[TriLUG] How to monitor mount point changes?

[Ed Blackman via TriLUG]

Is there a way to wait for mount point changes without polling?

I had an external disk fail over the weekend, and only noticed when jobs 
that accessed the disk failed with weird errors, causing me to have to 
debug back to the hardware failure.

As part of the response to the failures, Linux remounted the disk 
read-only.  I'd like to put monitoring in place so that if something 
similar happens in the future, I'll get a clear "/mountpoint remounted 
read-only" alert to point me more directly at what's wrong.

I can certainly create a systemd timer to run periodically and check for 
unexpected read-only filesystems.  But then I have to trade off the load 
of polling frequently vs a delay due to polling infrequently, which I 
would really prefer to block somehow waiting for changes to 
/proc/mounts.

I tried a systemd path unit looking for PathModified for /proc/mounts 
(also tried /proc/self/mounts based on some searching), but the 
associated service didn't get triggered when I mounted or unmounted a 
loopback device.

I tried using udevadm monitor, which did generate events on mount or 
unmount, but not on "mount -o remount,ro /mountpoint".

Searching didn't turn up anything else.  Is there some way to either 
directly do this?  If not, how do you get notified of such things?

-- 
Ed Blackman