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Re: [RFC 0/3] block/file-posix: Work around XFS bug
From: |
Max Reitz |
Subject: |
Re: [RFC 0/3] block/file-posix: Work around XFS bug |
Date: |
Mon, 28 Oct 2019 10:56:58 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 |
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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- Re: [RFC 0/3] block/file-posix: Work around XFS bug, (continued)
Re: [RFC 0/3] block/file-posix: Work around XFS bug, no-reply, 2019/10/25
Re: [RFC 0/3] block/file-posix: Work around XFS bug, Nir Soffer, 2019/10/26
Re: [RFC 0/3] block/file-posix: Work around XFS bug, Stefan Hajnoczi, 2019/10/27
Re: [RFC 0/3] block/file-posix: Work around XFS bug, Kevin Wolf, 2019/10/28
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Vladimir Sementsov-Ogievskiy, 2019/10/28
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Max Reitz, 2019/10/29
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Vladimir Sementsov-Ogievskiy, 2019/10/29
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Max Reitz, 2019/10/29
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Vladimir Sementsov-Ogievskiy, 2019/10/29
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Max Reitz, 2019/10/29