Possible io_uring regression with QEMU on Ubuntu's kernel

[Kamal Mostafa]

Hi-

Thanks very much for reporting this.  We picked up that patch ("io_uring:
don't mark S_ISBLK async work as unbounded") for our Ubuntu v5.8 kernel
from linux-stable/v5.10.31.  Since it's not clear that it's appropriate for
v5.8 (or even v5.10-stable?) we'll revert it from Ubuntu v5.8 if you can
confirm that actually fixes the problem.

Here's a test build of that (5.8.0-59 with that commit reverted).  The full
set of packages is provided, but you probably only actually need to install
the linux-image and linux-modules[-extra] deb's. We'll stand by for your
results:
https://kernel.ubuntu.com/~kamal/uringrevert0/

Thanks again,

 -Kamal Mostafa (Canonical Kernel Team)

On Wed, Jun 30, 2021 at 1:47 AM Juhyung Park <qkrwngud825 at gmail.com> wrote:

> Hi everyone.
>
> With the latest Ubuntu 20.04's HWE kernel 5.8.0-59, I'm noticing some
> weirdness when using QEMU/libvirt with the following storage
> configuration:
>
> <disk type="block" device="disk">
>   <driver name="qemu" type="raw" cache="none" io="io_uring"
> discard="unmap" detect_zeroes="unmap"/>
>   <source
> dev="/dev/disk/by-id/md-uuid-df271a1e:9dfb7edb:8dc4fbb8:c43e652f-part1"
> index="1"/>
>   <backingStore/>
>   <target dev="vda" bus="virtio"/>
>   <alias name="virtio-disk0"/>
>   <address type="pci" domain="0x0000" bus="0x07" slot="0x00"
> function="0x0"/>
> </disk>
>
> QEMU version is 5.2+dfsg-9ubuntu3 and libvirt version is 7.0.0-2ubuntu2.
>
> The guest VM is unable to handle I/O properly with io_uring, and
> nuking io="io_uring" fixes the issue.
> On one machine (EPYC 7742), the partition table cannot be read and on
> another (Ryzen 9 3950X), ext4 detects weirdness with journaling and
> ultimately remounts the guest disk to R/O:
>
> [    2.712321] virtio_blk virtio5: [vda] 3906519775 512-byte logical
> blocks (2.00 TB/1.82 TiB)
> [    2.714054] vda: detected capacity change from 0 to 2000138124800
> [    2.963671] blk_update_request: I/O error, dev vda, sector 0 op
> 0x0:(READ) flags 0x0 phys_seg 1 prio class 0
> [    2.964909] Buffer I/O error on dev vda, logical block 0, async page
> read
> [    2.966021] blk_update_request: I/O error, dev vda, sector 1 op
> 0x0:(READ) flags 0x0 phys_seg 1 prio class 0
> [    2.967177] Buffer I/O error on dev vda, logical block 1, async page
> read
> [    2.968330] blk_update_request: I/O error, dev vda, sector 2 op
> 0x0:(READ) flags 0x0 phys_seg 1 prio class 0
> [    2.969504] Buffer I/O error on dev vda, logical block 2, async page
> read
> [    2.970767] blk_update_request: I/O error, dev vda, sector 3 op
> 0x0:(READ) flags 0x0 phys_seg 1 prio class 0
> [    2.971624] Buffer I/O error on dev vda, logical block 3, async page
> read
> [    2.972170] blk_update_request: I/O error, dev vda, sector 4 op
> 0x0:(READ) flags 0x0 phys_seg 1 prio class 0
> [    2.972728] Buffer I/O error on dev vda, logical block 4, async page
> read
> [    2.973308] blk_update_request: I/O error, dev vda, sector 5 op
> 0x0:(READ) flags 0x0 phys_seg 1 prio class 0
> [    2.973920] Buffer I/O error on dev vda, logical block 5, async page
> read
> [    2.974496] blk_update_request: I/O error, dev vda, sector 6 op
> 0x0:(READ) flags 0x0 phys_seg 1 prio class 0
> [    2.975093] Buffer I/O error on dev vda, logical block 6, async page
> read
> [    2.975685] blk_update_request: I/O error, dev vda, sector 7 op
> 0x0:(READ) flags 0x0 phys_seg 1 prio class 0
> [    2.976295] Buffer I/O error on dev vda, logical block 7, async page
> read
> [    2.980074] blk_update_request: I/O error, dev vda, sector 0 op
> 0x0:(READ) flags 0x0 phys_seg 1 prio class 0
> [    2.981104] Buffer I/O error on dev vda, logical block 0, async page
> read
> [    2.981786] blk_update_request: I/O error, dev vda, sector 1 op
> 0x0:(READ) flags 0x0 phys_seg 1 prio class 0
> [    2.982083] ixgbe 0000:06:00.0: Multiqueue Enabled: Rx Queue count
> = 63, Tx Queue count = 63 XDP Queue count = 0
> [    2.982442] Buffer I/O error on dev vda, logical block 1, async page
> read
> [    2.983642] ldm_validate_partition_table(): Disk read failed.
>
> Kernel 5.8.0-55 is fine, and the only io_uring-related change between
> 5.8.0-55 and 5.8.0-59 is the commit 4b982bd0f383 ("io_uring: don't
> mark S_ISBLK async work as unbounded").
>
> The weird thing is that this commit was first introduced with v5.12,
> but neither the mainline v5.12.0 or v5.13.0 is affected by this issue.
>
> I guess one of these commits following the backported commit from
> v5.12 fixes the issue, but that's just a guess. It might be another
> earlier commit:
> c7d95613c7d6 io_uring: fix early sqd_list removal sqpoll hangs
> 9728463737db io_uring: fix rw req completion
> 6ad7f2332e84 io_uring: clear F_REISSUE right after getting it
> e82ad4853948 io_uring: fix !CONFIG_BLOCK compilation failure
> 230d50d448ac io_uring: move reissue into regular IO path
> 07204f21577a io_uring: fix EIOCBQUEUED iter revert
> 696ee88a7c50 io_uring/io-wq: protect against sprintf overflow
>
> It would be much appreciated if Jens could give pointers to Canonical
> developers on how to fix the issue, and hopefully a suggestion to
> prevent this from happening again.
>
> Thanks,
> Regards
>
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