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

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Message-Id: <200708202055.03058.nigel@nigel.suspend2.net>
Date:	Mon, 20 Aug 2007 20:55:01 +1000
From:	Nigel Cunningham <nigel@...el.suspend2.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Should GFP_ATOMIC fail when we're below low watermark?

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.

Nigel
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