linux-kernel - Re: [PATCH v2] mm: Warn about costly page allocation

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Date:	Tue, 10 Jul 2012 18:02:06 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Minchan Kim <minchan@...nel.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH v2] mm: Warn about costly page allocation

On Tue, 10 Jul 2012, Minchan Kim wrote:

> > So I dunno, this all looks like we have a kernel problem and we're
> > throwing our problem onto hopelessly ill-equipped users of that kernel?
> 
> As you know, this patch isn't for solving regular high-order allocations.
> As I wrote down, The problem is that we removed lumpy reclaim without any
> notification for user who might have used it implicitly.

And so now they're running with CONFIG_DEBUG_VM to try to figure out why 
they have seen a regression, which is required for your patch to have an 
effect?

> If such user disable compaction which is a replacement of lumpy reclaim,
> their system might be broken in real practice while test is passing.
> So, the goal is that let them know it in advance so that I expect they can
> test it stronger than old.
> 

So what are they supposed to do?  Enable CONFIG_COMPACTION as soon as they 
see the warning?  When they have seen the warning a specific number of 
times?  How much is "very few" high-order allocations over what time 
period?  This is what anybody seeing these messages for the first time is 
going to ask.

> Although they see the page allocation failure with compaction, it would
> be very helpful reports. It means we need to make compaction more
> aggressive about reclaiming pages.
> 

If CONFIG_COMPACTION is disabled, then how will making compaction more 
aggressive about reclaiming pages help?

Should we consider enabling CONFIG_COMPACTION in defconfig?  If not, would 
it be possible with a different extfrag_threshold (and more aggressive 
when things like THP are enabled)?
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