Re: Problems with c8bb23cbdbe3 on ppc64le

[Max Reitz]

On 11.10.19 09:49, Max Reitz wrote:
> On 10.10.19 18:15, Anton Nefedov wrote:
>> On 10/10/2019 6:17 PM, Max Reitz wrote:
>>> Hi everyone,
>>>
>>> (CCs just based on tags in the commit in question)
>>>
>>> I have two bug reports which claim problems of qcow2 on XFS on ppc64le
>>> machines since qemu 4.1.0.  One of those is about bad performance
>>> (sorry, is isn’t public :-/), the other about data corruption
>>> (https://bugzilla.redhat.com/show_bug.cgi?id=1751934).
>>>
>>> It looks like in both cases reverting c8bb23cbdbe3 solves the problem
>>> (which optimized COW of unallocated areas).
>>>
>>> I think I’ve looked at every angle but can‘t find what could be wrong
>>> with it.  Do any of you have any idea? :-/
>>>
>>
>> hi,
>>
>> oh, that patch strikes again..
>>
>> I don't quite follow, was this bug confirmed to happen on x86? Comment 8
>> (https://bugzilla.redhat.com/show_bug.cgi?id=1751934#c8) mentioned that
>> (or was that mixed up with the old xfsctl bug?)
> 
> I think that was mixed up with the xfsctl bug, yes.
> 
>> Regardless of the platform, does it reproduce? That's comforting
>> already; worst case we can trace each and every request then (unless it
>> will stop to reproduce this way).
> 
> I haven’t been able to reproduce it yet (wrestling with the test system
> and getting ppc64 machines provisioned), but as far as I know it
> reproduces reliably on ppc64, but only there.
> 
>> Also, perhaps it's worth to try to replace fallocate with write(0)?
>> Either in qcow2 (in the patch, bdrv_co_pwrite_zeroes -> bdrv_co_pwritev)
>> or in the file driver. It might hint whether it's misbehaving fallocate
>> (in qemu or in kernel) or something else.
> 
> Good idea, that should at least tell us something about the corruption.

OK, after a week of debugging I’m not really much wiser.

One thing I know is that I can see the issue on x86-64 now, but not on
ext4, only XFS.

Replacing the zero-write with actually writing zeroes fixes it, but I
still don’t know whether that’s because of the kernel or because the
write is just slower or takes another code path...

The only thing I could narrow it down to is this:

The issue persists if handle_alloc_space() writes zeroes (with a
narrowed aligned zero-write with NO_FALLBACK) only to the non-COW area,
and I keep skip_cow to be false.

So there seems to be some kind of interaction between the zero-write and
the following write of data.  I don’t know what kind of interaction that
is, though.  I have tried to write a test case in qemu-img (basically
rewriting qemu-img bench), but failed so far.

It certainly looks like a kernel issue, but without a simpler reproducer
I just cannot tell.

Max

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