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

From: "Lukáš Doktor" <ldoktor@redhat.com>
To: Anton Nefedov <anton.nefedov@virtuozzo.com>,
	QEMU Developers <qemu-devel@nongnu.org>,
	"qemu-block@nongnu.org" <qemu-block@nongnu.org>,
	Max Reitz <mreitz@redhat.com>, Andrew Jones <drjones@redhat.com>
Subject: Re: [Qemu-devel] Broken aarch64 by qcow2: skip writing zero buffers to empty COW areas [v2]
Date: Wed, 21 Aug 2019 18:23:47 +0200	[thread overview]
Message-ID: <db5bc7fa-a943-24fa-700b-9a675bdda79a@redhat.com> (raw)
In-Reply-To: <b9129921-91cb-4339-0a3e-eb73ae5f6390@virtuozzo.com>

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Dne 21. 08. 19 v 17:49 Anton Nefedov napsal(a):
> On 21/8/2019 5:14 PM, 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?
>>
>> My reproducer is running kickstart installation of RHEL-8 from DVD on aarch64 gicv3 machine, which never finishes since this commit, where anaconda complains about package installation, occasionally there are also XFS metadata corruption messages on serial console:
>>
> 
> hi,
> 
> this looks scary :( I doubt that it can have anything to do with aarch64
> but rather a really tricky timing (or, possibly, a broken environment
> like broken fallocate() on a host? who knows..)
> 
> Is it always the same machine you observe this issue on? Did you try
> others?
> 
> I just wonder if it's worth to try to reproduce it on my machine
> (and I don't have aarch64 on hand now). I can probably come up with
> some torture test that will continuously write to qcow2 with random
> offsets/sizes and verify the result.
> 
> If you could kindly reproduce it again then we can probably start with
> enabling qemu traces by appending
>   " -trace bdrv* -trace qcow2* -trace file=/some_huge_partition/qemu.log"
> to the command line.
> 
> Beware that it's going to produce a huge amount of logs.
> 
> Also, the corrupted image and the serial log will be required for
> investigation.
> 
> thanks,
> 
> /Anton
> 

Hello Anton,

I have only tried that on a single machine, but colleague of mine reported similar issues even on TCG installing Fedora using x86_64 host. I'll try to reproduce it on my x86_64 box which should simplify the debugging.

Lukáš

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