[PATCH v3 0/4] mm: introduce snapshot_page()

[PATCH v3 0/4] mm: introduce snapshot_page()

[Luiz Capitulino]

Hi,

The series introduction will follow the changelog. This is against bc9ff192a6c9 .

Changelog
=========

v2 -> v3
  - Small snapshot_page() refactor to simplify and fix stack-out-of-bugs bug
    reported by Harry Yoo
  - Use clear_compound_head() in snapshot_page() (Shivank)
  - Read head pointer from ps->page_snapshot() for tail page case (Shivank)
  - Improve comments and macro name in snapshot_page() (David)

v1 -> v2
  - Include is_huge_zero_pfn() patch and use it (David)
  - Move free page detection to snapshot_page() (David)
  - Changelog improvements (Shivank)
  - Added Acked-bys

RFC -> v1
  - Include <linux/page_idle.h> to avoid build error on sh arch

Introduction
============

This series introduces snapshot_page(), a helper function that can be used
to create a snapshot of a struct page and its associated struct folio.

This function is intended to help callers with a consistent view of a
a folio while reducing the chance of encountering partially updated or
inconsistent state, such as during folio splitting which could lead to
crashes and BUG_ON()s being triggered.

David Hildenbrand (1):
  mm/memory: introduce is_huge_zero_pfn() and use it in
    vm_normal_page_pmd()

Luiz Capitulino (3):
  mm/util: introduce snapshot_page()
  proc: kpagecount: use snapshot_page()
  fs: stable_page_flags(): use snapshot_page()

 fs/proc/page.c          | 50 +++++++++++++++-----------
 include/linux/huge_mm.h | 12 ++++++-
 include/linux/mm.h      | 19 ++++++++++
 mm/debug.c              | 42 +++-------------------
 mm/memory.c             |  2 +-
 mm/util.c               | 79 +++++++++++++++++++++++++++++++++++++++++
 6 files changed, 144 insertions(+), 60 deletions(-)

-- 
2.50.0

[PATCH v3 1/4] mm/memory: introduce is_huge_zero_pfn() and use it in vm_normal_page_pmd()

[Luiz Capitulino]

From: David Hildenbrand <david@redhat.com>

Let's avoid working with the PMD when not required. If
vm_normal_page_pmd() would be called on something that is not a present
pmd, it would already be a bug (pfn possibly garbage).

While at it, let's support passing in any pfn covered by the huge zero
folio by masking off PFN bits -- which should be rather cheap.

Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
---
 include/linux/huge_mm.h | 12 +++++++++++-
 mm/memory.c             |  2 +-
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 2f190c90192d..59e93fba15f4 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -486,9 +486,14 @@ static inline bool is_huge_zero_folio(const struct folio *folio)
 	return READ_ONCE(huge_zero_folio) == folio;
 }
 
+static inline bool is_huge_zero_pfn(unsigned long pfn)
+{
+	return READ_ONCE(huge_zero_pfn) == (pfn & ~(HPAGE_PMD_NR - 1));
+}
+
 static inline bool is_huge_zero_pmd(pmd_t pmd)
 {
-	return pmd_present(pmd) && READ_ONCE(huge_zero_pfn) == pmd_pfn(pmd);
+	return pmd_present(pmd) && is_huge_zero_pfn(pmd_pfn(pmd));
 }
 
 struct folio *mm_get_huge_zero_folio(struct mm_struct *mm);
@@ -636,6 +641,11 @@ static inline bool is_huge_zero_folio(const struct folio *folio)
 	return false;
 }
 
+static inline bool is_huge_zero_pfn(unsigned long pfn)
+{
+	return false;
+}
+
 static inline bool is_huge_zero_pmd(pmd_t pmd)
 {
 	return false;
diff --git a/mm/memory.c b/mm/memory.c
index b0cda5aab398..3a765553bacb 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -687,7 +687,7 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
 
 	if (pmd_devmap(pmd))
 		return NULL;
-	if (is_huge_zero_pmd(pmd))
+	if (is_huge_zero_pfn(pfn))
 		return NULL;
 	if (unlikely(pfn > highest_memmap_pfn))
 		return NULL;
-- 
2.50.0

Re: [PATCH v3 1/4] mm/memory: introduce is_huge_zero_pfn() and use it in vm_normal_page_pmd()

[David Hildenbrand]

On 14.07.25 15:16, Luiz Capitulino wrote:
> From: David Hildenbrand <david@redhat.com>
> 
> Let's avoid working with the PMD when not required. If
> vm_normal_page_pmd() would be called on something that is not a present
> pmd, it would already be a bug (pfn possibly garbage).
> 
> While at it, let's support passing in any pfn covered by the huge zero
> folio by masking off PFN bits -- which should be rather cheap.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Reviewed-by: Oscar Salvador <osalvador@suse.de>
> Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
> ---
>   include/linux/huge_mm.h | 12 +++++++++++-
>   mm/memory.c             |  2 +-
>   2 files changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 2f190c90192d..59e93fba15f4 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -486,9 +486,14 @@ static inline bool is_huge_zero_folio(const struct folio *folio)
>   	return READ_ONCE(huge_zero_folio) == folio;
>   }
>   
> +static inline bool is_huge_zero_pfn(unsigned long pfn)
> +{
> +	return READ_ONCE(huge_zero_pfn) == (pfn & ~(HPAGE_PMD_NR - 1));
> +}
> +
>   static inline bool is_huge_zero_pmd(pmd_t pmd)
>   {
> -	return pmd_present(pmd) && READ_ONCE(huge_zero_pfn) == pmd_pfn(pmd);
> +	return pmd_present(pmd) && is_huge_zero_pfn(pmd_pfn(pmd));
>   }
>   
>   struct folio *mm_get_huge_zero_folio(struct mm_struct *mm);
> @@ -636,6 +641,11 @@ static inline bool is_huge_zero_folio(const struct folio *folio)
>   	return false;
>   }
>   
> +static inline bool is_huge_zero_pfn(unsigned long pfn)
> +{
> +	return false;
> +}
> +
>   static inline bool is_huge_zero_pmd(pmd_t pmd)
>   {
>   	return false;
> diff --git a/mm/memory.c b/mm/memory.c
> index b0cda5aab398..3a765553bacb 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -687,7 +687,7 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
>   
>   	if (pmd_devmap(pmd))
>   		return NULL;

This likely doesn't apply as-is on top of mm-unstable / mm-new (and 
likely also not mm-stable).

Should be trivial to fixup, though.

-- 
Cheers,

David / dhildenb

[PATCH v3 2/4] mm/util: introduce snapshot_page()

[Luiz Capitulino]

This commit refactors __dump_page() into snapshot_page().

snapshot_page() tries to take a faithful snapshot of a page and its
folio representation. The snapshot is returned in the struct
page_snapshot parameter along with additional flags that are best
retrieved at snapshot creation time to reduce race windows.

This function is intended to be used by callers that need a stable
representation of a struct page and struct folio so that pointers
or page information doesn't change while working on a page.

The idea and original implementation of snapshot_page() comes from
Matthew Wilcox with suggestions for improvements from David Hildenbrand.
All bugs and misconceptions are mine.

Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
---
 include/linux/mm.h | 19 +++++++++++
 mm/debug.c         | 42 +++---------------------
 mm/util.c          | 79 ++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 102 insertions(+), 38 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 0ef2ba0c667a..929e2330af13 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -4184,4 +4184,23 @@ static inline bool page_pool_page_is_pp(struct page *page)
 }
 #endif
 
+#define PAGE_SNAPSHOT_FAITHFUL (1 << 0)
+#define PAGE_SNAPSHOT_PG_BUDDY (1 << 1)
+#define PAGE_SNAPSHOT_PG_IDLE  (1 << 2)
+
+struct page_snapshot {
+	struct folio folio_snapshot;
+	struct page page_snapshot;
+	unsigned long pfn;
+	unsigned long idx;
+	unsigned long flags;
+};
+
+static inline bool snapshot_page_is_faithful(const struct page_snapshot *ps)
+{
+	return ps->flags & PAGE_SNAPSHOT_FAITHFUL;
+}
+
+void snapshot_page(struct page_snapshot *ps, const struct page *page);
+
 #endif /* _LINUX_MM_H */
diff --git a/mm/debug.c b/mm/debug.c
index 907382257062..7349330ea506 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -129,47 +129,13 @@ static void __dump_folio(struct folio *folio, struct page *page,
 
 static void __dump_page(const struct page *page)
 {
-	struct folio *foliop, folio;
-	struct page precise;
-	unsigned long head;
-	unsigned long pfn = page_to_pfn(page);
-	unsigned long idx, nr_pages = 1;
-	int loops = 5;
-
-again:
-	memcpy(&precise, page, sizeof(*page));
-	head = precise.compound_head;
-	if ((head & 1) == 0) {
-		foliop = (struct folio *)&precise;
-		idx = 0;
-		if (!folio_test_large(foliop))
-			goto dump;
-		foliop = (struct folio *)page;
-	} else {
-		foliop = (struct folio *)(head - 1);
-		idx = folio_page_idx(foliop, page);
-	}
+	struct page_snapshot ps;
 
-	if (idx < MAX_FOLIO_NR_PAGES) {
-		memcpy(&folio, foliop, 2 * sizeof(struct page));
-		nr_pages = folio_nr_pages(&folio);
-		if (nr_pages > 1)
-			memcpy(&folio.__page_2, &foliop->__page_2,
-			       sizeof(struct page));
-		foliop = &folio;
-	}
-
-	if (idx > nr_pages) {
-		if (loops-- > 0)
-			goto again;
+	snapshot_page(&ps, page);
+	if (!snapshot_page_is_faithful(&ps))
 		pr_warn("page does not match folio\n");
-		precise.compound_head &= ~1UL;
-		foliop = (struct folio *)&precise;
-		idx = 0;
-	}
 
-dump:
-	__dump_folio(foliop, &precise, pfn, idx);
+	__dump_folio(&ps.folio_snapshot, &ps.page_snapshot, ps.pfn, ps.idx);
 }
 
 void dump_page(const struct page *page, const char *reason)
diff --git a/mm/util.c b/mm/util.c
index 0b270c43d7d1..f270bf42465b 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -25,6 +25,7 @@
 #include <linux/sizes.h>
 #include <linux/compat.h>
 #include <linux/fsnotify.h>
+#include <linux/page_idle.h>
 
 #include <linux/uaccess.h>
 
@@ -1171,3 +1172,81 @@ int compat_vma_mmap_prepare(struct file *file, struct vm_area_struct *vma)
 	return 0;
 }
 EXPORT_SYMBOL(compat_vma_mmap_prepare);
+
+static void set_ps_flags(struct page_snapshot *ps, const struct folio *folio,
+			 const struct page *page)
+{
+	/*
+	 * Only the first page of a high-order buddy page has PageBuddy() set.
+	 * So we have to check manually whether this page is part of a high-
+	 * order buddy page.
+	 */
+	if (PageBuddy(page))
+		ps->flags |= PAGE_SNAPSHOT_PG_BUDDY;
+	else if (page_count(page) == 0 && is_free_buddy_page(page))
+		ps->flags |= PAGE_SNAPSHOT_PG_BUDDY;
+
+	if (folio_test_idle(folio))
+		ps->flags |= PAGE_SNAPSHOT_PG_IDLE;
+}
+
+/**
+ * snapshot_page() - Create a snapshot of a struct page
+ * @ps: Pointer to a struct page_snapshot to store the page snapshot
+ * @page: The page to snapshot
+ *
+ * Create a snapshot of the page and store both its struct page and struct
+ * folio representations in @ps.
+ *
+ * Note that creating a faithful snapshot may fail if the compound
+ * state of the page keeps changing (e.g., due to a folio split). In
+ * this case, ps->faithful is set to false, and the snapshot assumes
+ * that @page refers to a single page.
+ */
+void snapshot_page(struct page_snapshot *ps, const struct page *page)
+{
+	unsigned long head, nr_pages = 1;
+	struct folio *foliop;
+	int loops = 5;
+
+	ps->pfn = page_to_pfn(page);
+	ps->flags = PAGE_SNAPSHOT_FAITHFUL;
+
+again:
+	memset(&ps->folio_snapshot, 0, sizeof(struct folio));
+	memcpy(&ps->page_snapshot, page, sizeof(*page));
+	head = ps->page_snapshot.compound_head;
+	if ((head & 1) == 0) {
+		ps->idx = 0;
+		foliop = (struct folio *)&ps->page_snapshot;
+		if (!folio_test_large(foliop)) {
+			set_ps_flags(ps, page_folio(page), page);
+			memcpy(&ps->folio_snapshot, foliop,
+			       sizeof(struct page));
+			return;
+		}
+		foliop = (struct folio *)page;
+	} else {
+		foliop = (struct folio *)(head - 1);
+		ps->idx = folio_page_idx(foliop, page);
+	}
+
+	if (ps->idx < MAX_FOLIO_NR_PAGES) {
+		memcpy(&ps->folio_snapshot, foliop, 2 * sizeof(struct page));
+		nr_pages = folio_nr_pages(&ps->folio_snapshot);
+		if (nr_pages > 1)
+			memcpy(&ps->folio_snapshot.__page_2, &foliop->__page_2,
+			       sizeof(struct page));
+		set_ps_flags(ps, foliop, page);
+	}
+
+	if (ps->idx > nr_pages) {
+		if (loops-- > 0)
+			goto again;
+		clear_compound_head(&ps->page_snapshot);
+		foliop = (struct folio *)&ps->page_snapshot;
+		memcpy(&ps->folio_snapshot, foliop, sizeof(struct page));
+		ps->flags = 0;
+		ps->idx = 0;
+	}
+}
-- 
2.50.0

Re: [PATCH v3 2/4] mm/util: introduce snapshot_page()

[David Hildenbrand]

[...]

>   
> -dump:
> -	__dump_folio(foliop, &precise, pfn, idx);
> +	__dump_folio(&ps.folio_snapshot, &ps.page_snapshot, ps.pfn, ps.idx);

Nit that can be cleaned up later on top:

We should probably call this

__dump_page_snapshot() and then just pass ... the page_snapshot.

>   }
>   
>   void dump_page(const struct page *page, const char *reason)
> diff --git a/mm/util.c b/mm/util.c
> index 0b270c43d7d1..f270bf42465b 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -25,6 +25,7 @@
>   #include <linux/sizes.h>
>   #include <linux/compat.h>
>   #include <linux/fsnotify.h>
> +#include <linux/page_idle.h>
>   
>   #include <linux/uaccess.h>
>   
> @@ -1171,3 +1172,81 @@ int compat_vma_mmap_prepare(struct file *file, struct vm_area_struct *vma)
>   	return 0;
>   }
>   EXPORT_SYMBOL(compat_vma_mmap_prepare);
> +
> +static void set_ps_flags(struct page_snapshot *ps, const struct folio *folio,
> +			 const struct page *page)
> +{
> +	/*
> +	 * Only the first page of a high-order buddy page has PageBuddy() set.
> +	 * So we have to check manually whether this page is part of a high-
> +	 * order buddy page.
> +	 */
> +	if (PageBuddy(page))
> +		ps->flags |= PAGE_SNAPSHOT_PG_BUDDY;
> +	else if (page_count(page) == 0 && is_free_buddy_page(page))
> +		ps->flags |= PAGE_SNAPSHOT_PG_BUDDY;
> +
> +	if (folio_test_idle(folio))
> +		ps->flags |= PAGE_SNAPSHOT_PG_IDLE;
> +}
> +
> +/**
> + * snapshot_page() - Create a snapshot of a struct page
> + * @ps: Pointer to a struct page_snapshot to store the page snapshot
> + * @page: The page to snapshot
> + *
> + * Create a snapshot of the page and store both its struct page and struct
> + * folio representations in @ps.
> + *
> + * Note that creating a faithful snapshot may fail if the compound

Maybe highlight that this is not really expected to happen, ever.

> + * state of the page keeps changing (e.g., due to a folio split). In
> + * this case, ps->faithful is set to false, and the snapshot assumes

There is no ps->faithful.

