linux-ext4 - Re: [PATCH] ext4: flag as supporting buffered async reads

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Date:   Fri, 4 Sep 2020 08:51:24 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     linux-ext4@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ext4: flag as supporting buffered async reads

On 9/3/20 9:55 PM, Theodore Y. Ts'o wrote:
> Hi Jens,
> 
> Sorry, the past few months have been *crazy* insane.  Between Linux
> Plumbers Conference organizing, having to deal with some interns at
> $WORK, performance reviews, etc.., etc., it's been a perfect storm.
> As a result, I was late getting the primary ext4 pull request to
> Linus, and he's already yelled at me for a late pull request.
> 
> As a result, I'm being super paranoid about asking Linus for anything,
> even a one-time change, if it's not a bug fix.  And what you are
> asking for isn't a bug fix, but enabling a new feature.

That's fine, we blew past the 5.9 window long ago imho. As mentioned,
back in May we could have solved this by just adding your acked-by
or reviewed-by to the patch like we did for xfs/btrfs. That removes the
ext4 tree dependency. Obviously that's no longer a concern for 5.10, but
I need to know if we're going one of two routes:

a) I'm queueing this up, in which case I'll still need that ack/review
b) you're queueing it up, in which case you can just grab it

I just don't want to end up in the same situation for 5.10, and then
we're pushing this out 2 releases at that point...

> Worse, right now, -rc1 and -rc2 is causing random crashes in my
> gce-xfstests framework.  Sometimes it happens before we've run even a
> single xfstests; sometimes it happens after we have successfully
> completed all of the tests, and we're doing a shutdown of the VM under
> test.  Other times it happens in the middle of a test run.  Given that
> I'm seeing this at -rc1, which is before my late ext4 pull request to
> Linus, it's probably not an ext4 related bug.  But it also means that
> I'm partially blind in terms of my kernel testing at the moment.  So I
> can't even tell Linus that I've run lots of tests and I'm 100%
> confident your one-line change is 100% safe.
> 
> Next week after Labor Day, I should be completely done with the
> performance review writeups that are on my plate, and I will hopefully
> have a bit more time to try to debug these mysterious failures.  Or
> maybe someone else will be able to figure out what the heck is going
> wrong, and perhaps -rc3 will make all of these failures go away.
> 
> I'm sorry your frustrated; I'm frustrated too!  But until I can find
> the time to do a full bisect (v5.8 works just fine.  v5.9-rc1 is
> flakey as hell when booting my test config in a GCE VM), I'm not in a
> position to send anything to Linus.

That looks nasty, good luck! Hopefully the bisect will be helpful. FWIW,
in the testing I've done (with this patch) on ext4 and with XFS, I
haven't seen anything odd. That's using real hardware though, and qemu
on the laptop, no GCE.

-- 
Jens Axboe