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

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Message-ID: <20171128204525.ijpis74t75f4bbsc@thunk.org>
Date:   Tue, 28 Nov 2017 15:45:26 -0500
From:   Theodore Ts'o <tytso@....edu>
To:     Ashlie Martinez <ashmrtn@...xas.edu>
Cc:     Vijay Chidambaram <vvijay03@...il.com>,
        Ext4 <linux-ext4@...r.kernel.org>
Subject: Re: ext4 fix for interaction between i_size, fallocate, and delalloc
 after a crash

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