

On Wed, Nov 11, 2026 at 10:39:17AM -0800, Usama Arif wrote:
> On Wed, 11 Nov 2026 13:18:36 +0000 "Lorenzo Stoakes (ARM)" <ljs@kernel.org> wrote:
>
> > On Wed, Nov 11, 2026 at 03:38:29AM -0800, Usama Arif wrote:
> > > do_swap_page() checks PageHWPoison() on the faulting page, but the
> > > large-folio batching path then maps the rest of the folio without looking
> > > at the other subpages. A poisoned subpage that is not the one being
> > > faulted gets mapped anyway, and the fault that would have reported it
> > > never happens.
> > >
> > > Skip batching for such a folio and let each page take its own fault,
> > > where the existing check catches it.
> > >
> > > Scan the subpages rather than testing the folio-level PG_has_hwpoisoned:
> > > memory_failure() sets PageHWPoison() on the subpage first and only sets
> > > the folio flag much later, and the bail-outs in between - for example
> > > get_hwpoison_page() losing a page lifecycle race - return with the
> > > subpage marked and the folio flag never set.
> > >
> > > Fixes: 508758960b8d ("mm: swap: entirely map large folios found in swapcache")
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: Usama Arif <usama.arif@linux.dev>
> > > ---
> > >  mm/internal.h | 15 +++++++++++++++
> > >  mm/memory.c   |  4 ++++
> > >  2 files changed, 19 insertions(+)
> > >
> > > diff --git a/mm/internal.h b/mm/internal.h
> > > index cd3675c044e0a..e61c02f4019b0 100644
> > > --- a/mm/internal.h
> > > +++ b/mm/internal.h
> > > @@ -28,6 +28,21 @@ struct folio_batch;
> > >  int __copy_remote_mm_str(struct mm_struct *mm, unsigned long addr,
> > >  		void *buf, int len, unsigned int gup_flags);
> > >
> > > +/*
> > > + * Check every page of the folio for HWPoison, rather than relying on the
> > > + * folio-level PG_has_hwpoisoned, which memory_failure() can leave clear.
> > > + */
> > > +static inline bool folio_has_any_hwpoisoned_page(const struct folio *folio)
> > > +{
> > > +	long nr = folio_nr_pages(folio);
> > > +	long i;
> > > +
> > > +	for (i = 0; i < nr; i++)
> > > +		if (PageHWPoison(folio_page(folio, i)))
> > > +			return true;
> > > +	return false;
> > > +}
> > > +
> > >  /* mm/workingset.c */
> > >  bool workingset_test_recent(void *shadow, bool file, bool *workingset,
> > >  			    bool flush);
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index d477a4f986bc8..4b7d456332200 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -5158,6 +5158,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > >  		    swap_pte_batch(folio_ptep, nr, folio_pte) != nr)
> > >  			goto check_folio;
> > >
> > > +		/* Let a poisoned subpage take its own fault. */
> > > +		if (folio_has_any_hwpoisoned_page(folio))
> > > +			goto check_folio;
> > > +
>
> Hello Lorenzo,
>
> Thanks for the reviews!
>
> >
> > Hm, isn't scanning every single page in the folio potentially quite costly
> > here?  I thought the point of batching was to improve performance overall?
>
> Swapping in requires checking every subpage, so there would be a cost.
> I guess trading it off for correctness :(

(we don't say subpage apparently :)

Yeah but the cost is now paid by literally everybody...

And...

>
> >
> > It seems broken to have stuff potentially interact with hwpoison but then
> > to only set the folio-level flag much later?
> >
>
> PG_has_hwpoisoned lives in the second page, so memory_failure() only
> sets it once it holds a folio reference, otherwise it could race with a
> THP split. If get_hwpoison_page() can't get a reference,
> memory_failure() gives up with only the subpage marked. The comment
> above folio_set_has_hwpoisoned() in memory_failure() says that is fine:
>
>   Don't need care about the above error handling paths for
>   get_hwpoison_page() since they handle either free page
>   or unhandlable page.
>
> But a large folio that is briefly off the LRU, e.g. isolated by
> reclaim, takes those paths too and can still be mapped afterwards.

