linux-kernel - Re: [PATCH 04/10] mm/ksm: add identical

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Date: Tue, 4 Jun 2024 10:25:52 +0200
From: David Hildenbrand <david@...hat.com>
To: alexs@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 izik.eidus@...ellosystems.com, willy@...radead.org, aarcange@...hat.com,
 chrisw@...s-sol.org, hughd@...gle.com
Subject: Re: [PATCH 04/10] mm/ksm: add identical_folio func

On 04.06.24 06:24, alexs@...nel.org wrote:
> From: "Alex Shi (tencent)" <alexs@...nel.org>
> 
> To replace pages_identical() func and further reduce the 'page' using in
> try_to_merge_one_page function.
> 
> Signed-off-by: Alex Shi (tencent) <alexs@...nel.org>
> ---
>   include/linux/mm.h | 17 +++++++++++++++++
>   mm/ksm.c           |  2 +-
>   2 files changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 9849dfda44d4..2d1f16ffc6ac 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4164,6 +4164,23 @@ static inline void setup_nr_node_ids(void) {}
>   
>   extern int memcmp_pages(struct page *page1, struct page *page2);
>   
> +static inline int identical_folio(struct folio *folio1, struct folio *folio2)
> +{
> +	int i, nr1, nr2, ret = 0;
> +
> +	nr1 = folio_nr_pages(folio1);
> +	nr2 = folio_nr_pages(folio2);
> +	if (nr1 != nr2)
> +		return 0;
> +
> +	for (i = 0; i < nr1; i++) {
> +		ret = memcmp_pages(folio_page(folio1, i), folio_page(folio2, i));
> +		if (ret)
> +			break;
> +	}
> +
> +	return !ret;
> +}

We never get there with large folios. So handling large folios is 
completely unnecessary.

Keep it simple: either VM_WARN_ON_ONCE when a large folio is passed, or 
simply return "false". Then, check only a single page.

-- 
Cheers,

David / dhildenb