linux-kernel - Re: memcg/kmem panics

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Date:   Wed, 19 Jun 2019 16:04:28 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: memcg/kmem panics

On Wed, Jun 19, 2019 at 3:50 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> I have a bit of a grievance to file.  :)
>
> I'm seeing "Cannot create slab..." panic()s coming from
> kmem_cache_open() when trying to create memory cgroups on a Fedora
> system running 5.2-rc's.  The panic()s happen when failing to create
> memcg-specific slabs because the memcg code passes through the
> root_cache->flags, which can include SLAB_PANIC.
>
> I haven't tracked down the root cause yet, or where this behavior
> started.  But, the end-user experience is that systemd tries to create a
> cgroup and ends up with a kernel panic.  That's rather sad, especially
> for the poor sod that's trying to debug it.
>
> Should memcg_create_kmem_cache() be, perhaps filtering out SLAB_PANIC
> from root_cache->flags, for instance?  That might make the system a bit
> less likely to turn into a doorstop if and when something goes mildly
> wrong.  I've hacked out the panic()s and the system actually seems to
> boot OK.

You must be using CONFIG_SLUB and I see that in kmem_cache_open() in
SLUB doing a SLAB_PANIC check. I think we should remove that
altogether from SLUB as SLAB does not do this and failure in memcg
kmem cache creation can and should be handled gracefully. I can send a
patch to remove that check.

Shakeel