...now you're narrowing this cost to literally a vague edge case.

Can we not just do the scan _only in this case_?

>
> > Also why are we swapping out folios containing hwpoison pages? Shouldn't we
> > migrate away from the hwpoison? :) (I'm not really familiar with the hw
> > poison stuff)
> >
> >
>
> The error hits a folio that is already in the swap
> cache, e.g. while reclaim has it off the LRU to write it out, and
> get_hwpoison_page() fails for any folio that is off the LRU. Normally
> memory_failure() would split the folio and isolate the bad page (for
> an uncorrected error there is nothing to migrate; that is soft
> offline). Here it does neither, and the folio stays in the swap cache
> with only the subpage marked.

Yeah this just seems like the swapcache code being broken in general.

>
> While looking at this I also noticed that the fallback drops a freshly
> read (not yet anon) folio from the swap cache and frees it with the
> poisoned page in it. We should actually actually also check folio_test_anon()
> before checking folio_has_any_hwpoisoned_page().

Ugh lord, how much crap is handled improperly re: hw poisoning. Such a mess...

--
Cheers, Lorenzo

Re: [PATCH 1/3] mm: don't PTE-batch a swap-in over a hardware-poisoned subpage

[David Hildenbrand (Arm)]

On 11/11/26 14:18, Lorenzo Stoakes (ARM) wrote:
> On Wed, Nov 11, 2026 at 03:38:29AM -0800, Usama Arif wrote:
>> do_swap_page() checks PageHWPoison() on the faulting page, but the
>> large-folio batching path then maps the rest of the folio without looking
>> at the other subpages. A poisoned subpage that is not the one being
>> faulted gets mapped anyway, and the fault that would have reported it
>> never happens.
>>
>> Skip batching for such a folio and let each page take its own fault,
>> where the existing check catches it.
>>
>> Scan the subpages rather than testing the folio-level PG_has_hwpoisoned:
>> memory_failure() sets PageHWPoison() on the subpage first and only sets
>> the folio flag much later, and the bail-outs in between - for example
>> get_hwpoison_page() losing a page lifecycle race - return with the
>> subpage marked and the folio flag never set.
>>
>> Fixes: 508758960b8d ("mm: swap: entirely map large folios found in swapcache")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Usama Arif <usama.arif@linux.dev>
>> ---
>>  mm/internal.h | 15 +++++++++++++++
>>  mm/memory.c   |  4 ++++
>>  2 files changed, 19 insertions(+)
>>
>> diff --git a/mm/internal.h b/mm/internal.h
>> index cd3675c044e0a..e61c02f4019b0 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -28,6 +28,21 @@ struct folio_batch;
>>  int __copy_remote_mm_str(struct mm_struct *mm, unsigned long addr,
>>  		void *buf, int len, unsigned int gup_flags);
>>
>> +/*
>> + * Check every page of the folio for HWPoison, rather than relying on the
>> + * folio-level PG_has_hwpoisoned, which memory_failure() can leave clear.
>> + */
>> +static inline bool folio_has_any_hwpoisoned_page(const struct folio *folio)
>> +{
>> +	long nr = folio_nr_pages(folio);
>> +	long i;
>> +
>> +	for (i = 0; i < nr; i++)
>> +		if (PageHWPoison(folio_page(folio, i)))
>> +			return true;
>> +	return false;
>> +}
>> +
>>  /* mm/workingset.c */
>>  bool workingset_test_recent(void *shadow, bool file, bool *workingset,
>>  			    bool flush);
>> diff --git a/mm/memory.c b/mm/memory.c
>> index d477a4f986bc8..4b7d456332200 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -5158,6 +5158,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>  		    swap_pte_batch(folio_ptep, nr, folio_pte) != nr)
>>  			goto check_folio;
>>
>> +		/* Let a poisoned subpage take its own fault. */
>> +		if (folio_has_any_hwpoisoned_page(folio))
>> +			goto check_folio;
>> +
> 
> Hm, isn't scanning every single page in the folio potentially quite costly
> here?  I thought the point of batching was to improve performance overall?

