Re: [PATCH 09/15] mm/swap: avoid calling swp_swap_info when try to check SWP_STABLE_WRITES

["NeilBrown" <neilb@suse.de>]

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@huawei.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
> 
>