linux-kernel - Re: [PATCH 2/2] mm/slub: disable merging after enabling debug in runtime

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Date:	Tue, 14 Jul 2015 23:21:52 +0300
From:	Konstantin Khlebnikov <koct9i@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm/slub: disable merging after enabling debug in runtime

On Tue, Jul 14, 2015 at 9:11 PM, Christoph Lameter <cl@...ux.com> wrote:
> On Tue, 14 Jul 2015, Konstantin Khlebnikov wrote:
>
>> Enabling debug in runtime breaks creation of new kmem caches:
>> they have incompatible flags thus cannot be merged but unique
>> names are taken by existing caches.
>
> What breaks?

The same commands from first patch:

# echo 1 | tee /sys/kernel/slab/*/sanity_checks
# modprobe configfs

loading configfs now fails (without crashing kernel though) because of
"sysfs: cannot create duplicate filename '/kernel/slab/:t-0000096'"

Of course we could rename sysfs entry when enable debug options
but that requires much more code than my "stop merging" solution.

>
> Caches may already have been merged and thus the question is what to do
> about a cache that has multiple aliases if a runtime option is requested.
> The solution that slub implements is to only allow a limited number of
> debug operations to be enabled. Those then will appear to affect all
> aliases of course.
>
> Creating additional caches later may create additional
> aliasing which will then restrict what options can be changed.
>
> Other operations are also restricted depending on the number of objects
> stored in a cache. A cache with zero objects can be easily reconfigured.
> If there are objects then modifications that impact object size are not
> allowed anymore.
>
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