What does the 10 mean in "inotify_add_watch(6, /dev/dm-4, 10) failed ..."

catkin · 09-03-2013, 01:05 AM

The full error message text, from syslog is

Code:

inotify_add_watch(6, /dev/dm-4, 10) failed: No such file or directory

According to the inotify_add_watch man page, the 10 is a bitmask documented in the inotify (7) man page with meanings as assigned in /usr/include/linux/inotify.h.

If it is hexadecimal it means "Unwrittable file closed" (sic)
If it is decimal it is 0x8 and 0x2 and means ""Unwrittable file closed" (sic) and "File was modified"
If it is binary it means "File was modified"

None of those fit well with the presumed intention of detecting a u/mount event to a device-mapper device (in this case an LVM snapshot). The 3rd interpretation would fit with detecting a write event.

The inotify_add_watch may be run by udev via a rule in /lib/udev/rules.d/80-udisks.rules, which is part of the udisks package:

Code:

# Make udevd synthesize a 'change' uevent when last opener of a rw-fd closes the fd - this
# should be part of the device-mapper rules
KERNEL=="dm-*", OPTIONS+="watch"

Similar error messages appear for LVM snapshots which are successfully removed by lvremove; no such message appears for a snapshot that lvremove does not remove with message "Can't remove open logical volume "snap-var"".

I am trying to understand what is going on with the intention of finding a workaround; the best related discussion was found in a Red Hat bug report at https://bugzilla.redhat.com/show_bug.cgi?id=753105

In case it makes any difference, this is on an ubuntu 12.04 server with lvm2 2.02.66, udev 175-0 and udisks 1.0.4

smallpond · 09-03-2013, 06:58 PM

Most messages print numbers as decimal or else explicitly precede them with 0x for hex, so I think it is "File was modified or closed". Note that multiple mask bits are "or" not "and".

That udev rule seems a little iffy. It has comments: "hmm, this might be racy", and "TODO: this heuristic might need some work". So I would not be surprised at some problems. After the error do you have a /dev/dm-4?

catkin · 09-10-2013, 04:13 AM

Thanks smallpond

"After the error do you have a /dev/dm-4?" No, at least not when I look. I'm far from having a clear picture of what's going on but I interpret this to mean that lvremove has been able to remove the LVM volume because it caught it at a time when udev's udisk rule wasn't making it busy. This results in /dev/dm-4 being removed and hence the error message.