linux-kernel - Re: [stable 4.4, 4.9, 4.14, 4.19 LTS] Missing fix "memcg: fix a crash in wb

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Message-ID: <20210210180025.GE4035784@sasha-vm>
Date:   Wed, 10 Feb 2021 13:00:25 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable <stable@...r.kernel.org>, Theodore Tso <tytso@....edu>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Chris Mason <clm@...com>, Tejun Heo <tj@...nel.org>,
        Jens Axboe <axboe@...nel.dk>, rostedt <rostedt@...dmis.org>,
        Michael Jeanson <mjeanson@...icios.com>
Subject: Re: [stable 4.4, 4.9, 4.14, 4.19 LTS] Missing fix "memcg: fix a
 crash in wb_workfn when a device disappears"

On Wed, Feb 10, 2021 at 12:25:31PM -0500, Mathieu Desnoyers wrote:
>----- On Feb 10, 2021, at 12:09 PM, Greg Kroah-Hartman gregkh@...uxfoundation.org wrote:
>
>> On Wed, Feb 10, 2021 at 11:04:19AM -0500, Mathieu Desnoyers wrote:
>>> Hi,
>>>
>>> While reconciling the lttng-modules writeback instrumentation with its
>>> counterpart
>>> within the upstream Linux kernel, I notice that the following commit introduced
>>> in
>>> 5.6 is present in stable branches 5.4 and 5.5, but is missing from LTS stable
>>> branches
>>> for 4.4, 4.9, 4.14, 4.19:
>>>
>>> commit 68f23b89067fdf187763e75a56087550624fdbee
>>> ("memcg: fix a crash in wb_workfn when a device disappears")
>>>
>>> Considering that this fix was CC'd to the stable mailing list, is there any
>>> reason why it has not been integrated into those LTS branches ?
>>
>> Yes, it doesn't apply at all.  If you think this is needed, I will
>> gladly take backported and tested patches.
>>
>> But why do you think this is needed in older kernels?  Have you hit
>> this in real-life?
>
>No, I have not hit this in real-life. Looking at the patch commit message,
>the conditions needed to trigger this issue are very specific: memcg must
>be enabled, and a device must be hotremoved while writeback is going on,
>with writeback tracing active.
>
>AFAIU memcg was present in those LTS releases and devices can be hotremoved
>(please correct me if I'm wrong here), so all the preconditions appear to be
>met.
>
>Considering that I don't have the setup ready to reproduce this issue, I will
>have to defer to the original patch authors for a properly tested backport,
>if it happens to be relevant at all.
>
>I just though reporting what appears to be a missing fix in LTS branches
>would be the right thing to do.

Looks like it doesn't apply due to churn with tracepoints, I think it's
fixable. Let me try and get something for <=4.19.

-- 
Thanks,
Sasha