linux-kernel - Re: [PATCH] slub: track number of slabs irrespective of CONFIG_SLUB

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Date:   Thu, 21 Jun 2018 01:15:30 +0000
From:   Christopher Lameter <cl@...ux.com>
To:     Shakeel Butt <shakeelb@...gle.com>
cc:     "Jason A . Donenfeld" <Jason@...c4.com>,
        David Rientjes <rientjes@...gle.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] slub: track number of slabs irrespective of
 CONFIG_SLUB_DEBUG

On Wed, 20 Jun 2018, Shakeel Butt wrote:

> For !CONFIG_SLUB_DEBUG, SLUB does not maintain the number of slabs
> allocated per node for a kmem_cache. Thus, slabs_node() in
> __kmem_cache_empty(), __kmem_cache_shrink() and __kmem_cache_destroy()
> will always return 0 for such config. This is wrong and can cause issues
> for all users of these functions.


CONFIG_SLUB_DEBUG is set by default on almost all builds. The only case
where CONFIG_SLUB_DEBUG is switched off is when we absolutely need to use
the minimum amount of memory (embedded or some such thing).

> The right solution is to make slabs_node() work even for
> !CONFIG_SLUB_DEBUG. The commit 0f389ec63077 ("slub: No need for per node
> slab counters if !SLUB_DEBUG") had put the per node slab counter under
> CONFIG_SLUB_DEBUG because it was only read through sysfs API and the
> sysfs API was disabled on !CONFIG_SLUB_DEBUG. However the users of the
> per node slab counter assumed that it will work in the absence of
> CONFIG_SLUB_DEBUG. So, make the counter work for !CONFIG_SLUB_DEBUG.

Please do not do this. Find a way to avoid these checks. The
objective of a !CONFIG_SLUB_DEBUG configuration is to not compile in
debuggin checks etc etc in order to reduce the code/data footprint to the
minimum necessary while sacrificing debuggability etc etc.

Maybe make it impossible to disable CONFIG_SLUB_DEBUG if CGROUPs are in
use?