linux-kernel - Re: [PATCH] slab: Ignore internal flags in cache creation

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Date:	Wed, 26 Sep 2012 12:43:21 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	Christoph Lameter <cl@...ux.com>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, Michal Hocko <mhocko@...e.cz>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [PATCH] slab: Ignore internal flags in cache creation

On 09/26/2012 04:46 AM, David Rientjes wrote:
> On Tue, 25 Sep 2012, Christoph Lameter wrote:
> 
>>> No cache should ever pass those as a creation flags. We can just ignore
>>> this bit if it happens to be passed (such as when duplicating a cache in
>>> the kmem memcg patches)
>>
>> Acked-by: Christoph Lameter <cl@...ux.com>
>>
> 
> Nack, this is already handled by CREATE_MASK in the mm/slab.c allocator; 
> the flag extensions beyond those defined in the generic slab.h header are 
> implementation defined.  It may be true that SLAB uses a bit only 
> internally (and already protects it with a BUG_ON() in 
> __kmem_cache_create()) but that doesn't mean other implementations can't 
> use such a flag that would be a no-op on another allocator.
> 

So the problem I am facing here is that when I am creating caches from
memcg, I would very much like to reuse their flags fields. They are
stored in the cache itself, so this is not a problem. But slab also
stores that flag, leading to the precise BUG_ON() on CREATE_MASK that
you quoted.

In this context, passing this flag becomes completely valid, I just need
that to be explicitly masked out.

What is your suggestion to handle this ?

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