> On Aug 17, 2023, at 4:49 PM, Maxim Levitsky <[email protected]> wrote:
>
> У чт, 2023-08-17 у 15:58 +0000, Chuck Lever III пише:
>>> On Aug 17, 2023, at 11:57 AM, Chuck Lever <[email protected]> wrote:
>>>
>>>
>>>
>>>> On Aug 17, 2023, at 11:52 AM, Maxim Levitsky <[email protected]> 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 I run somewhat outdated qemu, but running the latest qemu doesn't make a difference.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/[email protected]/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