linux-kernel - Re: WARNING bisected (was Re: [PATCH v7 08/10] mm: rework non-root kmem

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Message-Id: <755e5365-3d98-10c0-ff8a-71dec992148d@de.ibm.com>
Date:   Fri, 22 Nov 2019 14:00:21 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Roman Gushchin <guro@...com>, Rik van Riel <riel@...com>
Cc:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "hannes@...xchg.org" <hannes@...xchg.org>,
        Kernel Team <Kernel-team@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "longman@...hat.com" <longman@...hat.com>,
        "shakeelb@...gle.com" <shakeelb@...gle.com>,
        "vdavydov.dev@...il.com" <vdavydov.dev@...il.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: WARNING bisected (was Re: [PATCH v7 08/10] mm: rework non-root
 kmem_cache lifecycle management)



On 21.11.19 23:09, Roman Gushchin wrote:
> On Thu, Nov 21, 2019 at 12:55:28PM -0800, Roman Gushchin wrote:
>> On Thu, Nov 21, 2019 at 12:43:01PM -0800, Rik van Riel wrote:
>>> On Thu, 2019-11-21 at 13:45 -0500, Roman Gushchin wrote:
>>>> On Thu, Nov 21, 2019 at 05:59:54PM +0100, Christian Borntraeger
>>>> wrote:
>>>>>
>>>>>
>>>>> Yes, rmmod has to be called directly after the guest shutdown to
>>>>> see the issue.
>>>>> See my 2nd mail.
>>>>
>>>> I see. Do you know, which kmem_cache it is? If not, can you, please,
>>>> figure it out?

I can try to figure out.

>>>>
>>>> I tried to reproduce the issue, but wasn't successful so far. So I
>>>> wonder
>>>> what can make your case special.
>>>
>>> I do not know either, but have a guess.
>>>
>>> My guess would be that either the slab object or the
>>> slab page is RCU freed, and the kmem_cache destruction
>>> is called before that RCU callback has completed.
>>>
>>
>> I've a reproducer, but it requires SLAB_TYPESAFE_BY_RCU to panic.
>> The only question is if it's the same or different issues.
>> As soon as I'll have a fix, I'll post it here to test.
> 
> Ah, no, the issue I've reproduced is already fixed by commit b749ecfaf6c5
> ("mm: memcg/slab: fix panic in __free_slab() caused by premature memcg pointer release").
> 
> Christian, can you, please, confirm that you have this one in your tree?

The problem still reproduces on the latest next kernel.
> 
> Also, can you, please, provide you config?

I can but this is an s390 config. 

> And you mentioned some panics, but didn't send any dmesg messages.
> Can you, please, provide them?

Those were all the WARN_ONs from percpu_ref_exit, I just had panic_on_warn enabled.

I attached my config and dmesg just in case for the next kernel of today.

View attachment "config.txt" of type "text/plain" (79462 bytes)

View attachment "dmesg.txt" of type "text/plain" (56836 bytes)