cleancache: forbid overriding cleancache_ops

Currently, cleancache_register_ops returns the previous value of
cleancache_ops to allow chaining.  However, chaining, as it is
implemented now, is extremely dangerous due to possible pool id
collisions.  Suppose, a new cleancache driver is registered after the
previous one assigned an id to a super block.  If the new driver assigns
the same id to another super block, which is perfectly possible, we will
have two different filesystems using the same id.  No matter if the new
driver implements chaining or not, we are likely to get data corruption
with such a configuration eventually.

This patch therefore disables the ability to override cleancache_ops
altogether as potentially dangerous.  If there is already cleancache
driver registered, all further calls to cleancache_register_ops will
return EBUSY.  Since no user of cleancache implements chaining, we only
need to make minor changes to the code outside the cleancache core.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Stefan Hengelein <ilendir@googlemail.com>
Cc: Florian Schmaus <fschmaus@gmail.com>
Cc: Andor Daam <andor.daam@googlemail.com>
Cc: Dan Magenheimer <dan.magenheimer@oracle.com>
Cc: Bob Liu <lliubbo@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 3 deletions

diff --git a/Documentation/vm/cleancache.txt b/Documentation/vm/cleancache.txt
index 01d7628..e4b49df 100644
--- a/Documentation/vm/cleancache.txt
+++ b/Documentation/vm/cleancache.txt
@@ -28,9 +28,7 @@ IMPLEMENTATION OVERVIEW
 A cleancache "backend" that provides transcendent memory registers itself
 to the kernel's cleancache "frontend" by calling cleancache_register_ops,
 passing a pointer to a cleancache_ops structure with funcs set appropriately.
-Note that cleancache_register_ops returns the previous settings so that
-chaining can be performed if desired. The functions provided must conform to
-certain semantics as follows:
+The functions provided must conform to certain semantics as follows:
 
 Most important, cleancache is "ephemeral".  Pages which are copied into
 cleancache have an indefinite lifetime which is completely unknowable