linux-kernel - Re: [15/17] SLUB: Support virtual fallback via SLAB

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Message-Id: <1191316774.13204.50.camel@twins>
Date:	Tue, 02 Oct 2007 11:19:34 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Christoph Lameter <clameter@....com>, nickpiggin@...oo.com.au,
	hch@....de, mel@...net.ie, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, dgc@....com, jens.axboe@...cle.com
Subject: Re: [15/17] SLUB: Support virtual fallback via SLAB_VFALLBACK

On Mon, 2007-10-01 at 14:30 -0700, Andrew Morton wrote:
> On Mon, 1 Oct 2007 13:55:29 -0700 (PDT)
> Christoph Lameter <clameter@....com> wrote:
> 
> > On Sat, 29 Sep 2007, Andrew Morton wrote:
> > 
> > > > atomic allocations. And with SLUB using higher order pages, atomic !0
> > > > order allocations will be very very common.
> > > 
> > > Oh OK.
> > > 
> > > I thought we'd already fixed slub so that it didn't do that.  Maybe that
> > > fix is in -mm but I don't think so.
> > > 
> > > Trying to do atomic order-1 allocations on behalf of arbitray slab caches
> > > just won't fly - this is a significant degradation in kernel reliability,
> > > as you've very easily demonstrated.
> > 
> > Ummm... SLAB also does order 1 allocations. We have always done them.
> > 
> > See mm/slab.c
> > 
> > /*
> >  * Do not go above this order unless 0 objects fit into the slab.
> >  */
> > #define BREAK_GFP_ORDER_HI      1
> > #define BREAK_GFP_ORDER_LO      0
> > static int slab_break_gfp_order = BREAK_GFP_ORDER_LO;
> 
> Do slab and slub use the same underlying page size for each slab?
> 
> Single data point: the CONFIG_SLAB boxes which I have access to here are
> using order-0 for radix_tree_node, so they won't be failing in the way in
> which Peter's machine is.
> 
> I've never ever before seen reports of page allocation failures in the
> radix-tree node allocation code, and that's the bottom line.  This is just
> a drop-dead must-fix show-stopping bug.  We cannot rely upon atomic order-1
> allocations succeeding so we cannot use them for radix-tree nodes.  Nor for
> lots of other things which we have no chance of identifying.
> 
> Peter, is this bug -mm only, or is 2.6.23 similarly failing?

I'm mainly using -mm (so you have at least one tester :-), I think the
-mm specific SLUB patch that ups slub_min_order makes the problem -mm
specific, would have to test .23.

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