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

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Message-ID: <20090628150407.GA25076@localhost>
Date:	Sun, 28 Jun 2009 23:04:08 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Minchan Kim <minchan.kim@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	David Howells <dhowells@...hat.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>,
	"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>,
	"Barnes, Jesse" <jesse.barnes@...el.com>
Subject: Re: Found the commit that causes the OOMs

On Sun, Jun 28, 2009 at 10:49:52PM +0800, KOSAKI Motohiro wrote:
> >> In David's OOM case, there are two symptoms:
> >> 1) 70000 unaccounted/leaked pages as found by Andrew
> >>   (plus rather big number of PG_buddy and pagetable pages)
> >> 2) almost zero active_file/inactive_file; small inactive_anon;
> >>   many slab and active_anon pages.
> >>
> >> In the situation of (2), the slab cache is _under_ scanned. So David
> >> got OOM when vmscan should have squeezed some free pages from the slab
> >> cache. Which is one important side effect of MinChan's patch?
> >
> > My patch's side effect is (2).
> >
> > My guessing is following as.
> >
> > 1. The number of page scanned in shrink_slab is increased in shrink_page_list.
> > And it is doubled for mapped page or swapcache.
> > 2. shrink_page_list is called by shrink_inactive_list
> > 3. shrink_inactive_list is called by shrink_list
> >
> > Look at the shrink_list.
> > If inactive lru list is low, it always call shrink_active_list not
> > shrink_inactive_list in case of anon.
> > It means it doesn't increased sc->nr_scanned.
> > Then shrink_slab can't shrink enough slab pages.
> > So, David OOM have a lot of slab pages and active anon pages.
> >
> > Does it make sense ?
> > If it make sense, we have to change shrink_slab's pressure method.
> > What do you think ?
> 
> I'm confused.
> 
> if system have no swap, get_scan_ratio() always return anon=0%.
> Then, the numver of inactive_anon is not effect to sc.nr_scanned.

You are right. Hehe, so that's not a real side effect.
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