linux-ext4 - Re: REGRESSION: 37f4a24c2469: blk-mq: centralise related handling into blk_mq_get_driver

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Message-ID: <20200925011311.GJ482521@mit.edu>
Date:   Thu, 24 Sep 2020 21:13:11 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-ext4@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-block@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: REGRESSION: 37f4a24c2469: blk-mq: centralise related handling
 into blk_mq_get_driver_tag

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.  :-(

						- Ted