Re: REGRESSION: 37f4a24c2469: blk-mq: centralise related handling into blk_mq_get_driver_tag

[Ming Lei <ming.lei@redhat.com>]

On Thu, Sep 24, 2020 at 09:13:11PM -0400, Theodore Y. Ts'o wrote:
> On Thu, Sep 24, 2020 at 10:33:45AM -0400, Theodore Y. Ts'o wrote:
> > HOWEVER, thanks to a hint from a colleague at $WORK, and realizing
> > that one of the stack traces had virtio balloon in the trace, I
> > realized that when I switched the GCE VM type from e1-standard-2 to
> > n1-standard-2 (where e1 VM's are cheaper because they use
> > virtio-balloon to better manage host OS memory utilization), problem
> > has become, much, *much* rarer (and possibly has gone away, although
> > I'm going to want to run a lot more tests before I say that
> > conclusively) on my test setup.  At the very least, using an n1 VM
> > (which doesn't have virtio-balloon enabled in the hypervisor) is
> > enough to unblock ext4 development.
> 
> .... and I spoke too soon.  A number of runs using -rc6 are now
> failing even with the n1-standard-2 VM, so virtio-ballon may not be an
> indicator.
> 
> This is why debugging this is frustrating; it is very much a heisenbug
> --- although 5.8 seems to work completely reliably, as does commits
> before 37f4a24c2469.  Anything after that point will show random
> failures.  :-(

It does not make sense to mention 37f4a24c2469, which is reverted in
4e2f62e566b5. Later the patch in 37f4a24c2469 is fixed and re-commited
as 568f27006577.

However, I can _not_ reproduce the issue by running the same test on
kernel built from 568f27006577 directly.

Also you have confirmed that the issue can't be fixed after reverting
568f27006577 against v5.9-rc4.

Looks the real issue(slab list corruption) should be introduced between
568f27006577 and v5.9-rc4.


thanks,
Ming