zbud: add to mm/

zbud: add to mm/

zbud is an special purpose allocator for storing compressed pages.  It is
designed to store up to two compressed pages per physical page.  While
this design limits storage density, it has simple and deterministic
reclaim properties that make it preferable to a higher density approach
when reclaim will be used.

zbud works by storing compressed pages, or "zpages", together in pairs in
a single memory page called a "zbud page".  The first buddy is "left
justifed" at the beginning of the zbud page, and the last buddy is "right
justified" at the end of the zbud page.  The benefit is that if either
buddy is freed, the freed buddy space, coalesced with whatever slack space
that existed between the buddies, results in the largest possible free
region within the zbud page.

zbud also provides an attractive lower bound on density.  The ratio of
zpages to zbud pages can not be less than 1.  This ensures that zbud can
never "do harm" by using more pages to store zpages than the uncompressed
zpages would have used on their own.

This implementation is a rewrite of the zbud allocator internally used by
zcache in the driver/staging tree.  The rewrite was necessary to remove
some of the zcache specific elements that were ingrained throughout and
provide a generic allocation interface that can later be used by zsmalloc
and others.

This patch adds zbud to mm/ for later use by zswap.

Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Dan Magenheimer <dan.magenheimer@oracle.com>
Cc: Robert Jennings <rcj@linux.vnet.ibm.com>
Cc: Jenifer Hopper <jhopper@us.ibm.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Johannes Weiner <jweiner@redhat.com>
Cc: Larry Woodman <lwoodman@redhat.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Dave Hansen <dave@sr71.net>
Cc: Joe Perches <joe@perches.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Cody P Schafer <cody@linux.vnet.ibm.com>
Cc: Hugh Dickens <hughd@google.com>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>