Re: [PATCH] Revert "mm: vmscan: fix misused nr_reclaimed in shrink_mem_cgroup_zone()"

[Andrew Morton <akpm@linux-foundation.org>]

On Mon,  9 Apr 2012 12:42:04 -0700
Ying Han <yinghan@google.com> wrote:

> This reverts commit c38446cc65e1f2b3eb8630c53943b94c4f65f670.
> 
> Before the commit, the code makes senses to me but not after the commit. The
> "nr_reclaimed" is the number of pages reclaimed by scanning through the memcg's
> lru lists. The "nr_to_reclaim" is the target value for the whole function. For
> example, we like to early break the reclaim if reclaimed 32 pages under direct
> reclaim (not DEF_PRIORITY).
> 
> After the reverted commit, the target "nr_to_reclaim" is decremented each time
> by "nr_reclaimed" but we still use it to compare the "nr_reclaimed". It just
> doesn't make sense to me...
> 
> Signed-off-by: Ying Han <yinghan@google.com>
> ---
>  mm/vmscan.c |    7 +------
>  1 files changed, 1 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 33c332b..1a51868 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2107,12 +2107,7 @@ restart:
>  		 * with multiple processes reclaiming pages, the total
>  		 * freeing target can get unreasonably large.
>  		 */
> -		if (nr_reclaimed >= nr_to_reclaim)
> -			nr_to_reclaim = 0;
> -		else
> -			nr_to_reclaim -= nr_reclaimed;
> -
> -		if (!nr_to_reclaim && priority < DEF_PRIORITY)
> +		if (nr_reclaimed >= nr_to_reclaim && priority < DEF_PRIORITY)
>  			break;
>  	}
>  	blk_finish_plug(&plug);

This code is all within a loop: the "goto restart" thing.  We reset
nr_reclaimed to zero each time around that loop.  nr_to_reclaim is (or
rather, was) constant throughout the entire function.

Comparing nr_reclaimed (whcih is reset each time around the loop) to
nr_to_reclaim made no sense.

I think the code as it stands is ugly.  It would be better to make
nr_to_reclaim a const and to add another local total_reclaimed, and
compare that with nr_to_reclaim.  Or just stop resetting nr_reclaimed
each time around the loop.

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