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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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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