Re: [RFC 0/3] block/file-posix: Work around XFS bug

[Max Reitz]

On 28.10.19 11:07, Vladimir Sementsov-Ogievskiy wrote:
> 28.10.2019 12:56, Max Reitz wrote:
>> On 28.10.19 10:30, Max Reitz wrote:
>>> On 28.10.19 10:24, Max Reitz wrote:
>>>> On 27.10.19 13:35, Stefan Hajnoczi wrote:
>>>>> On Fri, Oct 25, 2019 at 11:58:46AM +0200, Max Reitz wrote:
>>>>>> As for how we can address the issue, I see three ways:
>>>>>> (1) The one presented in this series: On XFS with aio=native, we extend
>>>>>>      tracked requests for post-EOF fallocate() calls (i.e., write-zero
>>>>>>      operations) to reach until infinity (INT64_MAX in practice), mark
>>>>>>      them serializing and wait for other conflicting requests.
>>>>>>
>>>>>>      Advantages:
>>>>>>      + Limits the impact to very specific cases
>>>>>>        (And that means it wouldn’t hurt too much to keep this workaround
>>>>>>        even when the XFS driver has been fixed)
>>>>>>      + Works around the bug where it happens, namely in file-posix
>>>>>>
>>>>>>      Disadvantages:
>>>>>>      - A bit complex
>>>>>>      - A bit of a layering violation (should file-posix have access to
>>>>>>        tracked requests?)
>>>>>
>>>>> Your patch series is reasonable.  I don't think it's too bad.
>>>>>
>>>>> The main question is how to detect the XFS fix once it ships.  XFS
>>>>> already has a ton of ioctls, so maybe they don't mind adding a
>>>>> feature/quirk bit map ioctl for publishing information about bug fixes
>>>>> to userspace.  I didn't see another obvious way of doing it, maybe a
>>>>> mount option that the kernel automatically sets and that gets reported
>>>>> to userspace?
>>>>
>>>> I’ll add a note to the RH BZ.
>>>>
>>>>> If we imagine that XFS will not provide a mechanism to detect the
>>>>> presence of the fix, then could we ask QEMU package maintainers to
>>>>> ./configure --disable-xfs-fallocate-beyond-eof-workaround at some point
>>>>> in the future when their distro has been shipping a fixed kernel for a
>>>>> while?  It's ugly because it doesn't work if the user installs an older
>>>>> custom-built kernel on the host.  But at least it will cover 98% of
>>>>> users...
>>>>
>>>> :-/
>>>>
>>>> I don’t like it, but I suppose it would work.  We could also
>>>> automatically enable this disabling option in configure when we detect
>>>> uname to report a kernel version that must include the fix.  (This
>>>> wouldn’t work for kernel with backported fixes, but those disappear over
>>>> time...)
>>> I just realized that none of this is going to work for the gluster case
>>> brought up by Nir.  The affected kernel is the remote one and we have no
>>> insight into that.  I don’t think we can do ioctls to XFS over gluster,
>>> can we?
>>
>> On third thought, we could try to detect whether the file is on a remote
>> filesystem, and if so enable the workaround unconditionally.  I suppose
>> it wouldn’t hurt performance-wise, given that it’s a remote filesystem
>> anyway.
>>
> 
> I think, for remote, the difference may be even higher than for local, as cost
> of writing real zeroes through the wire vs fast zero command is high.

I was speaking of a workaround in general, and that includes the
workaround presented in this series.

> Really, can we live with simple config option, is it so bad?

The config option won’t work for remote hosts, though.  That’s exactly
the problem.

Max

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