[Xen-devel] block backend issues

[Marek Marczykowski <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

Hey,

I've faced strange problem with block devices. When trying to read some file
(from read-only ext3), everything looks good, except that file content is
corrupted! But this can be coincidence (that "failed" reads doesn't hit
filesystem metadata).
fsck in dom0 on filesystem image returns no errors.
fsck (with -nf flags) in domU on the device causes the kernel to output
"blkfront: flush disk cache: empty write xvdd op failed", "blkfront: xvdd:
barrier or flush: disable". And returns no filesystem errors. From that point,
file reads return correct file content. For most cases dropping block cache
(echo 3 > /proc/sys/vm/drop_caches) or remounting device also "fixes" the 
problem.

On RW device (with different size, filesystem and content), domU kernel
complains about EXT4 errors.
Doesn't observed such strange issues on device-mapper backed devices.

On 3.2.7 it worked, problem observed on 3.3.5 and 3.4 in dom0, regardless of
domU kernel (tried 3.2.7, 3.3.5, 3.4.0).

I've suspected feature-flush-cache/feature-barrier, but when disabled its
advertise in blkback code, problem still occurs.

Some details:
dom0: 3.4.0-1.pvops.qubes.x86_64 (vanilla 3.4 + Konrad's patches for ACPI S3)
domU: 3.3.5-1.pvops.qubes.x86_64 (vanilla 3.3.5 + Konrad's patches for ACPI S3)

domU:
# mount | grep /lib/modules
/dev/xvdd on /usr/lib/modules type ext3
(ro,relatime,errors=continue,barrier=1,data=ordered)
# pwd
/lib/modules/3.3.5-1.pvops.qubes.x86_64/kernel
# md5sum ./sound/usb/snd-usbmidi-lib.ko
fbc0aeb4dd5c0c3b041a5899a15c6566  ./sound/usb/snd-usbmidi-lib.ko
# ls -l ./sound/usb/snd-usbmidi-lib.ko
-rwxr--r-- 1 root root 38248 May 20 14:14 ./sound/usb/snd-usbmidi-lib.ko

dom0:
# mount|grep modules
/var/lib/qubes/vm-kernels/3.3.5/modules.img on /mnt/tmp type ext3
(ro,loop=/dev/loop10)
# pwd
/mnt/tmp/3.3.5-1.pvops.qubes.x86_64/kernel
# md5sum ./sound/usb/snd-usbmidi-lib.ko
9d2d3fedd4a357252e367fa8109c16ed  ./sound/usb/snd-usbmidi-lib.ko
# ls -l ./sound/usb/snd-usbmidi-lib.ko
-rwxr--r-- 1 root root 38248 May 20 14:14 ./sound/usb/snd-usbmidi-lib.ko

And block backend parameters:
# xenstore-ls /local/domain/0/backend/vbd/3/51760
frontend = "/local/domain/3/device/vbd/51760"
params = "/var/lib/qubes/vm-kernels/3.3.5/modules.img"
scripted = "1"
frontend-id = "3"
online = "1"
removable = "0"
bootable = "1"
state = "4"
dev = "xvdd"
type = "file"
mode = "r"
node = "/dev/loop4"
physical-device = "7:4"
hotplug-status = "connected"
feature-flush-cache = "1"
discard-granularity = "4096"
discard-alignment = "0"
discard-secure = "0"
feature-discard = "1"
feature-barrier = "1"
sectors = "409600"
info = "4"
sector-size = "512"

BTW 3.2.7 advertise feature-flush-cache=0 and feature-barrier=0 on this one
device (RO, loop backed). Don't know why, but seems irrelevant to this issue.

-- 
Best Regards / Pozdrawiam,
Marek Marczykowski
Invisible Things Lab

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