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

[Luiz Capitulino <luizcap@redhat.com>]

On 2025-07-08 12:59, Luiz Capitulino wrote:
> On 2025-07-08 01:49, Shivank Garg wrote:
>>
>>
>> On 7/8/2025 12:20 AM, 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)
>>> +#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.
>>> +     */
>>> +    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;
>>> +}
>>> +
>>> +/*
>>> + * 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);
>>                       ^^^^
>> should we use ps->page_snapshot here?
>> IIUC, page may change due to race.
> 
> You're right the race exists, but we can't make foliop point to the
> snapshot because we do pointer arithmetic and other operations that need
> the real page memory address to work properly.

While working on v3 I realized it should be fine (and correct) to use
the head pointer from ps->page_snapshot like the original __dump_page()
does (assuming this is what you meant for me to change). So, I changed
this for v3.

> 
>>> +        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;
>>
>> Should we use clear_compound_head() for clarity?
> 
> Yes, we can do that.
> 
> Andrew, since you applied this series already, can you squash the change
> below into this patch? If not, I can send v3.
> 
> diff --git a/mm/util.c b/mm/util.c
> index c38d213be83f..ce4200529772 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -1239,7 +1239,7 @@ void snapshot_page(struct page_snapshot *ps, const struct page *page)
>       if (ps->idx > nr_pages) {
>           if (loops-- > 0)
>               goto again;
> -        ps->page_snapshot.compound_head &= ~1UL;
> +        clear_compound_head(&ps->page_snapshot);
>           foliop = (struct folio *)&ps->page_snapshot;
>           ps->flags = 0;
>           ps->idx = 0;