linux-ext4 - Re: [patch 1/5] mm: add nofail variants of kmalloc kcalloc and kzalloc

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Date:	Wed, 25 Aug 2010 13:43:34 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Theodore Tso <tytso@....edu>, Jens Axboe <jaxboe@...ionio.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Neil Brown <neilb@...e.de>, Alasdair G Kergon <agk@...hat.com>,
	Chris Mason <chris.mason@...cle.com>,
	Steven Whitehouse <swhiteho@...hat.com>,
	Jan Kara <jack@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>,
	"linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
	"cluster-devel@...hat.com" <cluster-devel@...hat.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"reiserfs-devel@...r.kernel.org" <reiserfs-devel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [patch 1/5] mm: add nofail variants of kmalloc kcalloc and
 kzalloc

On Wed, 25 Aug 2010, Peter Zijlstra wrote:

> I'm not sure, but I think the cgroup thing doesn't account kernel
> allocations, in which case the above problem doesn't exist.
> 

Right, the only cgroup that does is cpusets because it binds the memory 
allocations to a set of nodes.

> For the cpuset case we punch through the cpuset constraints for kernel
> allocations (unless __GFP_HARDWALL).
> 

__GFP_HARDWALL doesn't mean that the allocation won't be constrained, this 
is a common misconception.  __GFP_HARDWALL only prevents us from looking 
at our cpuset.mem_exclusive flag and checking our nearest common ancestor 
cpuset if we can block.

The cpusets case is actually the easiest to fix: use GFP_ATOMIC.  
GFP_ATOMIC allocations aren't bound by any cpuset and, in the general 
case, can allocate below the min watermark because of 
ALLOC_HARD | ALLOC_HARDER in the page allocator which creates the notion 
of "memory reserves" available to these tasks.  Then, success really 
depends on the setting of the watermarks instead.
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