mm: frontswap: add frontswap header file

Frontswap is the alter ego of cleancache, the "yang" to cleancache's
"yin"... and more precisely frontswap is the provider of anonymous
pages to transcendent memory to nicely complement cleancache's providing
of clean pagecache pages to transcendent memory.  For optimal use
of transcendent memory, both are necessary... because a kernel
under memory pressure first reclaims clean pagecache pages and,
when under more memory pressure, starts swapping anonymous pages.

Frontswap and cleancache (which was merged at 3.0) are the "frontends"
and the only necessary changes to the core kernel for transcendent memory;
all other supporting code -- the "backends" -- is implemented as drivers.
See the LWN.net article "Transcendent memory in a nutshell" for a detailed
overview of frontswap and related kernel parts:
https://lwn.net/Articles/454795/

Frontswap code was first posted publicly in January 2009 and on LKML in
May 2009, and has remained functionally stable for nearly three years now.
It is barely invasive, touching only the swap subsystem and adds less
than 100 lines of code to existing swap subsystem code files.
It has improved syntactically substantially between V1 and this posting
of V14, thanks to the review of a few kernel developers, and has adapted
easily to at least one major swap subsystem change.  As of 3.4, there are
three in-tree users of frontswap patiently waiting for this patchset and
for CONFIG_FRONTSWAP to be enabled: zcache (staging driver merged at
2.6.39), Xen tmem (merged at 3.0 and 3.1) and RAMster (staging driver
merged at 3.4).  In addition, a RFC has been posted for a KVM backend.
The frontswap patchset has been in linux-next since next-110603.  Earlier
versions of frontswap already ship in the Oracle Unbreakable Enterprise Kernel
and SuSE SLES.

This patch, 1of4, provides the header file for the core code for frontswap
that interfaces between the hooks in the swap subsystem and a frontswap
backend via frontswap_ops.
mm: frontswap: core swap subsystem hooks and headers

This patch, 2of4, contains the changes to the core swap subsystem.
This includes:

(1) makes available core swap data structures (swap_lock, swap_list and
swap_info) that are needed by frontswap.c but we don't need to expose them
to the dozens of files that include swap.h so we create a new swapfile.h
just to extern-ify these and modify their declarations to non-static

(2) adds frontswap-related elements to swap_info_struct.  Frontswap_map
points to vzalloc'ed one-bit-per-swap-page metadata that indicates
whether the swap page is in frontswap or in the device and frontswap_pages
counts how many pages are in frontswap.

(3) adds hooks in the swap subsystem and extends try_to_unuse so that
frontswap_shrink can do a "partial swapoff".

Note that a failed frontswap_map allocation is safe... failure is noted
by lack of "FS" in the subsequent printk.
mm: frontswap: core frontswap functionality

This patch, 3of4, provides the core frontswap code that interfaces between
the hooks in the swap subsystem and a frontswap backend via frontswap_ops.
mm: frontswap: config and doc files

This patch 4of4 adds configuration and documentation files including a FAQ.

[v14: updated docs/FAQ to use zcache and RAMster as examples]
[v10: no change]
[v9: akpm@linux-foundation.org: sysfs->debugfs; no longer need Doc/ABI file]
[v8: rebase to 3.0-rc4]
[v7: rebase to 3.0-rc3]
[v6: rebase to 3.0-rc1]
[v5: change config default to n]
[v4: rebase to 2.6.39]
Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Acked-by: Jan Beulich <JBeulich@novell.com>
Acked-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Matthew Wilcox <matthew@wil.cx>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: Rik Riel <riel@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>