> + * that @page refers to a single page.
> + */
> +void snapshot_page(struct page_snapshot *ps, const struct page *page)
> +{
> +	unsigned long head, nr_pages = 1;
> +	struct folio *foliop;
> +	int loops = 5;
> +
> +	ps->pfn = page_to_pfn(page);
> +	ps->flags = PAGE_SNAPSHOT_FAITHFUL;
> +
> +again:
> +	memset(&ps->folio_snapshot, 0, sizeof(struct folio));
> +	memcpy(&ps->page_snapshot, page, sizeof(*page));
> +	head = ps->page_snapshot.compound_head;
> +	if ((head & 1) == 0) {
> +		ps->idx = 0;
> +		foliop = (struct folio *)&ps->page_snapshot;
> +		if (!folio_test_large(foliop)) {
> +			set_ps_flags(ps, page_folio(page), page);
> +			memcpy(&ps->folio_snapshot, foliop,
> +			       sizeof(struct page));
> +			return;
> +		}
> +		foliop = (struct folio *)page;
> +	} else {
> +		foliop = (struct folio *)(head - 1);
> +		ps->idx = folio_page_idx(foliop, page);
> +	}

Condition could be cleaned up by reversing both things

if (head & 1) {
	/* Tail page, lookup the actual head. */
	foliop = (struct folio *)(head - 1);
	ps->idx = folio_page_idx(foliop, page);
} else
	...
}

But you're just moving that code, so no need to do that now.


I think we could improve some of that in the future a bit to
make it even more faithful.

But for now this should be just fine.

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb

Re: [PATCH v3 2/4] mm/util: introduce snapshot_page()

[Luiz Capitulino]

On 2025-07-16 06:16, David Hildenbrand wrote:
> [...]
> 
>> -dump:
>> -    __dump_folio(foliop, &precise, pfn, idx);
>> +    __dump_folio(&ps.folio_snapshot, &ps.page_snapshot, ps.pfn, ps.idx);
> 
> Nit that can be cleaned up later on top:
> 
> We should probably call this
> 
> __dump_page_snapshot() and then just pass ... the page_snapshot.
> 
>>   }
>>   void dump_page(const struct page *page, const char *reason)
>> diff --git a/mm/util.c b/mm/util.c
>> index 0b270c43d7d1..f270bf42465b 100644
>> --- a/mm/util.c
>> +++ b/mm/util.c
>> @@ -25,6 +25,7 @@
>>   #include <linux/sizes.h>
>>   #include <linux/compat.h>
>>   #include <linux/fsnotify.h>
>> +#include <linux/page_idle.h>
>>   #include <linux/uaccess.h>
>> @@ -1171,3 +1172,81 @@ int compat_vma_mmap_prepare(struct file *file, struct vm_area_struct *vma)
>>       return 0;
>>   }
>>   EXPORT_SYMBOL(compat_vma_mmap_prepare);
>> +
>> +static void set_ps_flags(struct page_snapshot *ps, const struct folio *folio,
>> +             const struct page *page)
>> +{
>> +    /*
>> +     * Only the first page of a high-order buddy page has PageBuddy() set.
>> +     * So we have to check manually whether this page is part of a high-
>> +     * order buddy page.
>> +     */
>> +    if (PageBuddy(page))
>> +        ps->flags |= PAGE_SNAPSHOT_PG_BUDDY;
>> +    else if (page_count(page) == 0 && is_free_buddy_page(page))
>> +        ps->flags |= PAGE_SNAPSHOT_PG_BUDDY;
>> +
>> +    if (folio_test_idle(folio))
>> +        ps->flags |= PAGE_SNAPSHOT_PG_IDLE;
>> +}
>> +
>> +/**
>> + * snapshot_page() - Create a snapshot of a struct page
>> + * @ps: Pointer to a struct page_snapshot to store the page snapshot
>> + * @page: The page to snapshot
>> + *
>> + * Create a snapshot of the page and store both its struct page and struct
>> + * folio representations in @ps.
>> + *
>> + * Note that creating a faithful snapshot may fail if the compound
> 
> Maybe highlight that this is not really expected to happen, ever.
> 
>> + * state of the page keeps changing (e.g., due to a folio split). In
>> + * this case, ps->faithful is set to false, and the snapshot assumes
> 
> There is no ps->faithful.

Yes, good catch. This was from an earlier version.

Is it fine if I fix only this with a follow up patch for Andrew in this
thread or would you prefer that I post v4 with all the other changes as
well?

> 
>> + * that @page refers to a single page.
>> + */
>> +void snapshot_page(struct page_snapshot *ps, const struct page *page)
>> +{
>> +    unsigned long head, nr_pages = 1;
>> +    struct folio *foliop;
>> +    int loops = 5;
>> +
>> +    ps->pfn = page_to_pfn(page);
>> +    ps->flags = PAGE_SNAPSHOT_FAITHFUL;
>> +
>> +again:
>> +    memset(&ps->folio_snapshot, 0, sizeof(struct folio));
>> +    memcpy(&ps->page_snapshot, page, sizeof(*page));
>> +    head = ps->page_snapshot.compound_head;
>> +    if ((head & 1) == 0) {
>> +        ps->idx = 0;
>> +        foliop = (struct folio *)&ps->page_snapshot;
>> +        if (!folio_test_large(foliop)) {
>> +            set_ps_flags(ps, page_folio(page), page);
>> +            memcpy(&ps->folio_snapshot, foliop,
>> +                   sizeof(struct page));
>> +            return;
>> +        }
>> +        foliop = (struct folio *)page;
>> +    } else {
>> +        foliop = (struct folio *)(head - 1);
>> +        ps->idx = folio_page_idx(foliop, page);
>> +    }
> 
> Condition could be cleaned up by reversing both things
> 
> if (head & 1) {
>      /* Tail page, lookup the actual head. */
>      foliop = (struct folio *)(head - 1);
>      ps->idx = folio_page_idx(foliop, page);
> } else
>      ...
> }
> 
> But you're just moving that code, so no need to do that now.
> 
> 
> I think we could improve some of that in the future a bit to
> make it even more faithful.
> 
> But for now this should be just fine.
> 
> Acked-by: David Hildenbrand <david@redhat.com>
> 

Re: [PATCH v3 2/4] mm/util: introduce snapshot_page()

[David Hildenbrand]

On 16.07.25 19:36, Luiz Capitulino wrote:
> On 2025-07-16 06:16, David Hildenbrand wrote:
>> [...]
>>
>>> -dump:
>>> -    __dump_folio(foliop, &precise, pfn, idx);
>>> +    __dump_folio(&ps.folio_snapshot, &ps.page_snapshot, ps.pfn, ps.idx);
>>
>> Nit that can be cleaned up later on top:
>>
>> We should probably call this
>>
>> __dump_page_snapshot() and then just pass ... the page_snapshot.
>>
>>>    }
>>>    void dump_page(const struct page *page, const char *reason)
>>> diff --git a/mm/util.c b/mm/util.c
>>> index 0b270c43d7d1..f270bf42465b 100644
>>> --- a/mm/util.c
>>> +++ b/mm/util.c
>>> @@ -25,6 +25,7 @@
>>>    #include <linux/sizes.h>
>>>    #include <linux/compat.h>
>>>    #include <linux/fsnotify.h>
>>> +#include <linux/page_idle.h>
>>>    #include <linux/uaccess.h>
>>> @@ -1171,3 +1172,81 @@ int compat_vma_mmap_prepare(struct file *file, struct vm_area_struct *vma)
>>>        return 0;
>>>    }
>>>    EXPORT_SYMBOL(compat_vma_mmap_prepare);
>>> +
>>> +static void set_ps_flags(struct page_snapshot *ps, const struct folio *folio,
>>> +             const struct page *page)
>>> +{
>>> +    /*
>>> +     * Only the first page of a high-order buddy page has PageBuddy() set.
>>> +     * So we have to check manually whether this page is part of a high-
>>> +     * order buddy page.
>>> +     */
>>> +    if (PageBuddy(page))
>>> +        ps->flags |= PAGE_SNAPSHOT_PG_BUDDY;
>>> +    else if (page_count(page) == 0 && is_free_buddy_page(page))
>>> +        ps->flags |= PAGE_SNAPSHOT_PG_BUDDY;
>>> +
>>> +    if (folio_test_idle(folio))
>>> +        ps->flags |= PAGE_SNAPSHOT_PG_IDLE;
>>> +}
>>> +
>>> +/**
>>> + * snapshot_page() - Create a snapshot of a struct page
>>> + * @ps: Pointer to a struct page_snapshot to store the page snapshot
>>> + * @page: The page to snapshot
>>> + *
>>> + * Create a snapshot of the page and store both its struct page and struct
>>> + * folio representations in @ps.
>>> + *
>>> + * Note that creating a faithful snapshot may fail if the compound
>>
>> Maybe highlight that this is not really expected to happen, ever.
>>
>>> + * state of the page keeps changing (e.g., due to a folio split). In
>>> + * this case, ps->faithful is set to false, and the snapshot assumes
>>
>> There is no ps->faithful.
> 
> Yes, good catch. This was from an earlier version.
> 
> Is it fine if I fix only this with a follow up patch for Andrew in this
> thread or would you prefer that I post v4 with all the other changes as
> well?