Yeah, we don't really want another helper to check for hwpoison.

If we race with concurrent hwpoison, bad luck. The slapped on memory-failure
code is full of races. That needs a proper long-term solution, not more pain to
core-mm.

So let's please try reusing existing helpers that test the large-folio bit.

-- 
Cheers,

David

Re: [PATCH 1/3] mm: don't PTE-batch a swap-in over a hardware-poisoned subpage

[Lorenzo Stoakes (ARM)]

On Thu, Nov 12, 2026 at 09:56:39AM +0100, David Hildenbrand (Arm) wrote:
> On 11/11/26 14:18, Lorenzo Stoakes (ARM) wrote:
> > Hm, isn't scanning every single page in the folio potentially quite costly
> > here?  I thought the point of batching was to improve performance overall?
>
> Yeah, we don't really want another helper to check for hwpoison.
>
> If we race with concurrent hwpoison, bad luck. The slapped on memory-failure
> code is full of races. That needs a proper long-term solution, not more pain to
> core-mm.

Yeah exactly. See Usama's reply - it really does narrow into a weird edge
case. Scan there if you want to scan at all.

Otherwise tough luck yes.

>
> So let's please try reusing existing helpers that test the large-folio bit.

Yes please!

--
Cheers, Lorenzo

Re: [PATCH 1/3] mm: don't PTE-batch a swap-in over a hardware-poisoned subpage

[Zi Yan]

On 11 Nov 2026, at 6:38, Usama Arif wrote:

> do_swap_page() checks PageHWPoison() on the faulting page, but the
> large-folio batching path then maps the rest of the folio without looking
> at the other subpages. A poisoned subpage that is not the one being
> faulted gets mapped anyway, and the fault that would have reported it
> never happens.
>
> Skip batching for such a folio and let each page take its own fault,
> where the existing check catches it.
>
> Scan the subpages rather than testing the folio-level PG_has_hwpoisoned:
> memory_failure() sets PageHWPoison() on the subpage first and only sets
> the folio flag much later, and the bail-outs in between - for example
> get_hwpoison_page() losing a page lifecycle race - return with the
> subpage marked and the folio flag never set.
>
> Fixes: 508758960b8d ("mm: swap: entirely map large folios found in swapcache")
> Cc: stable@vger.kernel.org
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---
>  mm/internal.h | 15 +++++++++++++++
>  mm/memory.c   |  4 ++++
>  2 files changed, 19 insertions(+)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index cd3675c044e0a..e61c02f4019b0 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -28,6 +28,21 @@ struct folio_batch;
>  int __copy_remote_mm_str(struct mm_struct *mm, unsigned long addr,
>  		void *buf, int len, unsigned int gup_flags);
>
> +/*
> + * Check every page of the folio for HWPoison, rather than relying on the
> + * folio-level PG_has_hwpoisoned, which memory_failure() can leave clear.
> + */
> +static inline bool folio_has_any_hwpoisoned_page(const struct folio *folio)
> +{
> +	long nr = folio_nr_pages(folio);
> +	long i;
> +
> +	for (i = 0; i < nr; i++)
> +		if (PageHWPoison(folio_page(folio, i)))
> +			return true;
> +	return false;
> +}

It looks almost the same as page_range_has_hwpoisoned() in mm/huge_memory.c.
Maybe change the function to call page_range_has_hwpoisoned() instead.

