linux-kernel - Re: Found the commit that causes the OOMs

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Date:	Sat, 27 Jun 2009 17:36:30 +0200
From:	Johannes Weiner <hannes@...xchg.org>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	David Howells <dhowells@...hat.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	"riel@...hat.com" <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"tytso@....edu" <tytso@....edu>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"elladan@...imo.com" <elladan@...imo.com>,
	"npiggin@...e.de" <npiggin@...e.de>
Subject: Re: Found the commit that causes the OOMs

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@...xchg.org> 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.  It's:
> >>
> >>       commit 69c854817566db82c362797b4a6521d0b00fe1d8
> >>       Author: MinChan Kim <minchan.kim@...il.com>
> >>       Date:   Tue Jun 16 15:32:44 2009 -0700
> >>
> >>           vmscan: prevent shrinking of active anon lru list in case of no swap space V3
> >>
> >>           shrink_zone() can deactivate active anon pages even if we don't have a
> >>           swap device.  Many embedded products don't have a swap device.  So the
> >>           deactivation of anon pages is unnecessary.
> >>
> >>           This patch prevents unnecessary deactivation of anon lru pages.  But, it
> >>           don't prevent aging of anon pages to swap out.
> >>
> >>           Signed-off-by: Minchan Kim <minchan.kim@...il.com>
> >>           Acked-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> >>           Cc: Johannes Weiner <hannes@...xchg.org>
> >>           Acked-by: Rik van Riel <riel@...hat.com>
> >>           Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> >>           Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
> >>
> >> This exhibits the problem.  The previous commit:
> >>
> >>       commit 35282a2de4e5e4e173ab61aa9d7015886021a821
> >>       Author: Brice Goglin <Brice.Goglin@...-lyon.org>
> >>       Date:   Tue Jun 16 15:32:43 2009 -0700
> >>
> >>           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,
> >         * Even if we did not try to evict anon pages at all, we want to
> >         * rebalance the anon lru active/inactive ratio.
> >         */
> > -       if (inactive_anon_is_low(zone, sc))
> > +       if (inactive_anon_is_low(zone, sc) && nr_swap_pages > 0)
> >                shrink_active_list(SWAP_CLUSTER_MAX, zone, sc, priority, 0);
> >
> >        throttle_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.  I can imagine that it can not keep up for long.
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