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

[Max Reitz]

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.

Max

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