> On Aug 17, 2023, at 4:49 PM, Maxim Levitsky <mlevitsk@xxxxxxxxxx> wrote: > > У чт, 2023-08-17 у 15:58 +0000, Chuck Lever III пише: >>> On Aug 17, 2023, at 11:57 AM, Chuck Lever <chuck.lever@xxxxxxxxxx> wrote: >>> >>> >>> >>>> 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 I run somewhat outdated qemu, but running the latest qemu doesn't make a difference.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 >> >> In particular: >> >> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c96e2a695e00bca5487824d84b85aab6aa2c1891 > > I just tested it. It does help (qemu doesn't crash anymore) but it doesn't eliminate the issue (VM still doesn't boot) > > The VM now starts but it drops into the UEFI shell. > > Once again, disabling O_DIRECT helps (that is -aio=threads,cache=writeback) > > For the reference, few kernels ago, I had an unrelated bug (not even NFS related, it was happening locally as well), > which caused the exact same drop to the UEFI shell when using O_DIRECT: > > https://www.mail-archive.com/qemu-devel@xxxxxxxxxx/msg912549.html > > It was decided that this issue is a qemu issue because it relied on undefined kernel behavior which has changed, > so the qemu got patched to fix the issue on its side. > > Since sometimes I use an older qemu version, I had this kernel commit reverted for now, but to be sure I now had built a kernel > without the revert on both server and the client, and tested with the latest qemu which has the fix for the bug. > > I don't remember details of this unrelated bug, but if I remember correctly, qemu had trouble reading first 512 bytes of the virtual disk, when > the VM tried to do so to read the boot sector. Let's start with this (on the NFS server with c96e2a695e00 applied): # trace-cmd record -e sunrpc:svc_xdr\* -e sunrpc:svcsock\* -e nfsd:nfsd_read\* and run your failing disk accesses. ^C the "trace-cmd record" when the reproducer finishes, and send me the trace.dat file in private email. -- Chuck Lever
