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SubjectRe: Commit 'sunrpc: Use sendmsg(MSG_SPLICE_PAGES) rather then sendpage' broke O_DIRECT over NFS
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> On Aug 17, 2023, at 11:52 AM, Maxim Levitsky <mlevitsk@redhat.com> wrote:
>
> Hi!
>
> I just updated my developement systems to 6.5-rc6 (from 6.4) and now I can't start a VM
> with a disk which is mounted over the NFS.
>
> The VM has two qcow2 files, one depends on another and qemu opens both.
>
> This is the command line of qemu:
>
> -drive if=none,id=os_image,file=./disk_s1.qcow2,aio=native,discard=unmap,cache=none
>
> The disk_s1.qcow2 depends on disk_s0.qcow2
>
> However this is what I get:
>
> qemu-system-x86_64: -drive if=none,id=os_image,file=./disk_s1.qcow2,aio=native,discard=unmap,cache=none: Could not open backing file: Could not open './QFI?': No such file or directory
>
> 'QFI?' is qcow2 file signature, which signals that there might be some nasty corruption happening.
>
> The program was supposed to read a field inside the disk_s1.qcow2 file which should read 'disk_s0.qcow2'
> but instead it seems to read the first 4 bytes of the file.
>
>
> Bisect leads to the above commit. Reverting it was not possible due to many changes.
>
> Both the client and the server were tested with the 6.5-rc6 kernel, but once rebooting the server into
> the 6.4, the bug disappeared, thus I did a bisect on the server.
>
> When I tested a version before the offending commit on the server, the 6.5-rc6 client was able to work with it,
> which increases the chances that the bug is in nfsd.
>
> Switching qemu to use write back paging also helps (aio=threads,discard=unmap,cache=writeback)
> The client and the server (both 6.5-rc6) work with this configuration.
>
> Running the VM on the same machine (also 6.5-rc6) where the VM disk is located (thus avoiding NFS) works as well.
>
> I tested several VMs that I have, all are affected in the same way.
>
> I run somewhat outdated qemu, but running the latest qemu doesn't make a difference.
>
> I use nfs4.
>
> I can test patches and provide more info if needed.

Linus just merged a possible fix for this issue. See:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/ master


--
Chuck Lever


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