Re: [GIT PULL] bcachefs

Kent Overstreet
2023-06-28T08:06:22Z

On Tue, Jun 27, 2023 at 09:16:31PM -0600, Jens Axboe wrote:
> On 6/27/23 2:15?PM, Kent Overstreet wrote:
> >> to ktest/tests/xfstests/ and run it with -bcachefs, otherwise it kept
> >> failing because it assumed it was XFS.
> >>
> >> I suspected this was just a timing issue, and it looks like that's
> >> exactly what it is. Looking at the test case, it'll randomly kill -9
> >> fsstress, and if that happens while we have io_uring IO pending, then we
> >> process completions inline (for a PF_EXITING current). This means they
> >> get pushed to fallback work, which runs out of line. If we hit that case
> >> AND the timing is such that it hasn't been processed yet, we'll still be
> >> holding a file reference under the mount point and umount will -EBUSY
> >> fail.
> >>
> >> As far as I can tell, this can happen with aio as well, it's just harder
> >> to hit. If the fput happens while the task is exiting, then fput will
> >> end up being delayed through a workqueue as well. The test case assumes
> >> that once it's reaped the exit of the killed task that all files are
> >> released, which isn't necessarily true if they are done out-of-line.
> >
> > Yeah, I traced it through to the delayed fput code as well.
> >
> > I'm not sure delayed fput is responsible here; what I learned when I was
> > tracking this down has mostly fell out of my brain, so take anything I
> > say with a large grain of salt. But I believe I tested with delayed_fput
> > completely disabled, and found another thing in io_uring with the same
> > effect as delayed_fput that wasn't being flushed.
>
> I'm not saying it's delayed_fput(), I'm saying it's the delayed putting
> io_uring can end up doing. But yes, delayed_fput() is another candidate.

Sorry - was just working through my recollections/initial thought
process out loud

> >> For io_uring specifically, it may make sense to wait on the fallback
> >> work. The below patch does this, and should fix the issue. But I'm not
> >> fully convinced that this is really needed, as I do think this can
> >> happen without io_uring as well. It just doesn't right now as the test
> >> does buffered IO, and aio will be fully sync with buffered IO. That
> >> means there's either no gap where aio will hit it without O_DIRECT, or
> >> it's just small enough that it hasn't been hit.
> >
> > I just tried your patch and I still have generic/388 failing - it
> > might've taken a bit longer to pop this time.
>
> Yep see the same here. Didn't have time to look into it after sending
> that email today, just took a quick stab at writing a reproducer and
> ended up crashing bcachefs:

You must have hit an error before we finished initializing the
filesystem, the list head never got initialized. Patch for that will be
in the testing branch momentarily.

> > I wonder if there might be a better way of solving this though? For aio,
> > when a process is exiting we just synchronously tear down the ioctx,
> > including waiting for outstanding iocbs.
>
> aio is pretty trivial, because the only async it supports is O_DIRECT
> on regular files which always completes in finite time. io_uring has to
> cancel etc, so we need to do a lot more.

ahh yes, buffered IO would complicate things

> But the concept of my patch should be fine, but I think we must be
> missing a case. Which is why I started writing a small reproducer
> instead. I'll pick it up again tomorrow and see what is going on here.

Ok. Soon as you've got a patch I'll throw it at my CI, or I can point my
CI at your branch if you have one.