linux-kernel - Re: [PATCH v4] mm: SLAB freelist randomization

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Date:	Wed, 27 Apr 2016 10:39:29 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Thomas Garnier <thgarnie@...gle.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Kees Cook <keescook@...omium.org>, gthelen@...gle.com,
	labbott@...oraproject.org, kernel-hardening@...ts.openwall.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v4] mm: SLAB freelist randomization

On Tue, 26 Apr 2016, Andrew Morton wrote:

> : CONFIG_FREELIST_RANDOM bugs me a bit - "freelist" is so vague.
> : CONFIG_SLAB_FREELIST_RANDOM would be better.  I mean, what Kconfig
> : identifier could be used for implementing randomisation in
> : slub/slob/etc once CONFIG_FREELIST_RANDOM is used up?
>
> but this pearl appeared to pass unnoticed.

Ok. lets add SLAB here and then use this option for the other allocators
as well.

> > +	/* If it fails, we will just use the global lists */
> > +	cachep->random_seq = kcalloc(count, sizeof(freelist_idx_t), GFP_KERNEL);
> > +	if (!cachep->random_seq)
> > +		return -ENOMEM;
>
> OK, no BUG.  If this happens, kmem_cache_init_late() will go BUG
> instead ;)
>
> Questions for slab maintainers:
>
> What's going on with the gfp_flags in there?  kmem_cache_init_late()
> passes GFP_NOWAIT into enable_cpucache().
>
> a) why the heck does it do that?  It's __init code!

enable_cpucache() was called when a slab cache was reconfigured by writing to /proc/slabinfo.
That was changed awhile back when the memcg changes were made ot slab. So
now its ok to be made init code.

> Finally, all callers of enable_cpucache() (and hence of
> cache_random_seq_create()) are __init, so we're unnecessarily bloating
> up vmlinux.  Could someone please take a look at this as a separate
> thing?

Hmmm. Well if that is the case then lots of stuff could be straightened
out. Joonsoo?