Subject: Re: [PATCH] Should GFP_ATOMIC fail when we're below low watermark?
From: Peter Zijlstra <peterz@infradead.org>
To: nigel@suspend2.net
Cc: LKML <linux-kernel@vger.kernel.org>, Nick Piggin <piggin@cyberone.com.au>,
       Mel Gorman <mel@skynet.ie>
In-Reply-To: <200708202055.03058.nigel@nigel.suspend2.net>
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	 <1187600376.6114.186.camel@twins>
	 <200708202055.03058.nigel@nigel.suspend2.net>
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Date: Mon, 20 Aug 2007 13:06:01 +0200
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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 ?

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