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Date: Wed, 9 Apr 2014 03:39:47 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        "Serge E. Hallyn" <serge@hallyn.com>,
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        "J. Bruce Fields" <bfields@fieldses.org>,
        Fengguang Wu <fengguang.wu@intel.com>
Subject: Re: [GIT PULL] Detaching mounts on unlink for 3.15-rc1
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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?