> +
>  /* mm/workingset.c */
>  bool workingset_test_recent(void *shadow, bool file, bool *workingset,
>  			    bool flush);
> diff --git a/mm/memory.c b/mm/memory.c
> index d477a4f986bc8..4b7d456332200 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5158,6 +5158,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  		    swap_pte_batch(folio_ptep, nr, folio_pte) != nr)
>  			goto check_folio;
>
> +		/* Let a poisoned subpage take its own fault. */
> +		if (folio_has_any_hwpoisoned_page(folio))
> +			goto check_folio;
> +
>  		page_idx = idx;
>  		address = folio_start;
>  		ptep = folio_ptep;
> -- 
> 2.53.0-Meta


Best Regards,
Yan, Zi

[PATCH 2/3] fs/proc/task_mmu: don't read young/dirty from a non-present PMD in smaps

[Usama Arif]

smaps_pmd_entry() passes pmd_young(*pmd) and pmd_dirty(*pmd) to
smaps_account() for every PMD, including migration and device-private
entries, where those bits hold part of the swap offset rather than
accessed/dirty state. smaps_pte_entry() only reads them when the entry
is present.

On x86_64 the offset is stored inverted across bits 9-58, which overlap
_PAGE_SAVED_DIRTY, so pmd_dirty() returns garbage: a clean 2MB
device-private mapping is accounted as 2048 kB dirty, and lazyfree
accounting is suppressed. pgtable_64.h also warns that the hardware can
set the accessed and dirty bits on non-present entries by mistake.

Only read the bits when the PMD is present.

Fixes: c94b6923fa0a ("/proc/PID/smaps: Add PMD migration entry parsing")
Cc: stable@vger.kernel.org
Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 fs/proc/task_mmu.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 22361607020d1..c0d391186c2b8 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1019,13 +1019,15 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
 	struct vm_area_struct *vma = walk->vma;
 	bool locked = !!(vma->vm_flags & VM_LOCKED);
 	struct page *page = NULL;
-	bool present = false;
+	bool present = false, young = false, dirty = false;
 	struct folio *folio;
 
 	if (pmd_none(*pmd))
 		return;
 	if (pmd_present(*pmd)) {
 		page = vm_normal_page_pmd(vma, addr, *pmd);
+		young = pmd_young(*pmd);
+		dirty = pmd_dirty(*pmd);
 		present = true;
 	} else if (unlikely(thp_migration_supported())) {
 		const softleaf_t entry = softleaf_from_pmd(*pmd);
@@ -1045,8 +1047,7 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
 	else
 		mss->file_thp += HPAGE_PMD_SIZE;
 
-	smaps_account(mss, page, true, pmd_young(*pmd), pmd_dirty(*pmd),
-		      locked, present);
+	smaps_account(mss, page, true, young, dirty, locked, present);
 }
 #else
 static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
-- 
2.53.0-Meta

Re: [PATCH 2/3] fs/proc/task_mmu: don't read young/dirty from a non-present PMD in smaps

[Lorenzo Stoakes (ARM)]

On Wed, Nov 11, 2026 at 03:38:30AM -0800, Usama Arif wrote:
> smaps_pmd_entry() passes pmd_young(*pmd) and pmd_dirty(*pmd) to
> smaps_account() for every PMD, including migration and device-private
> entries, where those bits hold part of the swap offset rather than
> accessed/dirty state. smaps_pte_entry() only reads them when the entry
> is present.

Could say something something softleaf here ;)

>
> On x86_64 the offset is stored inverted across bits 9-58, which overlap
> _PAGE_SAVED_DIRTY, so pmd_dirty() returns garbage: a clean 2MB
> device-private mapping is accounted as 2048 kB dirty, and lazyfree
> accounting is suppressed. pgtable_64.h also warns that the hardware can
> set the accessed and dirty bits on non-present entries by mistake.
>
> Only read the bits when the PMD is present.
>
> Fixes: c94b6923fa0a ("/proc/PID/smaps: Add PMD migration entry parsing")
> Cc: stable@vger.kernel.org
> Signed-off-by: Usama Arif <usama.arif@linux.dev>

