linux-kernel - Re: [PATCH 09/15] mm/swap: avoid calling swp_swap_info when try to check SWP_STABLE

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Date:   Tue, 10 May 2022 10:28:04 +1000
From:   "NeilBrown" <neilb@...e.de>
To:     "Miaohe Lin" <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, willy@...radead.org, vbabka@...e.cz,
        dhowells@...hat.com, apopple@...dia.com, david@...hat.com,
        surenb@...gle.com, peterx@...hat.com, naoya.horiguchi@....com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linmiaohe@...wei.com
Subject: Re: [PATCH 09/15] mm/swap: avoid calling swp_swap_info when try to
 check SWP_STABLE_WRITES

On Mon, 09 May 2022, Miaohe Lin wrote:
> Use flags of si directly to check SWP_STABLE_WRITES to avoid possible
> READ_ONCE and thus save some cpu cycles.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/memory.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 9c3e7e6ac202..89dd15504f3d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3892,7 +3892,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  			 */
>  			exclusive = true;
>  		} else if (exclusive && PageWriteback(page) &&
> -			  (swp_swap_info(entry)->flags & SWP_STABLE_WRITES)) {
> +			  (si->flags & SWP_STABLE_WRITES)) {

Should this have a data_race() annotation.  Other bit-tests of si->flags
do because scan_swap_map_slots can update it asynchronously, but we know
that won't matter in practice.

Thanks,
NeilBrown


>  			/*
>  			 * This is tricky: not all swap backends support
>  			 * concurrent page modifications while under writeback.
> -- 
> 2.23.0
> 
>