Messages in this thread | | | Date | Tue, 27 Jun 2023 16:15:24 -0400 | From | Kent Overstreet <> | Subject | Re: [GIT PULL] bcachefs |
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On Tue, Jun 27, 2023 at 11:16:01AM -0600, Jens Axboe wrote: > On 6/26/23 8:59?PM, Jens Axboe wrote: > > On 6/26/23 8:05?PM, Kent Overstreet wrote: > >> On Mon, Jun 26, 2023 at 07:13:54PM -0600, Jens Axboe wrote: > >>> Doesn't reproduce for me with XFS. The above ktest doesn't work for me > >>> either: > >> > >> It just popped for me on xfs, but it took half an hour or so of looping > >> vs. 30 seconds on bcachefs. > > > > OK, I'll try and leave it running overnight and see if I can get it to > > trigger. > > I did manage to reproduce it, and also managed to get bcachefs to run > the test. But I had to add: > > diff --git a/check b/check > index 5f9f1a6bec88..6d74bd4933bd 100755 > --- a/check > +++ b/check > @@ -283,7 +283,7 @@ while [ $# -gt 0 ]; do > case "$1" in > -\? | -h | --help) usage ;; > > - -nfs|-afs|-glusterfs|-cifs|-9p|-fuse|-virtiofs|-pvfs2|-tmpfs|-ubifs) > + -nfs|-afs|-glusterfs|-cifs|-9p|-fuse|-virtiofs|-pvfs2|-tmpfs|-ubifs|-bcachefs) > FSTYP="${1:1}" > ;; > -overlay)
I wonder if this is due to an upstream fstests change I haven't seen yet, I'll have a look.
> 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.
> 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.
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.
delayed_fput, even though I believe not responsible here, seems sketchy to me because there doesn't seem to be a straightforward way to flush delayed fputs for a given _process_ - there's a single global work item, and we can only flush globally.
Would what aio does work here?
(disclaimer: I haven't studied the io_uring code so I haven't figured out the approach your patch is taking yet)
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