Allow Xen tmem shim to be built/loaded as a module.  Xen self-ballooning
and frontswap-selfshrinking are now also "lazily" initialized when the
Xen tmem shim is loaded as a module, unless explicitly disabled by
module parameters.

Note runtime dependency disallows loading if cleancache/frontswap lazy
initialization patches are not present.

If built-in (not built as a module), the original mechanism of enabling
via a kernel boot parameter is retained, but this should be considered
deprecated.

Note that module unload is explicitly not yet supported.

[v1: Removed the [CLEANCACHE|FRONTSWAP]_HAS_LAZY_INIT ifdef]
[v2: Squashed the xen/tmem: Remove the subsys call patch in]
[akpm@linux-foundation.org: fix build (disable_frontswap_selfshrinking undeclared)]
Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Bob Liu <lliubbo@gmail.com>
Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Cc: Andor Daam <andor.daam@googlemail.com>
Cc: Florian Schmaus <fschmaus@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Stefan Hengelein <ilendir@googlemail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Use 'bool' for boolean variables. Do proper section placement.
Eliminate an unnecessary export.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

This patch introduces two in-kernel drivers for Xen transcendent memory
("tmem") functionality that complement cleancache and frontswap.  Both
use control theory to dynamically adjust and optimize memory utilization.
Selfballooning controls the in-kernel Xen balloon driver, targeting a goal
value (vm_committed_as), thus pushing less frequently used clean
page cache pages (through the cleancache code) into Xen tmem where
Xen can balance needs across all VMs residing on the physical machine.
Frontswap-selfshrinking controls the number of pages in frontswap,
driving it towards zero (effectively doing a partial swapoff) when
in-kernel memory pressure subsides, freeing up RAM for other VMs.

More detail is provided in the header comment of xen-selfballooning.c.

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>

[v8: konrad.wilk@oracle.com: set default enablement depending on frontswap]
[v7: konrad.wilk@oracle.com: fix capitalization and punctuation in comments]
[v6: fix frontswap-selfshrinking initialization]
[v6: konrad.wilk@oracle.com: fix init pr_infos; add comments about swap]
[v5: konrad.wilk@oracle.com: add NULL to attr list; move inits up to decls]
[v4: dkiper@net-space.pl: use strict_strtoul plus a few syntactic nits]
[v3: konrad.wilk@oracle.com: fix potential divides-by-zero]
[v3: konrad.wilk@oracle.com: add many more comments, fix nits]
[v2: rebased to linux-3.0-rc1]
[v2: Ian.Campbell@citrix.com: reorganize as new file (xen-selfballoon.c)]
[v2: dkiper@net-space.pl: proper access to vm_committed_as]
[v2: dkiper@net-space.pl: accounting fixes]
Cc: Jan Beulich <JBeulich@novell.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: <xen-devel@lists.xensource.com>