linux-kernel - Re: [PATCH 5/5] swap: avoid ra statistic lost when swapin races

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Date:   Thu, 8 Dec 2022 19:14:29 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Kairui Song <kasong@...cent.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        David Hildenbrand <david@...hat.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 5/5] swap: avoid ra statistic lost when swapin races

On Fri, Dec 09, 2022 at 02:02:09AM +0800, Kairui Song wrote:
> From: Kairui Song <kasong@...cent.com>
> 
> __read_swap_cache_async should just call swap_cache_get_folio for trying
> to look up the swap cache. Because swap_cache_get_folio handles the
> readahead statistic, and clears the RA flag, looking up the cache
> directly will skip these parts.
> 
> And the comment no longer applies after commit 442701e7058b
> ("mm/swap: remove swap_cache_info statistics"), just remove them.

But what about the readahead stats?

> Fixes: 442701e7058b ("mm/swap: remove swap_cache_info statistics")
> Signed-off-by: Kairui Song <kasong@...cent.com>
> ---
>  mm/swap_state.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index eba388f67741..f39cfb62551d 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -418,15 +418,12 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
>  	for (;;) {
>  		int err;
>  		/*
> -		 * First check the swap cache.  Since this is normally
> -		 * called after swap_cache_get_folio() failed, re-calling
> -		 * that would confuse statistics.
> +		 * First check the swap cache in case of race.
>  		 */
>  		si = get_swap_device(entry);
>  		if (!si)
>  			return NULL;
> -		folio = filemap_get_folio(swap_address_space(entry),
> -						swp_offset(entry));
> +		folio = swap_cache_get_folio(entry, vma, addr);
>  		put_swap_device(si);
>  		if (folio)
>  			return folio_file_page(folio, swp_offset(entry));
> -- 
> 2.35.2
> 
>