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

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Message-ID: <CALvZod7Rf0FZHqYBPd1OTkVuvA5QRrkYQku40QJtS2--g6PrQQ@mail.gmail.com>
Date:   Fri, 22 Jun 2018 08:33:40 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Christoph Lameter <cl@...ux.com>,
        "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 <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, stable@...r.kernel.org
Subject: Re: [PATCH] slub: track number of slabs irrespective of CONFIG_SLUB_DEBUG

On Thu, Jun 21, 2018 at 8:01 AM Michal Hocko <mhocko@...nel.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