linux-kernel - Re: [PATCH] slab: implement kmalloc guard

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Date:	Thu, 11 Sep 2014 22:32:52 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Christoph Lameter <cl@...ux.com>
cc:	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	"Alasdair G. Kergon" <agk@...hat.com>,
	Mike Snitzer <msnitzer@...hat.com>,
	Milan Broz <gmazyland@...il.com>, kkolasa@...soft.pl,
	dm-devel@...hat.com
Subject: Re: [PATCH] slab: implement kmalloc guard



On Mon, 8 Sep 2014, Christoph Lameter wrote:

> On Mon, 8 Sep 2014, Mikulas Patocka wrote:
> 
> > I don't know what you mean. If someone allocates 10000 objects with sizes
> > from 1 to 10000, you can't have 10000 slab caches - you can't have a slab
> > cache for each used size. Also - you can't create a slab cache in
> > interrupt context.
> 
> Oh you can create them up front on bootup. And I think only the small
> sizes matter. Allocations >=8K are pushed to the page allocator anyways.

Only for SLUB. For SLAB, large allocations are still use SLAB caches up to 
4M. But anyway - having 8K preallocated slab caches is too much.

If you want to integrate this patch into the slab/slub subsystem, a better 
solution would be to store the exact size requested with kmalloc along the 
slab/slub object itself (before the preceding redzone). But it would 
result in duplicating the work - you'd have to repeat the logic in this 
patch three times - once for slab, once for slub and once for 
kmalloc_large/kmalloc_large_node.

I don't know if it would be better than this patch.

> > > We already have a redzone structure to check for writes over the end of
> > > the object. Lets use that.
> >
> > So, change all three slab subsystems to use that.
> 
> SLOB has no debugging features and I think that was intentional. We are
> trying to unify the debug checks etc. Some work on that would be
> appreciated. I think the kmalloc creation is already in slab_common.c

Mikulas
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