Re: Occasional 'WARNING: mm/backing-dev.c:636 at cgwb_release_workfn' when running mdadm

On Tue, 26 May 2026 at 15:14, Jan Kara <[email protected]> wrote:
>
> Hi!
>
> On Tue 26-05-26 13:01:11, Luca Boccassi wrote:
> > Hi, I've been told to report this here.
> >
> > In our CI test that runs some mdadm commands we occasionally (maybe 1
> > in 100 runs, not reproducible manually) see the following kernel warn:
> >
> > [ 15.769393] TEST-64-UDEV-STORAGE.sh[245]: + mdadm -v --stop /dev/md/mdparity
> > [ 15.786033] ------------[ cut here ]------------
> > [ 15.788632] !list_empty(&wb->b_attached)
> > [ 15.788634] WARNING: mm/backing-dev.c:636 at
>
> Thanks for report! Interesting. I haven't seen this yet but I suspect my
> rework of cgwb inode switching (happened in 6.18) could be responsible for
> this.
>
> Since the 7.0.11 kernel we've got a couple of fixups to this code. In
> particular 6689f01d6740 ("writeback: Fix use after free in
> inode_switch_wbs_work_fn()") and now also fixes sitting in
> https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git/log/?h=vfs-7.2.writeback
>
> Your script seems to be exercising mount/umount so the above fixes could
> indeed be related to your problem. If you can run with a kernel built from
> the above branch, it would be great if you can verify the problem got
> indeed fixed.

Thanks, it's very occasional so can't reproduce it manually, but once
that new kernel version starts appearing in arch/rawhide, I'll report
back if I see the same warn again