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;
MIME-Version: 1.0
References: <20180620224147.23777-1-shakeelb@google.com> <010001641fe92599-9006a895-d1ea-4881-a63c-f3749ff9b7b3-000000@email.amazonses.com>
 <20180621150122.GB13063@dhcp22.suse.cz>
In-Reply-To: <20180621150122.GB13063@dhcp22.suse.cz>
From:   Shakeel Butt <shakeelb@google.com>
Date:   Fri, 22 Jun 2018 08:33:40 -0700
Message-ID: <CALvZod7Rf0FZHqYBPd1OTkVuvA5QRrkYQku40QJtS2--g6PrQQ@mail.gmail.com>
Subject: Re: [PATCH] slub: track number of slabs irrespective of CONFIG_SLUB_DEBUG
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Christoph Lameter <cl@linux.com>,
        "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 <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>, stable@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, Jun 21, 2018 at 8:01 AM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Thu 21-06-18 01:15:30, Cristopher Lameter wrote:
> > 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).
>
> I thought those would be using SLOB rather than SLUB.
>
> >
> > > 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?
>
> Why don't we simply remove the config option altogether and make it
> enabled effectively.
>

Christopher, how do you want to proceed? I don't have any strong
opinion. I just don't want KASAN users kept broken for SLUB.

thanks,
Shakeel