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

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Date:   Wed, 29 Nov 2017 05:37:49 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Ashlie Martinez <ashmrtn@...xas.edu>
Cc:     "Theodore Ts'o" <tytso@....edu>,
        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 11:27 PM, Ashlie Martinez <ashmrtn@...xas.edu> wrote:
> On Tue, Nov 28, 2017 at 2:45 PM, Theodore Ts'o <tytso@....edu> wrote:
>> 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.
>
> Unfortunately this timing bug only reproduces on some machines. Xiao
> and I have been unable to reproduce this bug (I've tried kvm-xfstests,
> my own kvm VMs, VMs without kvm, VMs with/without virtio drivers, and
> another bare metal system). generic/456 basically sets up a race
> condition between a kernel flusher thread and triggering dm-flakey, so
> I think things like system load, core count, etc. might cause
> different test results.
>

For what it's worth, I wasn't able to reproduce on my kvm-xfstests machine
either. With 2 cores, virtio over LVM/SSD. Didn't try to play with parameters.

Amir.