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

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Message-ID: <alpine.DEB.2.00.1209261813300.7072@chino.kir.corp.google.com>
Date:	Wed, 26 Sep 2012 18:16:11 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Glauber Costa <glommer@...allels.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 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.
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