Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 21 Jun 2018 01:15:30 +0000
From:   Christopher Lameter <cl@linux.com>
To:     Shakeel Butt <shakeelb@google.com>
cc:     "Jason A . Donenfeld" <Jason@zx2c4.com>,
        David Rientjes <rientjes@google.com>,
        Pekka Enberg <penberg@kernel.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH] slub: track number of slabs irrespective of
 CONFIG_SLUB_DEBUG
In-Reply-To: <20180620224147.23777-1-shakeelb@google.com>
Message-ID: <010001641fe92599-9006a895-d1ea-4881-a63c-f3749ff9b7b3-000000@email.amazonses.com>
References: <20180620224147.23777-1-shakeelb@google.com>
User-Agent: Alpine 2.21 (DEB 202 2017-01-01)
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Feedback-ID: 1.us-east-1.fQZZZ0Xtj2+TD7V5apTT/NrT6QKuPgzCT/IC7XYgDKI=:AmazonSES
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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?