linux-kernel - Re: [PATCH] mm-slab: allocate kmem_cache with __GFP

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Date:	Wed, 20 Jul 2011 16:53:56 +0300 (EEST)
From:	Pekka Enberg <penberg@...nel.org>
To:	Konstantin Khlebnikov <khlebnikov@...allels.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Matt Mackall <mpm@...enic.com>,
	"mgorman@...e.de" <mgorman@...e.de>
Subject: Re: [PATCH] mm-slab: allocate kmem_cache with __GFP_REPEAT

On Wed, 20 Jul 2011, Konstantin Khlebnikov wrote:
>>>> The changelog isn't that convincing, really. This is kmem_cache_create()
>>>> so I'm surprised we'd ever get NULL here in practice. Does this fix some
>>>> problem you're seeing? If this is really an issue, I'd blame the page
>>>> allocator as GFP_KERNEL should just work.
>>> 
>>> nf_conntrack creates separate slab-cache for each net-namespace,
>>> this patch of course not eliminates the chance of failure, but makes it 
>>> more
>>> acceptable.
>> 
>> I'm still surprised you are seeing failures. mm/slab.c hasn't changed
>> significantly in a long time. Why hasn't anyone reported this before? I'd
>> still be inclined to shift the blame to the page allocator... Mel,
>> Christoph?
>> 
>> On Wed, 20 Jul 2011, Konstantin Khlebnikov wrote:
>>> struct kmem_size for slub is more compact, it uses pecpu-pointers instead 
>>> of
>>> dumb NR_CPUS-size array.
>>> probably better to fix this side...
>> 
>> So how big is 'struct kmem_cache' for your configuration anyway? Fixing
>> the per-cpu data structures would be nice but I'm guessing it'll be
>> slightly painful for mm/slab.c.
>
> With NR_CPUS=4096 and MAX_NUMNODES=512 its over 9k!
> so it require order-4 page, meanwhile PAGE_ALLOC_COSTLY_ORDER is 3

That's somewhat sad. I suppose I can just merge your patch unless other 
people object to it. I'd like a v2 with better changelog though.

 			Pekka
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