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

[Max Reitz <mreitz@redhat.com>]

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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...)

Max


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