Re: Commit 'sunrpc: Use sendmsg(MSG_SPLICE_PAGES) rather then sendpage' broke O_DIRECT over NFS

[Chuck Lever III]

> On Aug 17, 2023, at 11:52 AM, Maxim Levitsky <mlevitsk@xxxxxxxxxx> 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 I run somewhat outdated qemu, but running the latest qemu doesn't make a difference.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