Re: Found the commit that causes the OOMs

[Minchan Kim]

On Sun, Jun 28, 2009 at 12:36 AM, Johannes Weiner<hannes@xxxxxxxxxxx> wrote:
> On Sat, Jun 27, 2009 at 10:50:25PM +0900, Minchan Kim wrote:
>> Hi, Hannes.
>>
>> On Sat, Jun 27, 2009 at 9:54 PM, Johannes Weiner<hannes@xxxxxxxxxxx> wrote:
>> > On Sat, Jun 27, 2009 at 08:12:49AM +0100, David Howells wrote:
>> >>
>> >> I've managed to bisect things to find the commit that causes the OOMs. ÂIt's:
>> >>
>> >> Â Â Â commit 69c854817566db82c362797b4a6521d0b00fe1d8
>> >> Â Â Â Author: MinChan Kim <minchan.kim@xxxxxxxxx>
>> >> Â Â Â Date: Â Tue Jun 16 15:32:44 2009 -0700
>> >>
>> >> Â Â Â Â Â vmscan: prevent shrinking of active anon lru list in case of no swap space V3
>> >>
>> >> Â Â Â Â Â shrink_zone() can deactivate active anon pages even if we don't have a
>> >> Â Â Â Â Â swap device. ÂMany embedded products don't have a swap device. ÂSo the
>> >> Â Â Â Â Â deactivation of anon pages is unnecessary.
>> >>
>> >> Â Â Â Â Â This patch prevents unnecessary deactivation of anon lru pages. ÂBut, it
>> >> Â Â Â Â Â don't prevent aging of anon pages to swap out.
>> >>
>> >> Â Â Â Â Â Signed-off-by: Minchan Kim <minchan.kim@xxxxxxxxx>
>> >> Â Â Â Â Â Acked-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
>> >> Â Â Â Â Â Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
>> >> Â Â Â Â Â Acked-by: Rik van Riel <riel@xxxxxxxxxx>
>> >> Â Â Â Â Â Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> >> Â Â Â Â Â Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>> >>
>> >> This exhibits the problem. ÂThe previous commit:
>> >>
>> >> Â Â Â commit 35282a2de4e5e4e173ab61aa9d7015886021a821
>> >> Â Â Â Author: Brice Goglin <Brice.Goglin@xxxxxxxxxxxx>
>> >> Â Â Â Date: Â Tue Jun 16 15:32:43 2009 -0700
>> >>
>> >> Â Â Â Â Â migration: only migrate_prep() once per move_pages()
>> >>
>> >> survives 16 iterations of the LTP syscall testsuite without exhibiting the
>> >> problem.
>> >
>> > Here is the patch in question:
>> >
>> > diff --git a/mm/vmscan.c b/mm/vmscan.c
>> > index 7592d8e..879d034 100644
>> > --- a/mm/vmscan.c
>> > +++ b/mm/vmscan.c
>> > @@ -1570,7 +1570,7 @@ static void shrink_zone(int priority, struct zone *zone,
>> > Â Â Â Â * Even if we did not try to evict anon pages at all, we want to
>> > Â Â Â Â * rebalance the anon lru active/inactive ratio.
>> > Â Â Â Â */
>> > - Â Â Â if (inactive_anon_is_low(zone, sc))
>> > + Â Â Â if (inactive_anon_is_low(zone, sc) && nr_swap_pages > 0)
>> > Â Â Â Â Â Â Â Âshrink_active_list(SWAP_CLUSTER_MAX, zone, sc, priority, 0);
>> >
>> > Â Â Â Âthrottle_vm_writeout(sc->gfp_mask);
>> >
>> > When this was discussed, I think we missed that nr_swap_pages can
>> > actually get zero on swap systems as well and this should have been
>> > total_swap_pages - otherwise we also stop balancing the two anon lists
>> > when swap is _full_ which was not the intention of this change at all.
>>
>> At that time we considered it so that we didn't prevent anon list
>> aging for background reclaim.
>> Do you think it is not enough ?
>
> With a heavy multiprocess anon load, direct reclaimers will likely
> reuse the reclaimed pages for anon mappings, so you have a handful of
> processes shuffling pages on the active list and only one thread that
> tries to balance. ÂI can imagine that it can not keep up for long.

I agree. :)
total_swap_pages is better than nr_swap_pages although it isn't
related this problem.


>



-- 
Kinds regards,
Minchan Kim
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