Re: [PATCH 1/3] mm: introduce snapshot_page()

[Luiz Capitulino]

On 2025-06-26 17:39, Andrew Morton wrote:
> On Thu, 26 Jun 2025 14:16:51 -0400 Luiz Capitulino <luizcap@xxxxxxxxxx> 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.