Re: [GIT PULL] Detaching mounts on unlink for 3.15

[Al Viro]

Have you tried to profile something like umount -l on a large mount tree?
You variant causes a shitstorm of
	schedule work
	switch to workqueue
	do actual fs shutdown
	wake umount(8) up
	get through wait_for_completion()
for every bleeding vfsmount in there.  And no, it's *not* guaranteed to
be dominated by fs shutdown time.  Here's the case where it definitely
won't be:
	mkdir /tmp/a
	mount --rbind / /tmp/a
	umount -l /tmp/a

All vfsmounts involved are killed off with no fs shutdown.  And that's
*not* a rare case - exit of the last process in namespace is very likely
to look that way too.

That's far too heavy.
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