linux-kernel - Re: [GIT PULL] Detaching mounts on unlink for 3.15-rc1

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Message-ID: <20140409023947.GY18016@ZenIV.linux.org.uk>
Date:	Wed, 9 Apr 2014 03:39:47 +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>
Subject: Re: [GIT PULL] Detaching mounts on unlink for 3.15-rc1

On Wed, Apr 09, 2014 at 03:30:27AM +0100, Al Viro wrote:

> > When renaming or unlinking directory entries that are not mountpoints
> > no additional locks are taken so no performance differences can result,
> > and my benchmark reflected that.
> 
> It also means that d_invalidate() now might trigger fs shutdown.  Which
> has bloody huge stack footprint, for obvious reasons.  And d_invalidate()
> can be called with pretty deep stack - walk into wrong dentry while
> resolving a deeply nested symlink and there you go...

PS: I thought I actually replied with that point back a month or so ago,
but having checked sent-mail...  Looks like I had not.  My deep apologies.

FWIW, I think that overall this thing is a good idea, provided that we can
live with semantics changes.  The implementation is too optimistic, though -
at the very least, we want this work done upon namespace_unlock() held
back until we are not too deep in stack.  task_work_add() fodder, perhaps?
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