Re: [PATCH] mm: remove incorrect swap_count() from try_to_unuse()

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Wed, 28 Oct 2009 20:34:38 +0000 (GMT)
Hugh Dickins <hugh.dickins@tiscali.co.uk> wrote:

> From: Bo Liu <bo-liu@hotmail.com>
> 
> In try_to_unuse(), swcount is a local copy of *swap_map, including the
> SWAP_HAS_CACHE bit; but a wrong comparison against swap_count(*swap_map),
> which masks off the SWAP_HAS_CACHE bit, succeeded where it should fail.
> 
Ah, okay...

> That had the effect of resetting the mm from which to start searching
> for the next swap page, to an irrelevant mm instead of to an mm in which
> this swap page had been found: which may increase search time by ~20%.
> But we're used to swapoff being slow, so never noticed the slowdown.
> 
> Remove that one spurious use of swap_count(): Bo Liu thought it merely
> redundant, Hugh rewrote the description since it was measurably wrong.
> 
> Signed-off-by: Bo Liu <bo-liu@hotmail.com>
> Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
> Cc: stable@kernel.org

Sorry for my misunderstanding.

Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

> ---
> 
>  mm/swapfile.c |    3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> --- 2.6.32-rc5/mm/swapfile.c	2009-10-05 04:20:31.000000000 +0100
> +++ linux/mm/swapfile.c	2009-10-28 19:31:43.000000000 +0000
> @@ -1151,8 +1151,7 @@ static int try_to_unuse(unsigned int typ
>  				} else
>  					retval = unuse_mm(mm, entry, page);
>  
> -				if (set_start_mm &&
> -				    swap_count(*swap_map) < swcount) {
> +				if (set_start_mm && *swap_map < swcount) {
>  					mmput(new_start_mm);
>  					atomic_inc(&mm->mm_users);
>  					new_start_mm = mm;

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