I think the series was part of mm-new, but now I only spot it in 
mm-everything, weird. Maybe because of a conflict with the other 
stable_page_flags() change?

So whatever Andrew prefers, really :)

-- 
Cheers,

David / dhildenb

Re: [PATCH v3 2/4] mm/util: introduce snapshot_page()

[Andrew Morton]

On Wed, 16 Jul 2025 20:18:00 +0200 David Hildenbrand <david@redhat.com> wrote:

> On 16.07.25 19:36, Luiz Capitulino wrote:
> > Yes, good catch. This was from an earlier version.
> > 
> > Is it fine if I fix only this with a follow up patch for Andrew in this
> > thread or would you prefer that I post v4 with all the other changes as
> > well?
> 
> I think the series was part of mm-new, but now I only spot it in 
> mm-everything, weird. Maybe because of a conflict with the other 
> stable_page_flags() change?

I think I removed v2 from mm-unstable and added v3 to mm-new.  Maybe.

> So whatever Andrew prefers, really :)

Either is OK at this stage.  I do prefer little fixes so that I and
others can see what changed - it depends how tricky and large they are.

Re: [PATCH v3 2/4] mm/util: introduce snapshot_page()

[Luiz Capitulino]

On 2025-07-16 18:19, Andrew Morton wrote:
> On Wed, 16 Jul 2025 20:18:00 +0200 David Hildenbrand <david@redhat.com> wrote:
> 
>> On 16.07.25 19:36, Luiz Capitulino wrote:
>>> Yes, good catch. This was from an earlier version.
>>>
>>> Is it fine if I fix only this with a follow up patch for Andrew in this
>>> thread or would you prefer that I post v4 with all the other changes as
>>> well?
>>
>> I think the series was part of mm-new, but now I only spot it in
>> mm-everything, weird. Maybe because of a conflict with the other
>> stable_page_flags() change?
> 
> I think I removed v2 from mm-unstable and added v3 to mm-new.  Maybe.
> 
>> So whatever Andrew prefers, really :)
> 
> Either is OK at this stage.  I do prefer little fixes so that I and
> others can see what changed - it depends how tricky and large they are.

I liked some of the other suggestions from David so I'll send v4.

Re: [PATCH v3 2/4] mm/util: introduce snapshot_page()

[Luiz Capitulino]

On 2025-07-16 21:52, Luiz Capitulino wrote:
> On 2025-07-16 18:19, Andrew Morton wrote:
>> On Wed, 16 Jul 2025 20:18:00 +0200 David Hildenbrand <david@redhat.com> wrote:
>>
>>> On 16.07.25 19:36, Luiz Capitulino wrote:
>>>> Yes, good catch. This was from an earlier version.
>>>>
>>>> Is it fine if I fix only this with a follow up patch for Andrew in this
>>>> thread or would you prefer that I post v4 with all the other changes as
>>>> well?
>>>
>>> I think the series was part of mm-new, but now I only spot it in
>>> mm-everything, weird. Maybe because of a conflict with the other
>>> stable_page_flags() change?
>>
>> I think I removed v2 from mm-unstable and added v3 to mm-new.  Maybe.
>>
>>> So whatever Andrew prefers, really :)
>>
>> Either is OK at this stage.  I do prefer little fixes so that I and
>> others can see what changed - it depends how tricky and large they are.
> 
> I liked some of the other suggestions from David so I'll send v4.

I changed my mind again (sorry). I think the current version should be
good as is except for the doc bug, so here's a patch to fix it on top:

diff --git a/mm/util.c b/mm/util.c
index f270bf42465b..6e5b6ff1c90d 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1198,10 +1198,12 @@ static void set_ps_flags(struct page_snapshot *ps, const struct folio *folio,
   * Create a snapshot of the page and store both its struct page and struct
   * folio representations in @ps.
   *
- * Note that creating a faithful snapshot may fail if the compound
- * state of the page keeps changing (e.g., due to a folio split). In
- * this case, ps->faithful is set to false, and the snapshot assumes
- * that @page refers to a single page.
+ * A snapshot is marked as "faithful" if the compound state of @page was
+ * stable and allowed safe reconstruction of the folio representation. In
+ * rare cases where this is not possible (e.g. due to folio splitting),
+ * snapshot_page() falls back to treating @page as a single page and the
+ * snapshot is marked as "unfaithful". The snapshot_page_is_faithful()
+ * helper can be used to check for this condition.
   */
  void snapshot_page(struct page_snapshot *ps, const struct page *page)
  {

[PATCH v3 3/4] proc: kpagecount: use snapshot_page()

[Luiz Capitulino]

Currently, the call to folio_precise_page_mapcount() from kpage_read()
can race with a folio split. When the race happens we trigger a
VM_BUG_ON_FOLIO() in folio_entire_mapcount() (see splat below).

This commit fixes this race by using snapshot_page() so that we
retrieve the folio mapcount using a folio snapshot.

[ 2356.558576] page: refcount:1 mapcount:1 mapping:0000000000000000 index:0xffff85200 pfn:0x6f7c00
[ 2356.558748] memcg:ffff000651775780
[ 2356.558763] anon flags: 0xafffff60020838(uptodate|dirty|lru|owner_2|swapbacked|node=1|zone=2|lastcpupid=0xfffff)
[ 2356.558796] raw: 00afffff60020838 fffffdffdb5d0048 fffffdffdadf7fc8 ffff00064c1629c1
[ 2356.558817] raw: 0000000ffff85200 0000000000000000 0000000100000000 ffff000651775780
[ 2356.558839] page dumped because: VM_BUG_ON_FOLIO(!folio_test_large(folio))
[ 2356.558882] ------------[ cut here ]------------
[ 2356.558897] kernel BUG at ./include/linux/mm.h:1103!
[ 2356.558982] Internal error: Oops - BUG: 00000000f2000800 [#1]  SMP
[ 2356.564729] CPU: 8 UID: 0 PID: 1864 Comm: folio-split-rac Tainted: G S      W           6.15.0+ #3 PREEMPT(voluntary)
[ 2356.566196] Tainted: [S]=CPU_OUT_OF_SPEC, [W]=WARN
[ 2356.566814] Hardware name: Red Hat KVM, BIOS edk2-20241117-3.el9 11/17/2024
[ 2356.567684] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 2356.568563] pc : kpage_read.constprop.0+0x26c/0x290
[ 2356.569605] lr : kpage_read.constprop.0+0x26c/0x290
[ 2356.569992] sp : ffff80008fb739b0
[ 2356.570263] x29: ffff80008fb739b0 x28: ffff00064aa69580 x27: 00000000ff000000
[ 2356.570842] x26: 0000fffffffffff8 x25: ffff00064aa69580 x24: ffff80008fb73ae0
[ 2356.571411] x23: 0000000000000001 x22: 0000ffff86c6e8b8 x21: 0000000000000008
[ 2356.571978] x20: 00000000006f7c00 x19: 0000ffff86c6e8b8 x18: 0000000000000000
[ 2356.572581] x17: 3630303066666666 x16: 0000000000000003 x15: 0000000000001000
[ 2356.573217] x14: 00000000ffffffff x13: 0000000000000004 x12: 00aaaaaa00aaaaaa
[ 2356.577674] x11: 0000000000000000 x10: 00aaaaaa00aaaaaa x9 : ffffbf3afca6c300
[ 2356.578332] x8 : 0000000000000002 x7 : 0000000000000001 x6 : 0000000000000001
[ 2356.578984] x5 : ffff000c79812408 x4 : 0000000000000000 x3 : 0000000000000000
[ 2356.579635] x2 : 0000000000000000 x1 : ffff00064aa69580 x0 : 000000000000003e
[ 2356.580286] Call trace:
[ 2356.580524]  kpage_read.constprop.0+0x26c/0x290 (P)
[ 2356.580982]  kpagecount_read+0x28/0x40
[ 2356.581336]  proc_reg_read+0x38/0x100
[ 2356.581681]  vfs_read+0xcc/0x320
[ 2356.581992]  ksys_read+0x74/0x118
[ 2356.582306]  __arm64_sys_read+0x24/0x38
[ 2356.582668]  invoke_syscall+0x70/0x100
[ 2356.583022]  el0_svc_common.constprop.0+0x48/0xf8
[ 2356.583456]  do_el0_svc+0x28/0x40
[ 2356.583930]  el0_svc+0x38/0x118
[ 2356.584328]  el0t_64_sync_handler+0x144/0x168
[ 2356.584883]  el0t_64_sync+0x19c/0x1a0
[ 2356.585350] Code: aa0103e0 9003a541 91082021 97f813fc (d4210000)
[ 2356.586130] ---[ end trace 0000000000000000 ]---
[ 2356.587377] note: folio-split-rac[1864] exited with irqs disabled
[ 2356.588050] note: folio-split-rac[1864] exited with preempt_count 1

