Re: read corruption with qemu master io_uring engine / linux master / btrfs(?)

[Dominique MARTINET <dominique.martinet@atmark-techno.com>]

Dominique MARTINET wrote on Thu, Jun 30, 2022 at 09:41:01AM +0900:
> > I just tried your program, against the qemu/vmdk image you mentioned in the
> > first message, and after over an hour running I couldn't trigger any short
> > reads - this was on the integration misc-next branch.
> >
> > It's possible that to trigger the issue, one needs a particular file extent
> > layout, which will not be the same as yours after downloading and converting
> > the file.
> 
> Ugh. I've also been unable to reproduce on a test fs, despite filling it
> with small files and removing some to artificially fragment the image,
> so I guess I really do have something on these "normal" filesystems...
> 
> Is there a way to artificially try to recreate weird layouts?
> I've also tried btrfs send|receive, but while it did preserve reflinked
> extents it didn't seem to do the trick.

I take that one back, I was able to reproduce with my filesystem riddled
with holes.
I was just looking at another distantly related problem that happened
with cp, but trying with busybox cat didn't reproduce it and got
confused:
https://lore.kernel.org/linux-btrfs/Yr1QwVW+sHWlAqKj@atmark-techno.com/T/#u


Anyway, here's a pretty ugly reproducer to create a file that made short
reads on a brand new FS:

# 50GB FS -> fill with 50GB of small files and remove 1/10
$ mkdir /mnt/d.{00..50}
$ for i in {00000..49999}; do
	dd if=/dev/urandom of=/mnt/d.${i:0:2}/test.$i bs=1M count=1 status=none;
  done
$ rm -f /mnt/d.*/*2
$ btrfs subvolume create ~/sendme
$ cp --reflink=always bigfile ~/sendme/bigfile
$ btrfs property set ~/sendme ro true
$ btrfs send ~/sendme | btrfs receive /mnt/receive

and /mnt/receive/bigfile did the trick for me.
This probably didn't need the send/receive at all, I just didn't try
plain copy again.

Anyway, happy to test any patch as said earlier, it's probably not worth
spending too much time on trying to reproduce on your end at this
point...

-- 
Dominique