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 implemenetation 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 | 20 +++++++++++++
 mm/debug.c         | 42 +++------------------------
 mm/util.c          | 72 ++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 96 insertions(+), 38 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 0ef2ba0c667a..4d83674e32a7 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -4184,4 +4184,24 @@ static inline bool page_pool_page_is_pp(struct page *page)
 }
 #endif
 
+#define PAGE_SNAPSHOT_FAITHFUL     (1 << 0)
+#define PAGE_SNAPSHOT_PG_HUGE_ZERO (1 << 1)
+#define PAGE_SNAPSHOT_PG_FREE      (1 << 2)
+#define PAGE_SNAPSHOT_PG_IDLE      (1 << 3)
+
+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 & 0x1;
+}
+
+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..4de427d48116 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 +1172,74 @@ int compat_vma_mmap_prepare(struct file *file, struct vm_area_struct *vma)
 	return 0;
 }
 EXPORT_SYMBOL(compat_vma_mmap_prepare);
+
+static void set_flags(struct page_snapshot *ps, const struct folio *folio,
+		      const struct page *page)
+{
+	if (is_huge_zero_folio(folio))
+		ps->flags |= PAGE_SNAPSHOT_PG_HUGE_ZERO;
+	if (folio_ref_count(folio) == 0 && is_free_buddy_page(page))
+		ps->flags |= PAGE_SNAPSHOT_PG_FREE;
+	if (folio_test_idle(folio))
+		ps->flags |= PAGE_SNAPSHOT_PG_IDLE;
+}
+
+/*
+ * Create a snapshot of a page and store its struct page and struct folio
+ * representations in a struct page_snapshot.
+ *
+ * @ps: struct page_snapshot to store the page snapshot
+ * @page: the page we want to snapshot
+ *
+ * Note that creating a faithful snapshot of a page may fail if the page
+ * compound keeps changing (eg. due to folio split). In this case we set
+ * ps->faithful to false and the snapshot will assume 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, folio;
+	int loops = 5;
+
+	ps->pfn = page_to_pfn(page);
+	ps->flags = PAGE_SNAPSHOT_FAITHFUL;
+
+again:
+	memcpy(&ps->page_snapshot, page, sizeof(*page));
+	head = ps->page_snapshot.compound_head;
+	if ((head & 1) == 0) {
+		foliop = (struct folio *)&ps->page_snapshot;
+		ps->idx = 0;
+		if (!folio_test_large(foliop)) {
+			set_flags(ps, page_folio(page), page);
+			goto out;
+		}
+		foliop = (struct folio *)page;
+	} else {
+		foliop = (struct folio *)(page->compound_head - 1);
+		ps->idx = folio_page_idx(foliop, page);
+	}
+
+	if (ps->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));
+		set_flags(ps, foliop, page);
+		foliop = &folio;
+	}
+
+	if (ps->idx > nr_pages) {
+		if (loops-- > 0)
+			goto again;
+		ps->page_snapshot.compound_head &= ~1UL;
+		foliop = (struct folio *)&ps->page_snapshot;
+		ps->flags = 0;
+		ps->idx = 0;
+	}
+
+out:
+	memcpy(&ps->folio_snapshot, foliop, sizeof(struct folio));
+}
-- 
2.49.0

On Thu, 26 Jun 2025 14:16:51 -0400 Luiz Capitulino <luizcap@redhat.com> wrote:

> 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 implemenetation of snapshot_page() comes from

tpyo!

> Matthew Wilcox with suggestions for improvements from David Hildenbrand.
> All bugs and misconceptions are mine.
>
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4184,4 +4184,24 @@ static inline bool page_pool_page_is_pp(struct page *page)
>  }
>  #endif
>  
> +#define PAGE_SNAPSHOT_FAITHFUL     (1 << 0)
> +#define PAGE_SNAPSHOT_PG_HUGE_ZERO (1 << 1)
> +#define PAGE_SNAPSHOT_PG_FREE      (1 << 2)
> +#define PAGE_SNAPSHOT_PG_IDLE      (1 << 3)
> +
> +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 & 0x1;'

	& PAGE_SNAPSHOT_FAITHFUL?

> +}
> +

All looks sane to me.  Small-system people (are there any left?) might
point out that all the new code could be under ifdef CONFIG_PROCFS?

I'll skip v1, see what reviewers have to say, thanks.

On 2025-06-26 17:39, Andrew Morton wrote:
> On Thu, 26 Jun 2025 14:16:51 -0400 Luiz Capitulino <luizcap@redhat.com> wrote:
> 
>> 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 implemenetation of snapshot_page() comes from
> 
> tpyo!
> 
>> Matthew Wilcox with suggestions for improvements from David Hildenbrand.
>> All bugs and misconceptions are mine.
>>
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -4184,4 +4184,24 @@ static inline bool page_pool_page_is_pp(struct page *page)
>>   }
>>   #endif
>>   
>> +#define PAGE_SNAPSHOT_FAITHFUL     (1 << 0)
>> +#define PAGE_SNAPSHOT_PG_HUGE_ZERO (1 << 1)
>> +#define PAGE_SNAPSHOT_PG_FREE      (1 << 2)
>> +#define PAGE_SNAPSHOT_PG_IDLE      (1 << 3)
>> +
>> +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 & 0x1;'
> 
> 	& PAGE_SNAPSHOT_FAITHFUL?
> 
>> +}
>> +
> 
> All looks sane to me.  Small-system people (are there any left?) might
> point out that all the new code could be under ifdef CONFIG_PROCFS?
> 
> I'll skip v1, see what reviewers have to say, thanks.

Yes, no rush. And I'll fix the things you pointed out for v2 (including
ifdef CONFIG_PROCFS). Thanks for the super quick feedback.