Damn, yikes. LGTM so:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  fs/proc/task_mmu.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 22361607020d1..c0d391186c2b8 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1019,13 +1019,15 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
>  	struct vm_area_struct *vma = walk->vma;
>  	bool locked = !!(vma->vm_flags & VM_LOCKED);
>  	struct page *page = NULL;
> -	bool present = false;
> +	bool present = false, young = false, dirty = false;
>  	struct folio *folio;
>
>  	if (pmd_none(*pmd))
>  		return;
>  	if (pmd_present(*pmd)) {
>  		page = vm_normal_page_pmd(vma, addr, *pmd);
> +		young = pmd_young(*pmd);
> +		dirty = pmd_dirty(*pmd);
>  		present = true;
>  	} else if (unlikely(thp_migration_supported())) {
>  		const softleaf_t entry = softleaf_from_pmd(*pmd);
> @@ -1045,8 +1047,7 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
>  	else
>  		mss->file_thp += HPAGE_PMD_SIZE;
>
> -	smaps_account(mss, page, true, pmd_young(*pmd), pmd_dirty(*pmd),
> -		      locked, present);
> +	smaps_account(mss, page, true, young, dirty, locked, present);
>  }
>  #else
>  static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
> --
> 2.53.0-Meta
>

--
Cheers, Lorenzo

Re: [PATCH 2/3] fs/proc/task_mmu: don't read young/dirty from a non-present PMD in smaps

[Baolin Wang]

On 11/11/26 7:38 PM, Usama Arif wrote:
> smaps_pmd_entry() passes pmd_young(*pmd) and pmd_dirty(*pmd) to
> smaps_account() for every PMD, including migration and device-private
> entries, where those bits hold part of the swap offset rather than
> accessed/dirty state. smaps_pte_entry() only reads them when the entry
> is present.
> 
> On x86_64 the offset is stored inverted across bits 9-58, which overlap
> _PAGE_SAVED_DIRTY, so pmd_dirty() returns garbage: a clean 2MB
> device-private mapping is accounted as 2048 kB dirty, and lazyfree
> accounting is suppressed. pgtable_64.h also warns that the hardware can
> set the accessed and dirty bits on non-present entries by mistake.
> 
> Only read the bits when the PMD is present.
> 
> Fixes: c94b6923fa0a ("/proc/PID/smaps: Add PMD migration entry parsing")
> Cc: stable@vger.kernel.org
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---

Make sense to me.
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>

Re: [PATCH 2/3] fs/proc/task_mmu: don't read young/dirty from a non-present PMD in smaps

[David Hildenbrand (Arm)]

On 11/11/26 12:38, Usama Arif wrote:
> smaps_pmd_entry() passes pmd_young(*pmd) and pmd_dirty(*pmd) to
> smaps_account() for every PMD, including migration and device-private
> entries, where those bits hold part of the swap offset rather than
> accessed/dirty state. smaps_pte_entry() only reads them when the entry
> is present.
> 
> On x86_64 the offset is stored inverted across bits 9-58, which overlap
> _PAGE_SAVED_DIRTY, so pmd_dirty() returns garbage: a clean 2MB
> device-private mapping is accounted as 2048 kB dirty, and lazyfree
> accounting is suppressed. pgtable_64.h also warns that the hardware can
> set the accessed and dirty bits on non-present entries by mistake.
> 
> Only read the bits when the PMD is present.
> 
> Fixes: c94b6923fa0a ("/proc/PID/smaps: Add PMD migration entry parsing")
> Cc: stable@vger.kernel.org
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---

Yes, thanks!

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David

[PATCH 3/3] fs/proc/task_mmu: account device-private PMDs as present in smaps

[Usama Arif]

Device-private PMDs represent mapped pages, but smaps_pmd_entry()
passes present=false to smaps_account(). That treats them like migration
entries: mapcounts are ignored, PSS is charged in full, and exclusive
mappings are accounted as shared.

