Re: ext4 fix for interaction between i_size, fallocate, and delalloc after a crash

Theodore Ts'o
2017-11-28T20:45:30Z

On Tue, Nov 28, 2017 at 07:04:54AM -0600, Ashlie Martinez wrote:
> No biggie, part of the reason this was so hard for me to wrap my head
> around is I don't have a physical machine that I can reproduce this on
> (and I never got around to getting a GCE instance to test on). Not
> being able to poke around a reproducing system makes it a little bit
> harder for me to reason about :)

This does reproduce easily using kvm-xfstests[1]; using gce-xfstests
was not necessary. That's actually how I debugged it, since kvm
starts up in under 5 seconds, while starting up a cloud VM takes a bit
longer. So if you want a quick edit/compile/debug cycle, or if you
attach a debugger to the running kernel, using kvm-xfstests is the
right tool to use. 99% of the command syntax and test appliance
implementation is the same between kvm-xfstests and gce-xfstests.

[1] https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md

I've been trying to promote the use of kvm-xfstests for researchers
who are interested in doing file system work. So if you can help
promote {kvm,gce}-xfstests amongst your fellow students and
professors, that would be great!

You can run the reproducer automatically via "kvm-xfstests -c 4k
generic/456". But you can also run "kvm-xfstests shell", and then run
the following commands;

kvm-xfstests# export FSTESTSET=generic/456
kvm-xfstests# ./runtests.sh

You can then edit the test script to add debugging commands; it can be
found in /root/xfstests/tests/generic/456 and then rerun the tests
using the "./runtests.sh" script.

Sorry, the only editor available is /bin/ed. If you want to use some
other editor, and are willing to build your own test-appliance VM
image instead of just downloading the rebuilt test applinace image,
you can add it to the xfstests-packages file in the
kvm-xfstests/test-appliance directory, and generate your own test
appliance. See [2] for more details.

[2] https://github.com/tytso/xfstests-bld/blob/master/Documentation/building-rootfs.md

This is actually how I figured out what was happening; I added
commands such as "debugfs -R 'stat <11>'" so I could see was going on
with the file system before the _flakey_drop_and_remount statement,
and then varied the number of operations in the fsx operations to
replay list.

Regards,

- Ted