linux-kernel - Re: [PATCH] mm: avoid livelock on !__GFP

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Message-ID: <20111101122940.GC25123@suse.de>
Date:	Tue, 1 Nov 2011 12:29:40 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Colin Cross <ccross@...roid.com>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org
Subject: Re: [PATCH] mm: avoid livelock on !__GFP_FS allocations

On Tue, Oct 25, 2011 at 10:39:34PM +0300, Pekka Enberg wrote:
> Hi Mel,
> 
> On Tue, Oct 25, 2011 at 2:23 PM, Mel Gorman <mgorman@...e.de> wrote:
> > I see what you mean with GFP_NOIO but there is an important difference
> > between GFP_NOIO and suspend.  A GFP_NOIO low-order allocation currently
> > implies __GFP_NOFAIL as commented on in should_alloc_retry(). If no progress
> > is made, we call wait_iff_congested() and sleep for a bit. As the system
> > is running, kswapd and other process activity will proceed and eventually
> > reclaim enough pages for the GFP_NOIO allocation to succeed. In a running
> > system, GFP_NOIO can stall for a period of time but your patch will cause
> > the allocation to fail. While I expect callers return ENOMEM or handle
> > the situation properly with a wait-and-retry loop, there will be
> > operations that fail that used to succeed. This is why I'd prefer it was
> > a suspend-specific fix unless we know there is a case where a machine
> > livelocks due to a GFP_NOIO allocation looping forever and even then I'd
> > wonder why kswapd was not helping.
> 
> I'm not that happy about your patch because it's going to the
> direction where the page allocator is special-casing for suspension.

Suspend really is a special case. While I'd prefer to avoid special
casing it like this, I prefer it a *lot* more than failing GFP_NOIO
allocations that used to succeed.

-- 
Mel Gorman
SUSE Labs
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