[pve-devel] [PATCH v2 qemu-server] fix #4099: disable io_uring for virtual disks on CIFS storages

[Fiona Ebner]

Since kernel 5.15, there is an issue with io_uring when used in
combination with CIFS [0]. Unfortunately, the kernel developers did
not suggest any way to resolve the issue and didn't comment on my
proposed one. So for now, just disable io_uring when the storage is
CIFS, like is done for other storage types that had problematic
interactions.

It is rather easy to reproduce when writing large amounts of data
within the VM. I used
dd if=/dev/urandom of=file bs=1M count=1000
to reproduce it consistently, but your mileage may vary.

Some forum reports about users running into the issue [1][2][3].

[0]: https://www.spinics.net/lists/linux-cifs/msg26734.html
[1]: https://forum.proxmox.com/threads/109848/
[2]: https://forum.proxmox.com/threads/110464/
[3]: https://forum.proxmox.com/threads/111382/

Signed-off-by: Fiona Ebner <f.ebner at proxmox.com>
---

Changes from v1:
    * Add 'fix #4099'-prefix to commit title.

 PVE/QemuServer.pm | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/PVE/QemuServer.pm b/PVE/QemuServer.pm
index 4e85dd02..513a248f 100644
--- a/PVE/QemuServer.pm
+++ b/PVE/QemuServer.pm
@@ -1665,8 +1665,12 @@ sub print_drive_commandline_full {
     # sometimes, just plain disable...
     my $lvm_no_io_uring = $scfg && $scfg->{type} eq 'lvm';
 
+    # io_uring causes problems when used with CIFS since kernel 5.15
+    # Some discussion: https://www.spinics.net/lists/linux-cifs/msg26734.html
+    my $cifs_no_io_uring = $scfg && $scfg->{type} eq 'cifs';
+
     if (!$drive->{aio}) {
-	if ($io_uring && !$rbd_no_io_uring && !$lvm_no_io_uring) {
+	if ($io_uring && !$rbd_no_io_uring && !$lvm_no_io_uring && !$cifs_no_io_uring) {
 	    # io_uring supports all cache modes
 	    $opts .= ",aio=io_uring";
 	} else {
-- 
2.30.2