Re: [PATCH mmotm] vmscan: handle may_swap more strictly (Re: [PATCH mmotm] vmscan: fix may_swap handling for memcg)

[Minchan Kim <minchan.kim@gmail.com>]

Hi, KOSAKI.

As you know, this problem caused by if condition(priority) in shrink_zone.
Let me have a question.

Why do we have to prevent scan value calculation when the priority is zero ?
As I know, before split-lru, we didn't do it.

Is there any specific issue in case of the priority is zero ?

On Tue, Jun 9, 2009 at 4:20 PM, KOSAKI
Motohiro<kosaki.motohiro@jp.fujitsu.com> wrote:
>> > > and, too many recliaming pages is not only memcg issue. I don't think this
>> > > patch provide generic solution.
>> > >
>> > Ah, you're right. It's not only memcg issue.
>> >
>> How about this one ?
>>
>> ===
>> From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
>>
>> Commit 2e2e425989080cc534fc0fca154cae515f971cf5 ("vmscan,memcg: reintroduce
>> sc->may_swap) add may_swap flag and handle it at get_scan_ratio().
>>
>> But the result of get_scan_ratio() is ignored when priority == 0,
>> so anon lru is scanned even if may_swap == 0 or nr_swap_pages == 0.
>> IMHO, this is not an expected behavior.
>>
>> As for memcg especially, because of this behavior many and many pages are
>> swapped-out just in vain when oom is invoked by mem+swap limit.
>>
>> This patch is for handling may_swap flag more strictly.
>>
>> Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
>
> Looks great.
> your patch doesn't only improve memcg, bug also improve noswap system.
>
> Thanks.
>        Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
>
>
>
>> ---
>>  mm/vmscan.c |   18 +++++++++---------
>>  1 files changed, 9 insertions(+), 9 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 2ddcfc8..bacb092 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1407,13 +1407,6 @@ static void get_scan_ratio(struct zone *zone, struct scan_control *sc,
>>       unsigned long ap, fp;
>>       struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc);
>>
>> -     /* If we have no swap space, do not bother scanning anon pages. */
>> -     if (!sc->may_swap || (nr_swap_pages <= 0)) {
>> -             percent[0] = 0;
>> -             percent[1] = 100;
>> -             return;
>> -     }
>> -
>>       anon  = zone_nr_pages(zone, sc, LRU_ACTIVE_ANON) +
>>               zone_nr_pages(zone, sc, LRU_INACTIVE_ANON);
>>       file  = zone_nr_pages(zone, sc, LRU_ACTIVE_FILE) +
>> @@ -1511,15 +1504,22 @@ static void shrink_zone(int priority, struct zone *zone,
>>       enum lru_list l;
>>       unsigned long nr_reclaimed = sc->nr_reclaimed;
>>       unsigned long swap_cluster_max = sc->swap_cluster_max;
>> +     int noswap = 0;
>>
>> -     get_scan_ratio(zone, sc, percent);
>> +     /* If we have no swap space, do not bother scanning anon pages. */
>> +     if (!sc->may_swap || (nr_swap_pages <= 0)) {
>> +             noswap = 1;
>> +             percent[0] = 0;
>> +             percent[1] = 100;
>> +     } else
>> +             get_scan_ratio(zone, sc, percent);
>>
>>       for_each_evictable_lru(l) {
>>               int file = is_file_lru(l);
>>               unsigned long scan;
>>
>>               scan = zone_nr_pages(zone, sc, l);
>> -             if (priority) {
>> +             if (priority || noswap) {
>>                       scan >>= priority;
>>                       scan = (scan * percent[file]) / 100;
>>               }
>
>
>
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-- 
Kinds regards,
Minchan Kim

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