linux-kernel - Re: [RFC 04/11] mm: remove SWAP_MLOCK check for SWAP

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Date:   Tue, 7 Mar 2017 17:26:43 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, kernel-team@....com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [RFC 04/11] mm: remove SWAP_MLOCK check for SWAP_SUCCESS in ttu

On Thu, Mar 02, 2017 at 03:39:18PM +0900, Minchan Kim wrote:
> If the page is mapped and rescue in ttuo, page_mapcount(page) == 0 cannot
> be true so page_mapcount check in ttu is enough to return SWAP_SUCCESS.
> IOW, SWAP_MLOCK check is redundant so remove it.
> 
> Signed-off-by: Minchan Kim <minchan@...nel.org>
> ---
>  mm/rmap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 3a14013..0a48958 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1523,7 +1523,7 @@ int try_to_unmap(struct page *page, enum ttu_flags flags)
>  	else
>  		ret = rmap_walk(page, &rwc);
>  
> -	if (ret != SWAP_MLOCK && !page_mapcount(page))
> +	if (!page_mapcount(page))

Hm. I think there's bug in current code.
It should be !total_mapcount(page) otherwise it can be false-positive if
there's THP mapped with PTEs.

And in this case ret != SWAP_MLOCK is helpful to cut down some cost.
Althouth it should be fine to remove it, I guess.

-- 
 Kirill A. Shutemov