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

[Ying Han <yinghan@google.com>]

On Mon, Apr 9, 2012 at 12:50 PM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> 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.

Ok, I will resend the patch w/ the "total_reclaimed" change.

--Ying

Or just stop resetting nr_reclaimed
> each time around the loop.
>

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