linux-kernel - Re: [patch] vmscan: fix zone shrinking exit when scan work is done

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Date:	Wed, 9 Feb 2011 12:05:50 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Mel Gorman <mel@....ul.ie>, Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>,
	Michal Hocko <mhocko@...e.cz>,
	Kent Overstreet <kent.overstreet@...il.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [patch] vmscan: fix zone shrinking exit when scan work is done

On Wed, 9 Feb 2011 19:28:46 +0100
Andrea Arcangeli <aarcange@...hat.com> wrote:

> On Wed, Feb 09, 2011 at 04:46:56PM +0000, Mel Gorman wrote:
> > On Wed, Feb 09, 2011 at 04:46:06PM +0100, Johannes Weiner wrote:
> > > Hi,
> > > 
> > > I think this should fix the problem of processes getting stuck in
> > > reclaim that has been reported several times.
> > 
> > I don't think it's the only source but I'm basing this on seeing
> > constant looping in balance_pgdat() and calling congestion_wait() a few
> > weeks ago that I haven't rechecked since. However, this looks like a
> > real fix for a real problem.
> 
> Agreed. Just yesterday I spent some time on the lumpy compaction
> changes after wondering about Michal's khugepaged 100% report, and I
> expected some fix was needed in this area (as I couldn't find any bug
> in khugepaged yet, so the lumpy compaction looked the next candidate
> for bugs).
> 
> I've also been wondering about the !nr_scanned check in
> should_continue_reclaim too but I didn't look too much into the caller
> (I was tempted to remove it all together). I don't see how checking
> nr_scanned can be safe even after we fix the caller to avoid passing
> non-zero values if "goto restart".
> 
> nr_scanned is incremented even for !page_evictable... so it's not
> really useful to insist, just because we scanned something, in my
> view. It looks bogus... So my proposal would be below.
> 
> ====
> Subject: mm: stop checking nr_scanned in should_continue_reclaim
> 
> From: Andrea Arcangeli <aarcange@...hat.com>
> 
> nr_scanned is incremented even for !page_evictable... so it's not
> really useful to insist, just because we scanned something.

So if reclaim has scanned 100% !page_evictable pages,
should_continue_reclaim() can return true and we keep on scanning?

That sounds like it's both good and bad :( Is this actually a problem? 
What sort of behaviour could it cause and under what circumstances?

Johannes's patch is an obvious bugfix and I'll run with it for now, but
please let's have a further think abut the impact of the
!page_evictable pages.

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