Reported-by: syzbot+3d7dc5eaba6b932f8535@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/67812fbd.050a0220.d0267.0030.GAE@google.com/
Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
---
 fs/proc/page.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/fs/proc/page.c b/fs/proc/page.c
index 999af26c7298..936f8bbe5a6f 100644
--- a/fs/proc/page.c
+++ b/fs/proc/page.c
@@ -43,6 +43,22 @@ static inline unsigned long get_max_dump_pfn(void)
 #endif
 }
 
+static u64 get_kpage_count(const struct page *page)
+{
+	struct page_snapshot ps;
+	u64 ret;
+
+	snapshot_page(&ps, page);
+
+	if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT))
+		ret = folio_precise_page_mapcount(&ps.folio_snapshot,
+						  &ps.page_snapshot);
+	else
+		ret = folio_average_page_mapcount(&ps.folio_snapshot);
+
+	return ret;
+}
+
 static ssize_t kpage_read(struct file *file, char __user *buf,
 		size_t count, loff_t *ppos,
 		enum kpage_operation op)
@@ -75,10 +91,7 @@ static ssize_t kpage_read(struct file *file, char __user *buf,
 				info = stable_page_flags(page);
 				break;
 			case KPAGE_COUNT:
-				if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT))
-					info = folio_precise_page_mapcount(page_folio(page), page);
-				else
-					info = folio_average_page_mapcount(page_folio(page));
+				info = get_kpage_count(page);
 				break;
 			case KPAGE_CGROUP:
 				info = page_cgroup_ino(page);
-- 
2.50.0

Re: [PATCH v3 3/4] proc: kpagecount: use snapshot_page()

[David Hildenbrand]

On 14.07.25 15:16, Luiz Capitulino wrote:
> Currently, the call to folio_precise_page_mapcount() from kpage_read()
> can race with a folio split. When the race happens we trigger a
> VM_BUG_ON_FOLIO() in folio_entire_mapcount() (see splat below).
> 
> This commit fixes this race by using snapshot_page() so that we
> retrieve the folio mapcount using a folio snapshot.
> 
> [ 2356.558576] page: refcount:1 mapcount:1 mapping:0000000000000000 index:0xffff85200 pfn:0x6f7c00
> [ 2356.558748] memcg:ffff000651775780
> [ 2356.558763] anon flags: 0xafffff60020838(uptodate|dirty|lru|owner_2|swapbacked|node=1|zone=2|lastcpupid=0xfffff)
> [ 2356.558796] raw: 00afffff60020838 fffffdffdb5d0048 fffffdffdadf7fc8 ffff00064c1629c1
> [ 2356.558817] raw: 0000000ffff85200 0000000000000000 0000000100000000 ffff000651775780
> [ 2356.558839] page dumped because: VM_BUG_ON_FOLIO(!folio_test_large(folio))
> [ 2356.558882] ------------[ cut here ]------------
> [ 2356.558897] kernel BUG at ./include/linux/mm.h:1103!
> [ 2356.558982] Internal error: Oops - BUG: 00000000f2000800 [#1]  SMP
> [ 2356.564729] CPU: 8 UID: 0 PID: 1864 Comm: folio-split-rac Tainted: G S      W           6.15.0+ #3 PREEMPT(voluntary)
> [ 2356.566196] Tainted: [S]=CPU_OUT_OF_SPEC, [W]=WARN
> [ 2356.566814] Hardware name: Red Hat KVM, BIOS edk2-20241117-3.el9 11/17/2024
> [ 2356.567684] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> [ 2356.568563] pc : kpage_read.constprop.0+0x26c/0x290
> [ 2356.569605] lr : kpage_read.constprop.0+0x26c/0x290
> [ 2356.569992] sp : ffff80008fb739b0
> [ 2356.570263] x29: ffff80008fb739b0 x28: ffff00064aa69580 x27: 00000000ff000000
> [ 2356.570842] x26: 0000fffffffffff8 x25: ffff00064aa69580 x24: ffff80008fb73ae0
> [ 2356.571411] x23: 0000000000000001 x22: 0000ffff86c6e8b8 x21: 0000000000000008
> [ 2356.571978] x20: 00000000006f7c00 x19: 0000ffff86c6e8b8 x18: 0000000000000000
> [ 2356.572581] x17: 3630303066666666 x16: 0000000000000003 x15: 0000000000001000
> [ 2356.573217] x14: 00000000ffffffff x13: 0000000000000004 x12: 00aaaaaa00aaaaaa
> [ 2356.577674] x11: 0000000000000000 x10: 00aaaaaa00aaaaaa x9 : ffffbf3afca6c300
> [ 2356.578332] x8 : 0000000000000002 x7 : 0000000000000001 x6 : 0000000000000001
> [ 2356.578984] x5 : ffff000c79812408 x4 : 0000000000000000 x3 : 0000000000000000
> [ 2356.579635] x2 : 0000000000000000 x1 : ffff00064aa69580 x0 : 000000000000003e
> [ 2356.580286] Call trace:
> [ 2356.580524]  kpage_read.constprop.0+0x26c/0x290 (P)
> [ 2356.580982]  kpagecount_read+0x28/0x40
> [ 2356.581336]  proc_reg_read+0x38/0x100
> [ 2356.581681]  vfs_read+0xcc/0x320
> [ 2356.581992]  ksys_read+0x74/0x118
> [ 2356.582306]  __arm64_sys_read+0x24/0x38
> [ 2356.582668]  invoke_syscall+0x70/0x100
> [ 2356.583022]  el0_svc_common.constprop.0+0x48/0xf8
> [ 2356.583456]  do_el0_svc+0x28/0x40
> [ 2356.583930]  el0_svc+0x38/0x118
> [ 2356.584328]  el0t_64_sync_handler+0x144/0x168
> [ 2356.584883]  el0t_64_sync+0x19c/0x1a0
> [ 2356.585350] Code: aa0103e0 9003a541 91082021 97f813fc (d4210000)
> [ 2356.586130] ---[ end trace 0000000000000000 ]---
> [ 2356.587377] note: folio-split-rac[1864] exited with irqs disabled
> [ 2356.588050] note: folio-split-rac[1864] exited with preempt_count 1
> 
> Reported-by: syzbot+3d7dc5eaba6b932f8535@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/67812fbd.050a0220.d0267.0030.GAE@google.com/
> Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
> ---
>   fs/proc/page.c | 21 +++++++++++++++++----
>   1 file changed, 17 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/proc/page.c b/fs/proc/page.c
> index 999af26c7298..936f8bbe5a6f 100644
> --- a/fs/proc/page.c
> +++ b/fs/proc/page.c
> @@ -43,6 +43,22 @@ static inline unsigned long get_max_dump_pfn(void)
>   #endif
>   }
>   
> +static u64 get_kpage_count(const struct page *page)
> +{
> +	struct page_snapshot ps;
> +	u64 ret;
> +
> +	snapshot_page(&ps, page);

Curious, if the snapshot is not faithful, maybe we simply want to return 
0 or sth. like that?

