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Re: [Qemu-block] Broken aarch64 by qcow2: skip writing zero buffers to e

From: Max Reitz
Subject: Re: [Qemu-block] Broken aarch64 by qcow2: skip writing zero buffers to empty COW areas [v2]
Date: Thu, 22 Aug 2019 14:09:23 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0

(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? :-)


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.)


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