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

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Date:	Thu, 17 Apr 2014 14:23:27 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Rob Landley <rob@...dley.net>,
	Miklos Szeredi <miklos@...redi.hu>,
	Christoph Hellwig <hch@...radead.org>,
	Karel Zak <kzak@...hat.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Fengguang Wu <fengguang.wu@...el.com>, tytso@....edu
Subject: Re: [GIT PULL] Detaching mounts on unlink for 3.15

Al Viro <viro@...IV.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

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