linux-kernel - Re: [PATCH] slub: correctly bootstrap boot caches

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Date:	Fri, 22 Feb 2013 19:09:06 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Christoph Lameter <cl@...ux.com>
CC:	<linux-mm@...ck.org>, <cgroups@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Pekka Enberg <penberg@...nel.org>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH] slub: correctly bootstrap boot caches

On 02/22/2013 07:00 PM, Christoph Lameter wrote:
> On Fri, 22 Feb 2013, Glauber Costa wrote:
> 
>> Although not verified in practice, I also point out that it is not safe to scan
>> the full list only when debugging is on in this case. As unlikely as it is, it
>> is theoretically possible for the pages to be full. If they are, they will
>> become unreachable. Aside from scanning the full list, we also need to make
>> sure that the pages indeed sit in there: the easiest way to do it is to make
>> sure the boot caches have the SLAB_STORE_USER debug flag set.
> 
> SLAB_STORE_USER typically increases the size of the managed object. It is
> not available when slab debugging is not compiled in. There is no list of
> full slab objects that is maintained in the non debug case and if the
> allocator is compiled without debug support also the code to manage full
> lists will not be present.
> 
> Only one or two kmem_cache item is allocated in the bootstrap code and so
> far the size of the objects was signficantly smaller than page size. So
> the slab pages will be on the partial lists. Why are your slab management
> structures so large that a page can no longer contain multiple objects?
> 
They are not.

As I've mentioned in the description, the real bug is from partial slabs
being temporarily in the cpu_slab during a recent allocation and
therefore unreachable through the partial list.

I've just read the code, and it seemed to me that theoretically that
could happen. I agree with you that this is an unlikely scenario and if
you prefer I can resend the patch without that part.

Would that be preferable ?

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