----- On Feb 10, 2021, at 12:09 PM, Greg Kroah-Hartman gregkh@linuxfoundation.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.

Thanks,

Mathieu

thanks,

greg k-h

On Wed, Feb 10, 2021 at 01:00:25PM -0500, Sasha Levin wrote:

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.

I've queued a backport of that patch (via two prereq patches), thanks!