linux-kernel - Re: [PATCH] slub: Don't panic for memcg kmem cache creation failure

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Date:   Thu, 20 Jun 2019 17:35:16 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Christoph Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH] slub: Don't panic for memcg kmem cache creation failure

On Thu 20-06-19 07:44:27, Shakeel Butt wrote:
> On Wed, Jun 19, 2019 at 10:50 PM Michal Hocko <mhocko@...nel.org> wrote:
> >
> > On Wed 19-06-19 16:25:14, Shakeel Butt wrote:
> > > Currently for CONFIG_SLUB, if a memcg kmem cache creation is failed and
> > > the corresponding root kmem cache has SLAB_PANIC flag, the kernel will
> > > be crashed. This is unnecessary as the kernel can handle the creation
> > > failures of memcg kmem caches.
> >
> > AFAICS it will handle those by simply not accounting those objects
> > right?
> >
> 
> The memcg kmem cache creation is async. The allocation has already
> been decided not to be accounted on creation trigger. If memcg kmem
> cache creation is failed, it will fail silently and the next
> allocation will trigger the creation process again.

Ohh, right I forgot that it will get retried. This would be useful to
mention in the changelog as it is not straightforward from reading just
the particular function.

> > > Additionally CONFIG_SLAB does not
> > > implement this behavior. So, to keep the behavior consistent between
> > > SLAB and SLUB, removing the panic for memcg kmem cache creation
> > > failures. The root kmem cache creation failure for SLAB_PANIC correctly
> > > panics for both SLAB and SLUB.
> >
> > I do agree that panicing is really dubious especially because it opens
> > doors to shut the system down from a restricted environment. So the
> > patch makes sesne to me.
> >
> > I am wondering whether SLAB_PANIC makes sense in general though. Why is
> > it any different from any other essential early allocations? We tend to
> > not care about allocation failures for those on bases that the system
> > must be in a broken state to fail that early already. Do you think it is
> > time to remove SLAB_PANIC altogether?
> >
> 
> That would need some investigation into the history of SLAB_PANIC. I
> will look into it.

Well, I strongly suspect this is a relict from the past. I have hard
time to believe that the system would get to a usable state if many of
those caches would fail to allocate. And as Dave said in his reply it is
quite silly to give this weapon to a random driver hands. Everybody just
thinks his toy is the most important one...

-- 
Michal Hocko
SUSE Labs