Re: [PATCH v2 2/4] mm/util: introduce snapshot_page()

[David Hildenbrand <david@redhat.com>]

On 07.07.25 20:50, Luiz Capitulino 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 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          | 77 ++++++++++++++++++++++++++++++++++++++++++++++
>   3 files changed, 100 insertions(+), 38 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 0ef2ba0c667a..090968c6eebb 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_FREE      (1 << 1)

Can we call this "BUDDY" instead of FREE

There are other types of free pages -- in particular free hugetlb folios 
-- which won't be covered by this check. We really only care about 
"buddy" pages, which correspond to "free" pages.

That's what we check and alter expose :)

> +#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..c38d213be83f 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 +1172,79 @@ 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)
> +{
> +	/*
> +	 * Caveats on high order pages: PG_buddy and PG_slab will only be set
> +	 * on the head page.
> +	 */

Talking about slab here is a bit misleading, IIRC they are proper 
compound pages today. For PG_buddy (which was renamed to PGTY_buddy), we
are dealing with non-compound higher allocations.

So talking about "head" is also a bit misleading.

/*
  * 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_FREE;
> +	else if (page_count(page) == 0 && is_free_buddy_page(page))
> +		ps->flags |= PAGE_SNAPSHOT_PG_FREE;
> +
> +	if (folio_test_idle(folio))
> +		ps->flags |= PAGE_SNAPSHOT_PG_IDLE;
> +}
> +
> +/*

I would suggest writing proper kernel doc (you mostly have that already)

Something like

/**
  * snapshot_page() - Create a snapshot of a "struct page"
  * @ps: struct page_snapshot to store the page snapshot
  * @page: the page we want to snapshot
  *
  * Create a snapshot of a page and store its struct page and struct
  * folio ...

> + * 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.
> + */

-- 
Cheers,

David / dhildenb