[linux-lvm] Missing error handling in lv_snapshot_remove

[Bastian Blank <waldi@debian.org>]

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Hi

I tried to tackle a particular bug that shows up in Debian for some time
now. Some blamed the udev rules and I still can't completely rule them
out. But this triggers a much worse bug in the error cleanup of the
snapshot remove. I reproduced this with Debian/Linux 3.2.46/LVM 2.02.99
without udevd running and Fedora 19/LVM 2.02.98-10.fc19.

On snapshot removal, LVM first converts the device into a regular LV
(lv_remove_snapshot) and in a second step removes this LV
(lv_remove_single). Is there a reason for this two step removal? An
error during removal leaves a non-snapshot LV behind.

I hold the cow device open so it will run into the error condition:
| $ sleep 100 < /dev/mapper/vg-test_snap-cow&

Then try to remove the LV:
| $ lvremove vg/test_snap

lv_remove_snapshot first suspends all devices:

| #metadata/lv_manip.c:4429     Removing snapshot test_snap
| #libdm-deptree.c:1314     Suspending vg-test_base (253:8) with device flush
| #ioctl/libdm-iface.c:1724         dm suspend   (253:8) NFS    [16384] (*1)
| #libdm-common.c:210         Suspended device counter increased to 1
| #ioctl/libdm-iface.c:1724         dm info   (253:9) NF   [16384] (*1)
| #libdm-deptree.c:1314     Suspending vg-test_snap (253:9) with device flush
| #ioctl/libdm-iface.c:1724         dm suspend   (253:9) NFS    [16384] (*1)
| #libdm-common.c:210         Suspended device counter increased to 2
| #ioctl/libdm-iface.c:1724         dm info   (253:10) NF   [16384] (*1)
| #libdm-deptree.c:1314     Suspending vg-test_base-real (253:10) with device flush
| #ioctl/libdm-iface.c:1724         dm suspend   (253:10) NFS    [16384] (*1)
| #libdm-common.c:210         Suspended device counter increased to 3
| #ioctl/libdm-iface.c:1724         dm info   (253:11) NF   [16384] (*1)
| #libdm-deptree.c:1314     Suspending vg-test_snap-cow (253:11) with device flush
| #ioctl/libdm-iface.c:1724         dm suspend   (253:11) NFS    [16384] (*1)
| #libdm-common.c:210         Suspended device counter increased to 4

Commits the VG:

| #format_text/format-text.c:735         Committing vg metadata (1276) to /dev/xvdb header@4096

Resumes three of the devices, but not vg-test_base:

| #libdm-deptree.c:1263     Resuming vg-test_snap-cow (253:11)
| #ioctl/libdm-iface.c:1724         dm resume   (253:11) NF   [16384] (*1)
| #libdm-common.c:1338         vg-test_snap-cow: Stacking NODE_ADD (253,11) 0:6 0660 [trust_udev]
| #libdm-common.c:1348         vg-test_snap-cow: Stacking NODE_READ_AHEAD 0 (flags=0)
| #libdm-common.c:221         Suspended device counter reduced to 3
| #libdm-deptree.c:1263     Resuming vg-test_base-real (253:10)
| #ioctl/libdm-iface.c:1724         dm resume   (253:10) NF   [16384] (*1)
| #libdm-common.c:1338         vg-test_base-real: Stacking NODE_ADD (253,10) 0:6 0660 [trust_udev]
| #libdm-common.c:1348         vg-test_base-real: Stacking NODE_READ_AHEAD 0 (flags=0)
| #libdm-common.c:221         Suspended device counter reduced to 2
| #libdm-deptree.c:1263     Resuming vg-test_snap (253:9)
| #ioctl/libdm-iface.c:1724         dm resume   (253:9) NF   [16384] (*1)
| #libdm-common.c:1338         vg-test_snap: Stacking NODE_ADD (253,9) 0:6 0660 [trust_udev]
| #libdm-common.c:1348         vg-test_snap: Stacking NODE_READ_AHEAD 256 (flags=1)
| #libdm-common.c:221         Suspended device counter reduced to 1

Now it fails to do lv_activate on the cow device, because it is still
open:

| #libdm-deptree.c:1562   Unable to deactivate open vg-test_snap-cow (253:11)
| #metadata/snapshot_manip.c:291   Failed to activate test_snap.

And exits without further error handling and with one suspended device:

|  libdevmapper exiting with 1 device(s) still suspended.

Bastian

-- 
Beam me up, Scotty, there's no intelligent life down here!

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