Re: [REGRESSION] 6.7.1: md: raid5 hang and unresponsive system; successfully bisected

[Dan Moulding]

> I think we still want d6e035aad6c0 in 6.7.2. We may need to revert
> 0de40f76d567 on top of that. Could you please test it out? (6.7.1 +
> d6e035aad6c0 + revert 0de40f76d567.

I was operating under the assumption that the two commits were
intended to exist as a pair (the one reverts the old fix, because the
next commit has what is supposed to be a better fix). But since the
regression still exists, even with both patches applied, the old fix
must be reapplied to resolve the current regression.

But, as you've requested, I have tested 6.7.1 + d6e035aad6c0 + revert
0de40f76d567 and it seems fine. So I have no issue if you think it
makes sense to accept d6e035aad6c0 on its own, even though it would
break up the pair of commits.

> OTOH, I am not able to reproduce the issue. Could you please help
> get more information:
>   cat /proc/mdstat

Here is /proc/mdstat from one of the systems where I can reproduce it:

    $ cat /proc/mdstat
    Personalities : [raid6] [raid5] [raid4]
    md0 : active raid5 dm-0[4](J) sdc[3] sda[0] sdb[1]
          3906764800 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/3] [UUU]

    unused devices: <none>

dm-0 is an LVM logical volume which is backed by an NVMe SSD. The
others are run-of-the-mill SATA SSDs.

>  profile (perf, etc.) of the md thread

I might need a little more pointing in the direction of what exactly
to look for and under what conditions (i.e. should I run perf while
the thread is stuck in the 100% CPU loop? what kind of report should I
ask perf for?). Also, are there any debug options I could enable in
the kernel configuration that might help gather more information?
Maybe something in debugfs? I currently get absolutely no warnings or
errors in dmesg when the problem occurs.

Cheers,

-- Dan