On 6/28/22 3:08 AM, Dominique MARTINET wrote: > I don't have any good reproducer so it's a bit difficult to specify, > let's start with what I have... > > I've got this one VM which has various segfaults all over the place when > starting it with aio=io_uring for its disk as follow: > > qemu-system-x86_64 -drive file=qemu/atde-test,if=none,id=hd0,format=raw,cache=none,aio=io_uring \ > -device virtio-blk-pci,drive=hd0 -m 8G -smp 4 -serial mon:stdio -enable-kvm > > It also happens with virtio-scsi-blk: > -device virtio-scsi-pci,id=scsihw0 \ > -drive file=qemu/atde-test,if=none,id=drive-scsi0,format=raw,cache=none,aio=io_uring \ > -device scsi-hd,bus=scsihw0.0,channel=0,scsi-id=0,lun=0,drive=drive-scsi0,id=scsi0,bootindex=100 > > It also happened when the disk I was using was a qcow file backing up a > vmdk image (this VM's original disk is for vmware), so while I assume > qemu reading code and qemu-img convert code are similar I'll pretend > image format doesn't matter at this point... > It's happened with two such images, but I haven't been able to reproduce > with any other VMs yet. > > I can also reproduce this on a second host machine with a completely > different ssd (WD sata in one vs. samsung nvme), so probably not a > firmware bug. > > scrub sees no problem with my filesystems on the host. > > I've confirmed it happens with at least debian testing's 5.16.0-4-amd64 > and 5.17.0-1-amd64 kernels, as well as 5.19.0-rc4. > It also happens with both debian's 7.0.0 and the master branch > (v7.0.0-2031-g40d522490714) > > > These factors aside, anything else I tried changing made this bug no > longer reproduce: > - I'm not sure what the rule is but it sometimes doesn't happen when > running the VM twice in a row, sometimes it happens again. Making a > fresh copy with `cp --reflink=always` of my source image seems to be > reliable. > - it stops happening without io_uring > - it stops happening if I copy the disk image with --reflink=never > - it stops happening if I copy the disk image to another btrfs > partition, created in the same lv, so something about my partition > history matters?... > (I have ssd > GPT partitions > luks > lvm > btrfs with a single disk as > metadata DUP data single) > - I was unable to reproduce on xfs (with a reflink copy) either but I > also was only able to try on a new fs... > - I've never been able to reproduce on other VMs > > > If you'd like to give it a try, my reproducer source image is > --- > curl -O https://download.atmark-techno.com/atde/atde9-amd64-20220624.tar.xz > tar xf atde9-amd64-20220624.tar.xz > qemu-img convert -O raw atde9-amd64-20220624/atde9-amd64.vmdk atde-test > cp --reflink=always atde-test atde-test2 > --- > and using 'atde-test'. > For further attempts I've removed atde-test and copied back from > atde-test2 with cp --reflink=always. > This VM graphical output is borked, but ssh listens so something like > `-netdev user,id=net0,hostfwd=tcp::2227-:22 -device virtio-net-pci,netdev=net0` > and 'ssh -p 2227 -l atmark localhost' should allow login with password > 'atmark' or you can change vt on the console (root password 'root') > > I also had similar problems with atde9-amd64-20211201.tar.xz . > > > When reproducing I've had either segfaults in the initrd and complete > boot failures, or boot working and login failures but ssh working > without login shell (ssh ... -tt localhost sh) > that allowed me to dump content of a couple of corrupted files. > When I looked: > - /usr/lib/x86_64-linux-gnu/libc-2.31.so had zeroes instead of data from > offset 0xb6000 to 0xb7fff; rest of file was identical. > - /usr/bin/dmesg had garbadge from 0x05000 until 0x149d8 (end of file). > I was lucky and could match the garbage quickly: it is identical to the > content from 0x1000-0x109d8 within the disk itself. > > I've rebooted a few times and it looks like the corruption is identical > everytime for this machine as long as I keep using the same source file; > running from qemu-img convert again seems to change things a bit? > but whatever it is that is specific to these files is stable, even > through host reboots. > > > > I'm sorry I haven't been able to make a better reproducer, I'll keep > trying a bit more tomorrow but maybe someone has an idea with what I've > had so far :/ > > Perhaps at this point it might be simpler to just try to take qemu out > of the equation and issue many parallel reads to different offsets > (overlapping?) of a large file in a similar way qemu io_uring engine > does and check their contents? > > > Thanks, and I'll probably follow up a bit tomorrow even if no-one has > any idea, but even ideas of where to look would be appreciated. Not sure what's going on here, but I use qemu with io_uring many times each day and haven't seen anything odd. This is on ext4 and xfs however, I haven't used btrfs as the backing file system. I wonder if we can boil this down into a test case and try and figure out what is doing on here. -- Jens Axboe
