linux-kernel - Re: forced umount?

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Date:	Sat, 17 Mar 2007 12:53:37 +0200
From:	"Pekka Enberg" <penberg@...helsinki.fi>
To:	"Mike Snitzer" <snitzer@...il.com>
Cc:	"Jeremy Fitzhardinge" <jeremy@...p.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: forced umount?

On 3/17/07, Mike Snitzer <snitzer@...il.com> wrote:
> Thanks for the heads up; its good to see that Pekka Enberg's work has
> continued.  I actually stumbled onto that line of work earlier while
> searching for more info on Tigran Aivazian's forced unmount (badfs)
> patches:
> http://lwn.net/Articles/192632/

FYI, the revoke implementation have since been changed to follow the
badfs-style approach of the forced unmount patches. However, there are
some problems with the forced unmount patches that are now fixed in
the revoke implementation:

  - You can't use munmap() to take down shared memory mappings because the
    application can accidentally remap something completely different
to that region.
  - The ->f_light bits slow down other fget_light() users and  there's
a race between
    fcheck_files() and set_f_light().
  - The operation can live-lock if a malicious process keeps forking. The revoke
    implementation solves this by revoking in two passes: (1) take
down the descriptors
    and (2) take down the actual inodes.

                           Pekka
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