Ceph » rbd

rbd: fix up the layering warning message

A warning gets spewed for any image being probed, including parent
images. Set up a condition such that the warning message only gets
printed for the image being mapped, not any of its parents.

Also, I didn't like the way the warning ended up being so long.
Make it a terse warning instead. People experimenting with layering
will know what the message means.

Signed-off-by: Alex Elder <elder@inktank.com>

rbd: snap names are pointer to constant data

Make explicit that snapshot names don't change by making functions
return and take parameters that that point to const qualified data.

Signed-off-by: Alex Elder <elder@inktank.com>

rbd: don't revalidate so much

Whenever a header object event causes a mapped rbd image to refresh
its header information, revalidate_disk() is being called. This was
done in rbd_dev_refresh() outside the control mutex in order to
avoid a lock inversion. Although a an event like this might
indicate the image has changed size, most of the time it does not.

Record the image size before and after the refresh, and only
call revalidate_disk() if it changes.

Signed-off-by: Alex Elder <elder@inktank.com>