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

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Message-ID: <20140417202237.GA18016@ZenIV.linux.org.uk>
Date:	Thu, 17 Apr 2014 21:22:37 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
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

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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