linux-kernel - Re: [PATCH] Should GFP_ATOMIC fail when we're below low watermark?

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Message-ID: <20070821110223.GA11329@skynet.ie>
Date:	Tue, 21 Aug 2007 12:02:23 +0100
From:	mel@...net.ie (Mel Gorman)
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	nigel@...pend2.net, LKML <linux-kernel@...r.kernel.org>,
	Nick Piggin <piggin@...erone.com.au>
Subject: Re: [PATCH] Should GFP_ATOMIC fail when we're below low watermark?

On (20/08/07 13:06), Peter Zijlstra didst pronounce:
> On Mon, 2007-08-20 at 20:55 +1000, Nigel Cunningham wrote:
> > Hi.
> > 
> > On Monday 20 August 2007 18:59:36 Peter Zijlstra wrote:
> > > On Mon, 2007-08-20 at 18:38 +1000, Nigel Cunningham wrote:
> > > > Hi.
> > > > 
> > > > On Monday 20 August 2007 12:43:50 Peter Zijlstra wrote:
> > > > > On Mon, 2007-08-20 at 11:38 +1000, Nigel Cunningham wrote:
> > > > > > Hi all.
> > > > > > 
> > > > > > In current git (and for a while now), an attempt to allocate memory 
> > with 
> > > > > > GFP_ATOMIC will fail if we're below the low watermark level. The only 
> > way 
> > > > to 
> > > > > > access that memory that I can see (not that I've looked that hard) is 
> > to 
> > > > have 
> > > > > > PF_MEMALLOC set (ie from kswapd). I'm wondering if this behaviour is 
> > > > correct. 
> > > > > > Shouldn't GFP_ATOMIC allocations ignore watermarks too? How about 
> > > > GFP_KERNEL?
> > > > > > 
> > > > > > The following patch is a potential fix for GFP_ATOMIC.
> > > > > 
> > > > > Sorry, no.
> > > > > 
> > > > > GFP_ATOMIC must fail when below the watermark. GFP_KERNEL has __GFP_WAIT
> > > > > and hence can sleep and wait for reclaim so that should not be a problem
> > > > > (usually).
> > > > > 
> > > > > GFP_ATOMIC may not access the reserves because the reserves are needed
> > > > > to get out of OOM deadlocks within the VM. Consider the fact that
> > > > > freeing memory needs memory - if there is no memory free, you cannot
> > > > > free memory and you're pretty much stuck.
> > > > 
> > > > I guess, then, the question should be whether the watermark values are 
> > > > appropriate. Do we need high order allocations watermarked if this is the 
> > > > only purpose, particularly considering that whatever memory is allocated 
> > for 
> > > > this purpose is essentially useless 99.9% of the time?
> > > 
> > > Could you perhaps explain what you're trying to do? No matter what we
> > > do, GFP_ATOMIC will fail eventually, there is only so much one can do
> > > without blocking.
> > > 
> > > As for higher order allocations, until we have a full online defrag
> > > solution those too can fail at any moment (even with __GFP_WAIT).
> > 
> > I was just trying to make hibernation more reliable in sitations where there's 
> > low amounts of memory available. I guess the amount of memory that's reserved 
> > for this has increased, because some users have been reporting issues that 
> > hadn't appeared before. No problem. I'll work around it.
> 
> I think the last time the default reserves were changed was 2.6.12 or
> there about.
> 
> Perhaps Mel fiddled with it in .23-rc ?
> 

I haven't changed how this works as such. When ZONE_MOVABLE is
configured, then the reserves will be higher than normal because of how
sysctl_lowmem_reserve_ratio[] works. But in the normal cases like we're
dealing with here, the ratios remain unchanged.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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