Re: [Qemu-devel] Broken aarch64 by qcow2: skip writing zero buffers to empty COW areas [v2]

[Max Reitz <mreitz@redhat.com>]

On 22.08.19 17:40, Max Reitz wrote:
> On 22.08.19 17:25, Max Reitz wrote:
>> On 22.08.19 14:09, Max Reitz wrote:
>>> (CC-ing Paolo because of the XFS connection, and Stefan because why not.)
>>>
>>> On 22.08.19 13:27, Lukáš Doktor wrote:
>>>> Dne 21. 08. 19 v 19:51 Max Reitz napsal(a):
>>>>> On 21.08.19 16:14, Lukáš Doktor wrote:
>>>>>> Hello guys,
>>>>>>
>>>>>> First attempt was rejected due to zip attachment, let's try it again with just Avocado-vt debug.log and serial console log files attached.
>>>>>>
>>>>>> I bisected a regression on aarch64 all the way to this commit: "qcow2: skip writing zero buffers to empty COW areas" c8bb23cbdbe32f5c326365e0a82e1b0e68cdcd8a. Would you please have a look at it?
>>>>>
>>>>> I think I can see the issue on my x64 system (I don’t see the XFS
>>>>> corruption, but the installation fails because of some segfaults).
>>>>>
>>>>> I haven’t found a simpler way to reproduce the problem yet, though,
>>>>> which is a pain... :-/
>>>>>
>>>>> It looks like the problem disappears when I configure qemu with
>>>>> “--disable-xfsctl”.  Can you try that?
>>>>>
>>>>> Max
>>>>>
>>>>
>>>> Hello Max,
>>>>
>>>> yes, I'm getting the same behavior. With "--disable-xfsctl" it works well. Also looking at the option I understand why it only failed on aarch64 for me, I don't have libs installed on the other machines, therefor it was disabled by "./configure" there. Anyway I guess disabling it in my builds won't really fix the issue, right? :-)
>>>
>>> Thanks!
>>>
>>> No, it won’t, but it means the actual root of the problem is probably
>>> rather in some XFS-related code (be it because qemu uses it the wrong
>>> way or because of XFS kernel code) than in the pure qcow2 commit that
>>> made the problem surface by exercising it heavily.  (Or in an
>>> interaction between the two.)
>>
>> OK, I got a simpler reproducer now:
>>
>> $ ./qemu-img create -f qcow2 test.qcow2 1M
>> $ (for i in $(seq 15 -1 0); do \
>>        echo "aio_write -P 42 $((i * 64 + 1))k 62k"; \
>>    done) \
>>   | ./qemu-io test.qcow2
>> $ for i in $(seq 0 15); do \
>>       echo $i; \
>>       ofs=$((i * 64)); \
>>       ./qemu-io -c "read -P 0 ${ofs}k 1k" \
>>                 -c "read -P 42 $((ofs + 1))k 62k" \
>>                 -c "read -P 0 $((ofs + 63))k 1k" \
>>                 test.qcow2 \
>>           | grep 'verification'; \
>>   done
>>
>> On XFS with --enable-xfsctl, this basically always gives me some
>> verification failure somewhere.  (On tmpfs or with --disable-xfsctl, it
>> never fails.)
>>
>> So it seems to be related to I/O from back to front.
>>
>> (You can also reproduce it with a plain “qemu-img bench” invocation,
>> like “./qemu-img bench -w --pattern=42 -o 1k -S 64k -s 62k test.qcow2”
>> (on, say, a 4 GB image), but then the failure appears much later in the
>> image, because you have to wait from some requests to come in reverse
>> (by chance) first.)
> 
> The problem is the ftruncate() in xfs_write_zeroes().  It is possible
> for it to yield, then other requests come in, and the data they write
> may get discarded once the ftruncate() settles.

I’ve just sent a patch: “block/file-posix: Fix xfs_write_zeroes()”,
Message-ID <20190822162618.27670-1-mreitz@redhat.com>:
https://lists.nongnu.org/archive/html/qemu-block/2019-08/msg01148.html

Max