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

[ebiederm@xmission.com (Eric W. Biederman)]

Al Viro <viro@ZenIV.linux.org.uk> writes:

> 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.

The typically system has about 12 mounts and no mount namespaces.
Making umount of any kind a rare case.

Al my apologies for not picking your favorite solution and in choosing
not to audit every call to mntput in the tree this week I have made an
uncommon case slower.

These bug fixes are important, the code is correct, and it is
technically straight forward to remove the wait in mntput in most
cases.

I am very frustrated that when given lots of opportunities to look at
and comment on my code it is hard to get you to engage and comment
unless I send a pull request to Linus.

This last objection really looks to be a case of perfection getting in
the way of the good.

Eric