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

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Date:	Thu, 27 Sep 2012 10:59:24 +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 09/27/2012 05:16 AM, David Rientjes wrote:
> On Wed, 26 Sep 2012, Glauber Costa wrote:
> 
>> 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 ?
>>
> 
> I would suggest cachep->flags being used solely for the flags passed to 
> kmem_cache_create() and seperating out all "internal flags" based on the 
> individual slab allocator's implementation into a different field.  There 
> should be no problem with moving CFLGS_OFF_SLAB elsewhere, in fact, I just 
> removed a "dflags" field from mm/slab.c's kmem_cache that turned out never 
> to be used.  You could simply reintroduce a new "internal_flags" field and 
> use it at your discretion.
> 
I can do it with you both agree with the approach.

But I still don't see the big reason for your objection. If other
allocator start using those bits, they would not be passed to
kmem_cache_alloc anyway, right? So what would be the big problem in
masking them out before it?
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