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

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Message-Id: <20071001143047.238dfe49.akpm@linux-foundation.org>
Date:	Mon, 1 Oct 2007 14:30:47 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <clameter@....com>
Cc:	a.p.zijlstra@...llo.nl, 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, 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?
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