Match smaps_pte_entry() and mark device-private PMDs as present so
smaps_account() uses mapcounts for PSS and private/shared accounting.

Fixes: a30b48bf1b24 ("mm/migrate_device: implement THP migration of zone device pages")
Cc: stable@vger.kernel.org
Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 fs/proc/task_mmu.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index c0d391186c2b8..ad97532fc1184 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1032,8 +1032,11 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
 	} else if (unlikely(thp_migration_supported())) {
 		const softleaf_t entry = softleaf_from_pmd(*pmd);
 
-		if (softleaf_has_pfn(entry))
+		if (softleaf_has_pfn(entry)) {
+			if (softleaf_is_device_private(entry))
+				present = true;
 			page = softleaf_to_page(entry);
+		}
 	}
 	if (IS_ERR_OR_NULL(page))
 		return;
-- 
2.53.0-Meta

Re: [PATCH 3/3] fs/proc/task_mmu: account device-private PMDs as present in smaps

[Lorenzo Stoakes (ARM)]

On Wed, Nov 11, 2026 at 03:38:31AM -0800, Usama Arif wrote:
> Device-private PMDs represent mapped pages, but smaps_pmd_entry()
> passes present=false to smaps_account(). That treats them like migration
> entries: mapcounts are ignored, PSS is charged in full, and exclusive
> mappings are accounted as shared.
>
> Match smaps_pte_entry() and mark device-private PMDs as present so
> smaps_account() uses mapcounts for PSS and private/shared accounting.
>
> Fixes: a30b48bf1b24 ("mm/migrate_device: implement THP migration of zone device pages")
> Cc: stable@vger.kernel.org
> Signed-off-by: Usama Arif <usama.arif@linux.dev>

It feels like PMD level device private entries have been the source of a lot of
issues, maybe a good area to audit in general... :)

Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  fs/proc/task_mmu.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index c0d391186c2b8..ad97532fc1184 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1032,8 +1032,11 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
>  	} else if (unlikely(thp_migration_supported())) {
>  		const softleaf_t entry = softleaf_from_pmd(*pmd);
>
> -		if (softleaf_has_pfn(entry))
> +		if (softleaf_has_pfn(entry)) {
> +			if (softleaf_is_device_private(entry))
> +				present = true;
>  			page = softleaf_to_page(entry);
> +		}
>  	}
>  	if (IS_ERR_OR_NULL(page))
>  		return;
> --
> 2.53.0-Meta
>

--
Cheers, Lorenzo

Re: [PATCH 3/3] fs/proc/task_mmu: account device-private PMDs as present in smaps

[David Hildenbrand (Arm)]

On 11/11/26 12:38, Usama Arif wrote:
> Device-private PMDs represent mapped pages, but smaps_pmd_entry()
> passes present=false to smaps_account(). That treats them like migration
> entries: mapcounts are ignored, PSS is charged in full, and exclusive
> mappings are accounted as shared.
> 
> Match smaps_pte_entry() and mark device-private PMDs as present so
> smaps_account() uses mapcounts for PSS and private/shared accounting.
> 
> Fixes: a30b48bf1b24 ("mm/migrate_device: implement THP migration of zone device pages")
> Cc: stable@vger.kernel.org
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---
>  fs/proc/task_mmu.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index c0d391186c2b8..ad97532fc1184 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1032,8 +1032,11 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
>  	} else if (unlikely(thp_migration_supported())) {
>  		const softleaf_t entry = softleaf_from_pmd(*pmd);
>  
> -		if (softleaf_has_pfn(entry))
> +		if (softleaf_has_pfn(entry)) {
> +			if (softleaf_is_device_private(entry))
> +				present = true;
>  			page = softleaf_to_page(entry);
> +		}
>  	}

I think I even spelled that out somewhere at some point while touching that code
... or did I fix the smaps_pte_entry() thing at some point?

"present" is a bit misleading in general, because it's not pte_present() ...

Matches smaps_pte_entry()

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David