In any case

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb

Re: [PATCH v3 3/4] proc: kpagecount: use snapshot_page()

[Luiz Capitulino]

On 2025-07-16 06:17, David Hildenbrand wrote:
> On 14.07.25 15:16, Luiz Capitulino wrote:
>> Currently, the call to folio_precise_page_mapcount() from kpage_read()
>> can race with a folio split. When the race happens we trigger a
>> VM_BUG_ON_FOLIO() in folio_entire_mapcount() (see splat below).
>>
>> This commit fixes this race by using snapshot_page() so that we
>> retrieve the folio mapcount using a folio snapshot.
>>
>> [ 2356.558576] page: refcount:1 mapcount:1 mapping:0000000000000000 index:0xffff85200 pfn:0x6f7c00
>> [ 2356.558748] memcg:ffff000651775780
>> [ 2356.558763] anon flags: 0xafffff60020838(uptodate|dirty|lru|owner_2|swapbacked|node=1|zone=2|lastcpupid=0xfffff)
>> [ 2356.558796] raw: 00afffff60020838 fffffdffdb5d0048 fffffdffdadf7fc8 ffff00064c1629c1
>> [ 2356.558817] raw: 0000000ffff85200 0000000000000000 0000000100000000 ffff000651775780
>> [ 2356.558839] page dumped because: VM_BUG_ON_FOLIO(!folio_test_large(folio))
>> [ 2356.558882] ------------[ cut here ]------------
>> [ 2356.558897] kernel BUG at ./include/linux/mm.h:1103!
>> [ 2356.558982] Internal error: Oops - BUG: 00000000f2000800 [#1]  SMP
>> [ 2356.564729] CPU: 8 UID: 0 PID: 1864 Comm: folio-split-rac Tainted: G S      W           6.15.0+ #3 PREEMPT(voluntary)
>> [ 2356.566196] Tainted: [S]=CPU_OUT_OF_SPEC, [W]=WARN
>> [ 2356.566814] Hardware name: Red Hat KVM, BIOS edk2-20241117-3.el9 11/17/2024
>> [ 2356.567684] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>> [ 2356.568563] pc : kpage_read.constprop.0+0x26c/0x290
>> [ 2356.569605] lr : kpage_read.constprop.0+0x26c/0x290
>> [ 2356.569992] sp : ffff80008fb739b0
>> [ 2356.570263] x29: ffff80008fb739b0 x28: ffff00064aa69580 x27: 00000000ff000000
>> [ 2356.570842] x26: 0000fffffffffff8 x25: ffff00064aa69580 x24: ffff80008fb73ae0
>> [ 2356.571411] x23: 0000000000000001 x22: 0000ffff86c6e8b8 x21: 0000000000000008
>> [ 2356.571978] x20: 00000000006f7c00 x19: 0000ffff86c6e8b8 x18: 0000000000000000
>> [ 2356.572581] x17: 3630303066666666 x16: 0000000000000003 x15: 0000000000001000
>> [ 2356.573217] x14: 00000000ffffffff x13: 0000000000000004 x12: 00aaaaaa00aaaaaa
>> [ 2356.577674] x11: 0000000000000000 x10: 00aaaaaa00aaaaaa x9 : ffffbf3afca6c300
>> [ 2356.578332] x8 : 0000000000000002 x7 : 0000000000000001 x6 : 0000000000000001
>> [ 2356.578984] x5 : ffff000c79812408 x4 : 0000000000000000 x3 : 0000000000000000
>> [ 2356.579635] x2 : 0000000000000000 x1 : ffff00064aa69580 x0 : 000000000000003e
>> [ 2356.580286] Call trace:
>> [ 2356.580524]  kpage_read.constprop.0+0x26c/0x290 (P)
>> [ 2356.580982]  kpagecount_read+0x28/0x40
>> [ 2356.581336]  proc_reg_read+0x38/0x100
>> [ 2356.581681]  vfs_read+0xcc/0x320
>> [ 2356.581992]  ksys_read+0x74/0x118
>> [ 2356.582306]  __arm64_sys_read+0x24/0x38
>> [ 2356.582668]  invoke_syscall+0x70/0x100
>> [ 2356.583022]  el0_svc_common.constprop.0+0x48/0xf8
>> [ 2356.583456]  do_el0_svc+0x28/0x40
>> [ 2356.583930]  el0_svc+0x38/0x118
>> [ 2356.584328]  el0t_64_sync_handler+0x144/0x168
>> [ 2356.584883]  el0t_64_sync+0x19c/0x1a0
>> [ 2356.585350] Code: aa0103e0 9003a541 91082021 97f813fc (d4210000)
>> [ 2356.586130] ---[ end trace 0000000000000000 ]---
>> [ 2356.587377] note: folio-split-rac[1864] exited with irqs disabled
>> [ 2356.588050] note: folio-split-rac[1864] exited with preempt_count 1
>>
>> Reported-by: syzbot+3d7dc5eaba6b932f8535@syzkaller.appspotmail.com
>> Closes: https://lore.kernel.org/all/67812fbd.050a0220.d0267.0030.GAE@google.com/
>> Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
>> ---
>>   fs/proc/page.c | 21 +++++++++++++++++----
>>   1 file changed, 17 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/proc/page.c b/fs/proc/page.c
>> index 999af26c7298..936f8bbe5a6f 100644
>> --- a/fs/proc/page.c
>> +++ b/fs/proc/page.c
>> @@ -43,6 +43,22 @@ static inline unsigned long get_max_dump_pfn(void)
>>   #endif
>>   }
>> +static u64 get_kpage_count(const struct page *page)
>> +{
>> +    struct page_snapshot ps;
>> +    u64 ret;
>> +
>> +    snapshot_page(&ps, page);
> 
> Curious, if the snapshot is not faithful, maybe we simply want to return 0 or sth. like that?

I chose the flag because it's not exactly that the snapshot failed as it
tries to return something. But it's not a big deal to return failure
instead.

> 
> In any case
> 
> Acked-by: David Hildenbrand <david@redhat.com>
> 

Re: [PATCH v3 3/4] proc: kpagecount: use snapshot_page()

[David Hildenbrand]

