linux-kernel - Re: [PATCH 1/3] mm: introduce snapshot

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Message-Id: <20250626143929.a7f2bcbb8e90290f23af452e@linux-foundation.org>
Date: Thu, 26 Jun 2025 14:39:29 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Luiz Capitulino <luizcap@...hat.com>
Cc: david@...hat.com, willy@...radead.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, lcapitulino@...il.com, shivankg@....com
Subject: Re: [PATCH 1/3] mm: introduce snapshot_page()

On Thu, 26 Jun 2025 14:16:51 -0400 Luiz Capitulino <luizcap@...hat.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.