> 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 I run somewhat outdated qemu, but running the latest qemu doesn't make a difference.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
