linux-kernel - Re: 2.6.22 -mm merge plans: slub

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Date:	Thu, 3 May 2007 01:15:15 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <clameter@....com>
Cc:	Hugh Dickins <hugh@...itas.com>, haveblue@....com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: 2.6.22 -mm merge plans: slub

On Wed, 2 May 2007 10:25:47 -0700 (PDT) Christoph Lameter <clameter@....com> wrote:

> On Wed, 2 May 2007, Hugh Dickins wrote:
> 
> > I presume the answer is just to extend your quicklist work to
> > powerpc's lowest level of pagetables.  The only other architecture
> > which is using kmem_cache for them is arm26, which has
> > "#error SMP is not supported", so won't be giving this problem.
> 
> In the meantime we would need something like this to disable SLUB in this 
> particular configuration. Note that I have not tested this and the <= for
> the comparision with SPLIT_PTLOCK_CPUS may not work (Never seen such a
> construct in a Kconfig file but it is needed here).
> 
> 
> 
> PowerPC: Disable SLUB for configurations in which slab page structs are modified
> 
> PowerPC uses the slab allocator to manage the lowest level of the page table.
> In high cpu configurations we also use the page struct to split the page
> table lock. Disallow the selection of SLUB for that case.
> 
> [Not tested: I am not familiar with powerpc build procedures etc]
> 
> Signed-off-by: Christoph Lameter <clameter@....com>
> 
> Index: linux-2.6.21-rc7-mm2/arch/powerpc/Kconfig
> ===================================================================
> --- linux-2.6.21-rc7-mm2.orig/arch/powerpc/Kconfig	2007-05-02 10:07:34.000000000 -0700
> +++ linux-2.6.21-rc7-mm2/arch/powerpc/Kconfig	2007-05-02 10:13:37.000000000 -0700
> @@ -117,6 +117,19 @@ config GENERIC_BUG
>  	default y
>  	depends on BUG
>  
> +#
> +# Powerpc uses the slab allocator to manage its ptes and the
> +# page structs of ptes are used for splitting the page table
> +# lock for configurations supporting more than SPLIT_PTLOCK_CPUS.
> +#
> +# In that special configuration the page structs of slabs are modified.
> +# This setting disables the selection of SLUB as a slab allocator.
> +#
> +config ARCH_USES_SLAB_PAGE_STRUCT
> +	bool
> +	default y
> +	depends on SPLIT_PTLOCK_CPUS <= NR_CPUS
> +

That all seems to work as intended.

However with NR_CPUS=8 SPLIT_PTLOCK_CPUS=4, enabling SLUB=y crashes the
machine early in boot.  

Too early for netconsole, no serial console.  Wedges up uselessly with
CONFIG_XMON=n, does mysterious repeated uncontrollable exceptions with
CONFIG_XMON=y.  This is all fairly typical for a powerpc/G5 crash :(

However I was able to glimpse some stuff as it flew past.  Crash started in
flush_old_exec and ended in pgtable_free_tlb -> kmem_cache_free.  I don't know
how to do better than that I'm afraid, unless I'm to hunt down a PCIE serial
card, perhaps.

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