Re: [MD] Crash with 4.12+ kernel and high disk load -- bisected to 4ad23a976413: MD: use per-cpu counter for writes_pending

[David R]

I will apply this to my home server this evening (BST) and set off a  
check. Will have results tomorrow.

Thanks for the fix!

David


Quoting NeilBrown <neilb@xxxxxxxx>:

> On Mon, Aug 07 2017, Dominik Brodowski wrote:
>
> > Neil, Shaohua,
> >
> > following up on David R's bug message: I have observed something similar
> > on v4.12.[345] and v4.13-rc4, but not on v4.11. This is a RAID1 (on bare
> > metal partitions, /dev/sdaX and /dev/sdbY linked together). In case it
> > matters: Further upwards are cryptsetup, a DM volume group, then logical
> > volumes, and then filesystems (ext4, but also happened with xfs).
> >
> > In a tedious bisect (the bug wasn't as quickly reproducible as I would like,
> > but happened when I repeatedly created large lvs and filled them with some
> > content, while compiling kernels in parallel), I was able to track this
> > down to:
> >
> >
> > commit 4ad23a976413aa57fe5ba7a25953dc35ccca5b71
> > Author: NeilBrown <neilb@xxxxxxxx>
> > Date:   Wed Mar 15 14:05:14 2017 +1100
> >
> >     MD: use per-cpu counter for writes_pending
> >
> >     The 'writes_pending' counter is used to determine when the
> >     array is stable so that it can be marked in the superblock
> >     as "Clean".  Consequently it needs to be updated frequently
> >     but only checked for zero occasionally.  Recent changes to
> >     raid5 cause the count to be updated even more often - once
> >     per 4K rather than once per bio.  This provided
> >     justification for making the updates more efficient.
> >
> >     ...
>
> Thanks for the report... and for bisecting and for re-sending...
>
> I believe I have found the problem, and have sent a patch separately.
>
> If mddev->safemode == 1 and mddev->in_sync != 0, md_check_recovery()
> causes the thread that calls it to spin.
> Prior to the patch you found, that couldn't happen.  Now it can,
> so it needs to be handled more carefully.
>
> While I was examining the code, I found another bug - so that is a win!
>
> Thanks,
> NeilBrown