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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