On 16.07.25 19:41, Luiz Capitulino wrote:
> On 2025-07-16 06:17, David Hildenbrand wrote:
>> On 14.07.25 15:16, Luiz Capitulino wrote:
>>> Currently, the call to folio_precise_page_mapcount() from kpage_read()
>>> can race with a folio split. When the race happens we trigger a
>>> VM_BUG_ON_FOLIO() in folio_entire_mapcount() (see splat below).
>>>
>>> This commit fixes this race by using snapshot_page() so that we
>>> retrieve the folio mapcount using a folio snapshot.
>>>
>>> [ 2356.558576] page: refcount:1 mapcount:1 mapping:0000000000000000 index:0xffff85200 pfn:0x6f7c00
>>> [ 2356.558748] memcg:ffff000651775780
>>> [ 2356.558763] anon flags: 0xafffff60020838(uptodate|dirty|lru|owner_2|swapbacked|node=1|zone=2|lastcpupid=0xfffff)
>>> [ 2356.558796] raw: 00afffff60020838 fffffdffdb5d0048 fffffdffdadf7fc8 ffff00064c1629c1
>>> [ 2356.558817] raw: 0000000ffff85200 0000000000000000 0000000100000000 ffff000651775780
>>> [ 2356.558839] page dumped because: VM_BUG_ON_FOLIO(!folio_test_large(folio))
>>> [ 2356.558882] ------------[ cut here ]------------
>>> [ 2356.558897] kernel BUG at ./include/linux/mm.h:1103!
>>> [ 2356.558982] Internal error: Oops - BUG: 00000000f2000800 [#1]  SMP
>>> [ 2356.564729] CPU: 8 UID: 0 PID: 1864 Comm: folio-split-rac Tainted: G S      W           6.15.0+ #3 PREEMPT(voluntary)
>>> [ 2356.566196] Tainted: [S]=CPU_OUT_OF_SPEC, [W]=WARN
>>> [ 2356.566814] Hardware name: Red Hat KVM, BIOS edk2-20241117-3.el9 11/17/2024
>>> [ 2356.567684] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>>> [ 2356.568563] pc : kpage_read.constprop.0+0x26c/0x290
>>> [ 2356.569605] lr : kpage_read.constprop.0+0x26c/0x290
>>> [ 2356.569992] sp : ffff80008fb739b0
>>> [ 2356.570263] x29: ffff80008fb739b0 x28: ffff00064aa69580 x27: 00000000ff000000
>>> [ 2356.570842] x26: 0000fffffffffff8 x25: ffff00064aa69580 x24: ffff80008fb73ae0
>>> [ 2356.571411] x23: 0000000000000001 x22: 0000ffff86c6e8b8 x21: 0000000000000008
>>> [ 2356.571978] x20: 00000000006f7c00 x19: 0000ffff86c6e8b8 x18: 0000000000000000
>>> [ 2356.572581] x17: 3630303066666666 x16: 0000000000000003 x15: 0000000000001000
>>> [ 2356.573217] x14: 00000000ffffffff x13: 0000000000000004 x12: 00aaaaaa00aaaaaa
>>> [ 2356.577674] x11: 0000000000000000 x10: 00aaaaaa00aaaaaa x9 : ffffbf3afca6c300
>>> [ 2356.578332] x8 : 0000000000000002 x7 : 0000000000000001 x6 : 0000000000000001
>>> [ 2356.578984] x5 : ffff000c79812408 x4 : 0000000000000000 x3 : 0000000000000000
>>> [ 2356.579635] x2 : 0000000000000000 x1 : ffff00064aa69580 x0 : 000000000000003e
>>> [ 2356.580286] Call trace:
>>> [ 2356.580524]  kpage_read.constprop.0+0x26c/0x290 (P)
>>> [ 2356.580982]  kpagecount_read+0x28/0x40
>>> [ 2356.581336]  proc_reg_read+0x38/0x100
>>> [ 2356.581681]  vfs_read+0xcc/0x320
>>> [ 2356.581992]  ksys_read+0x74/0x118
>>> [ 2356.582306]  __arm64_sys_read+0x24/0x38
>>> [ 2356.582668]  invoke_syscall+0x70/0x100
>>> [ 2356.583022]  el0_svc_common.constprop.0+0x48/0xf8
>>> [ 2356.583456]  do_el0_svc+0x28/0x40
>>> [ 2356.583930]  el0_svc+0x38/0x118
>>> [ 2356.584328]  el0t_64_sync_handler+0x144/0x168
>>> [ 2356.584883]  el0t_64_sync+0x19c/0x1a0
>>> [ 2356.585350] Code: aa0103e0 9003a541 91082021 97f813fc (d4210000)
>>> [ 2356.586130] ---[ end trace 0000000000000000 ]---
>>> [ 2356.587377] note: folio-split-rac[1864] exited with irqs disabled
>>> [ 2356.588050] note: folio-split-rac[1864] exited with preempt_count 1
>>>
>>> Reported-by: syzbot+3d7dc5eaba6b932f8535@syzkaller.appspotmail.com
>>> Closes: https://lore.kernel.org/all/67812fbd.050a0220.d0267.0030.GAE@google.com/
>>> Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
>>> ---
>>>    fs/proc/page.c | 21 +++++++++++++++++----
>>>    1 file changed, 17 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/fs/proc/page.c b/fs/proc/page.c
>>> index 999af26c7298..936f8bbe5a6f 100644
>>> --- a/fs/proc/page.c
>>> +++ b/fs/proc/page.c
>>> @@ -43,6 +43,22 @@ static inline unsigned long get_max_dump_pfn(void)
>>>    #endif
>>>    }
>>> +static u64 get_kpage_count(const struct page *page)
>>> +{
>>> +    struct page_snapshot ps;
>>> +    u64 ret;
>>> +
>>> +    snapshot_page(&ps, page);
>>
>> Curious, if the snapshot is not faithful, maybe we simply want to return 0 or sth. like that?
> 
> I chose the flag because it's not exactly that the snapshot failed as it
> tries to return something. But it's not a big deal to return failure
> instead.

Yeah, for this case I would just return 0. But whatever you prefer!

-- 
Cheers,

David / dhildenb

[PATCH v3 4/4] fs: stable_page_flags(): use snapshot_page()

[Luiz Capitulino]

A race condition is possible in stable_page_flags() where user-space is
reading /proc/kpageflags concurrently to a folio split. This may lead to
oopses or BUG_ON()s being triggered.

To fix this, this commit uses snapshot_page() in stable_page_flags() so
that stable_page_flags() works with a stable page and folio snapshots
instead.

Note that stable_page_flags() makes use of some functions that require
the original page or folio pointer to work properly (eg.
is_free_budy_page() and folio_test_idle()). Since those functions can't
be used on the page snapshot, we replace their usage with flags that
were set by snapshot_page() for this purpose.

Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
---
 fs/proc/page.c | 29 +++++++++++++----------------
 1 file changed, 13 insertions(+), 16 deletions(-)

diff --git a/fs/proc/page.c b/fs/proc/page.c
index 936f8bbe5a6f..52e4f49c0638 100644
--- a/fs/proc/page.c
+++ b/fs/proc/page.c
@@ -147,6 +147,7 @@ static inline u64 kpf_copy_bit(u64 kflags, int ubit, int kbit)
 u64 stable_page_flags(const struct page *page)
 {
 	const struct folio *folio;
+	struct page_snapshot ps;
 	unsigned long k;
 	unsigned long mapping;
 	bool is_anon;
@@ -158,7 +159,9 @@ u64 stable_page_flags(const struct page *page)
 	 */
 	if (!page)
 		return 1 << KPF_NOPAGE;
-	folio = page_folio(page);
+
+	snapshot_page(&ps, page);
+	folio = &ps.folio_snapshot;
 
 	k = folio->flags;
 	mapping = (unsigned long)folio->mapping;
@@ -167,7 +170,7 @@ u64 stable_page_flags(const struct page *page)
 	/*
 	 * pseudo flags for the well known (anonymous) memory mapped pages
 	 */
-	if (page_mapped(page))
+	if (folio_mapped(folio))
 		u |= 1 << KPF_MMAP;
 	if (is_anon) {
 		u |= 1 << KPF_ANON;
@@ -179,7 +182,7 @@ u64 stable_page_flags(const struct page *page)
 	 * compound pages: export both head/tail info
 	 * they together define a compound page's start/end pos and order
 	 */
-	if (page == &folio->page)
+	if (ps.idx == 0)
 		u |= kpf_copy_bit(k, KPF_COMPOUND_HEAD, PG_head);
 	else
 		u |= 1 << KPF_COMPOUND_TAIL;
@@ -189,25 +192,19 @@ u64 stable_page_flags(const struct page *page)
 	         folio_test_large_rmappable(folio)) {
 		/* Note: we indicate any THPs here, not just PMD-sized ones */
 		u |= 1 << KPF_THP;
-	} else if (is_huge_zero_folio(folio)) {
+	} else if (is_huge_zero_pfn(ps.pfn)) {
 		u |= 1 << KPF_ZERO_PAGE;
 		u |= 1 << KPF_THP;
-	} else if (is_zero_folio(folio)) {
+	} else if (is_zero_pfn(ps.pfn)) {
 		u |= 1 << KPF_ZERO_PAGE;
 	}
 
-	/*
-	 * Caveats on high order pages: PG_buddy and PG_slab will only be set
-	 * on the head page.
-	 */
-	if (PageBuddy(page))
-		u |= 1 << KPF_BUDDY;
-	else if (page_count(page) == 0 && is_free_buddy_page(page))
+	if (ps.flags & PAGE_SNAPSHOT_PG_BUDDY)
 		u |= 1 << KPF_BUDDY;
 
-	if (PageOffline(page))
+	if (folio_test_offline(folio))
 		u |= 1 << KPF_OFFLINE;
-	if (PageTable(page))
+	if (folio_test_pgtable(folio))
 		u |= 1 << KPF_PGTABLE;
 	if (folio_test_slab(folio))
 		u |= 1 << KPF_SLAB;
@@ -215,7 +212,7 @@ u64 stable_page_flags(const struct page *page)
 #if defined(CONFIG_PAGE_IDLE_FLAG) && defined(CONFIG_64BIT)
 	u |= kpf_copy_bit(k, KPF_IDLE,          PG_idle);
 #else
-	if (folio_test_idle(folio))
+	if (ps.flags & PAGE_SNAPSHOT_PG_IDLE)
 		u |= 1 << KPF_IDLE;
 #endif
 
@@ -241,7 +238,7 @@ u64 stable_page_flags(const struct page *page)
 	if (u & (1 << KPF_HUGE))
 		u |= kpf_copy_bit(k, KPF_HWPOISON,	PG_hwpoison);
 	else
-		u |= kpf_copy_bit(page->flags, KPF_HWPOISON,	PG_hwpoison);
+		u |= kpf_copy_bit(ps.page_snapshot.flags, KPF_HWPOISON, PG_hwpoison);
 #endif
 
 	u |= kpf_copy_bit(k, KPF_RESERVED,	PG_reserved);
-- 
2.50.0

Re: [PATCH v3 4/4] fs: stable_page_flags(): use snapshot_page()

[David Hildenbrand]

On 14.07.25 15:16, Luiz Capitulino wrote:
> A race condition is possible in stable_page_flags() where user-space is
> reading /proc/kpageflags concurrently to a folio split. This may lead to
> oopses or BUG_ON()s being triggered.
> 
> To fix this, this commit uses snapshot_page() in stable_page_flags() so
> that stable_page_flags() works with a stable page and folio snapshots
> instead.
> 
> Note that stable_page_flags() makes use of some functions that require
> the original page or folio pointer to work properly (eg.
> is_free_budy_page() and folio_test_idle()). Since those functions can't
> be used on the page snapshot, we replace their usage with flags that
> were set by snapshot_page() for this purpose.
> 
> Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
> ---
>   fs/proc/page.c | 29 +++++++++++++----------------
>   1 file changed, 13 insertions(+), 16 deletions(-)
> 
> diff --git a/fs/proc/page.c b/fs/proc/page.c
> index 936f8bbe5a6f..52e4f49c0638 100644
> --- a/fs/proc/page.c
> +++ b/fs/proc/page.c
> @@ -147,6 +147,7 @@ static inline u64 kpf_copy_bit(u64 kflags, int ubit, int kbit)
>   u64 stable_page_flags(const struct page *page)
>   {
>   	const struct folio *folio;
> +	struct page_snapshot ps;
>   	unsigned long k;
>   	unsigned long mapping;
>   	bool is_anon;
> @@ -158,7 +159,9 @@ u64 stable_page_flags(const struct page *page)
>   	 */
>   	if (!page)
>   		return 1 << KPF_NOPAGE;
> -	folio = page_folio(page);
> +
> +	snapshot_page(&ps, page);
> +	folio = &ps.folio_snapshot;

Not sure what to do here if the snapshot is not faithful. Fortunately, 
it shouldn't happen often :)

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb

Re: [PATCH v3 0/4] mm: introduce snapshot_page()

[Shivank Garg]

On 7/14/2025 6:46 PM, Luiz Capitulino wrote:
> Hi,
> 
> The series introduction will follow the changelog. This is against bc9ff192a6c9 .
> 
> Changelog
> =========
> 
> v2 -> v3
>   - Small snapshot_page() refactor to simplify and fix stack-out-of-bugs bug
>     reported by Harry Yoo
>   - Use clear_compound_head() in snapshot_page() (Shivank)
>   - Read head pointer from ps->page_snapshot() for tail page case (Shivank)
>   - Improve comments and macro name in snapshot_page() (David)
> 
> v1 -> v2
>   - Include is_huge_zero_pfn() patch and use it (David)
>   - Move free page detection to snapshot_page() (David)
>   - Changelog improvements (Shivank)
>   - Added Acked-bys
> 
> RFC -> v1
>   - Include <linux/page_idle.h> to avoid build error on sh arch
> 
> Introduction
> ============
> 
> This series introduces snapshot_page(), a helper function that can be used
> to create a snapshot of a struct page and its associated struct folio.
> 
> This function is intended to help callers with a consistent view of a
> a folio while reducing the chance of encountering partially updated or
> inconsistent state, such as during folio splitting which could lead to
> crashes and BUG_ON()s being triggered.
> 
> David Hildenbrand (1):
>   mm/memory: introduce is_huge_zero_pfn() and use it in
>     vm_normal_page_pmd()
> 
> Luiz Capitulino (3):
>   mm/util: introduce snapshot_page()
>   proc: kpagecount: use snapshot_page()
>   fs: stable_page_flags(): use snapshot_page()
> 
>  fs/proc/page.c          | 50 +++++++++++++++-----------
>  include/linux/huge_mm.h | 12 ++++++-
>  include/linux/mm.h      | 19 ++++++++++
>  mm/debug.c              | 42 +++-------------------
>  mm/memory.c             |  2 +-
>  mm/util.c               | 79 +++++++++++++++++++++++++++++++++++++++++
>  6 files changed, 144 insertions(+), 60 deletions(-)
> 

The changes look good to me.
Thanks for addressing the feedback.

Reviewed-by: Shivank Garg <shivankg@amd.com>

Re: [PATCH v3 0/4] mm: introduce snapshot_page()

[Harry Yoo]

On Mon, Jul 14, 2025 at 09:16:50AM -0400, Luiz Capitulino wrote:
> Hi,
> 
> The series introduction will follow the changelog. This is against bc9ff192a6c9 .
> 
> Changelog
> =========
> 
> v2 -> v3
>   - Small snapshot_page() refactor to simplify and fix stack-out-of-bugs bug
>     reported by Harry Yoo
>   - Use clear_compound_head() in snapshot_page() (Shivank)
>   - Read head pointer from ps->page_snapshot() for tail page case (Shivank)
>   - Improve comments and macro name in snapshot_page() (David)
> 
> v1 -> v2
>   - Include is_huge_zero_pfn() patch and use it (David)
>   - Move free page detection to snapshot_page() (David)
>   - Changelog improvements (Shivank)
>   - Added Acked-bys
> 
> RFC -> v1
>   - Include <linux/page_idle.h> to avoid build error on sh arch
> 
> Introduction
> ============
> 
> This series introduces snapshot_page(), a helper function that can be used
> to create a snapshot of a struct page and its associated struct folio.
> 
> This function is intended to help callers with a consistent view of a
> a folio while reducing the chance of encountering partially updated or
> inconsistent state, such as during folio splitting which could lead to
> crashes and BUG_ON()s being triggered.
> 
> David Hildenbrand (1):
>   mm/memory: introduce is_huge_zero_pfn() and use it in
>     vm_normal_page_pmd()
> 
> Luiz Capitulino (3):
>   mm/util: introduce snapshot_page()
>   proc: kpagecount: use snapshot_page()
>   fs: stable_page_flags(): use snapshot_page()
> 
>  fs/proc/page.c          | 50 +++++++++++++++-----------
>  include/linux/huge_mm.h | 12 ++++++-
>  include/linux/mm.h      | 19 ++++++++++
>  mm/debug.c              | 42 +++-------------------
>  mm/memory.c             |  2 +-
>  mm/util.c               | 79 +++++++++++++++++++++++++++++++++++++++++
>  6 files changed, 144 insertions(+), 60 deletions(-)
> 
> --

Haha, I was waiting for this!

The previously reported KASAN splats have completely disappeared on my
testbed.

Please feel free to add,
Tested-by: Harry Yoo <harry.yoo@oracle.com>

-- 
Cheers,
Harry / Hyeonggon