zsmalloc: add Kconfig for enabling page table method

Zsmalloc has two methods 1) copy-based and 2) pte based to
access objects that span two pages.
You can see history why we supported two approach from [1].

But it was bad choice that adding hard coding to select arch
which want to use pte based method because there are lots of
SoC in an architecure and they can have different cache size,
CPU speed and so on so it would be better to expose it to user
as selectable Kconfig option like Andrew Morton suggested.

[1] https://lkml.org/lkml/2012/7/11/58

Acked-by: Nitin Gupta <ngupta@vflare.org>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
zsmalloc: add more comment

This patch adds lots of comments and it will help others
to review and enhance.

Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
Signed-off-by: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
zsmalloc: move it under mm

This patch moves zsmalloc under mm directory.

Before that, description will explain why we have needed custom
allocator.

Zsmalloc is a new slab-based memory allocator for storing compressed
pages.  It is designed for low fragmentation and high allocation success
rate on large object, but <= PAGE_SIZE allocations.

zsmalloc differs from the kernel slab allocator in two primary ways to
achieve these design goals.

zsmalloc never requires high order page allocations to back slabs, or
"size classes" in zsmalloc terms.  Instead it allows multiple
single-order pages to be stitched together into a "zspage" which backs
the slab.  This allows for higher allocation success rate under memory
pressure.

Also, zsmalloc allows objects to span page boundaries within the zspage.
This allows for lower fragmentation than could be had with the kernel
slab allocator for objects between PAGE_SIZE/2 and PAGE_SIZE.  With the
kernel slab allocator, if a page compresses to 60% of it original size,
the memory savings gained through compression is lost in fragmentation
because another object of the same size can't be stored in the leftover
space.

This ability to span pages results in zsmalloc allocations not being
directly addressable by the user.  The user is given an
non-dereferencable handle in response to an allocation request.  That
handle must be mapped, using zs_map_object(), which returns a pointer to
the mapped region that can be used.  The mapping is necessary since the
object data may reside in two different noncontigious pages.

The zsmalloc fulfills the allocation needs for zram perfectly

[sjenning@linux.vnet.ibm.com: borrow Seth's quote]
Signed-off-by: Minchan Kim <minchan@kernel.org>
Acked-by: Nitin Gupta <ngupta@vflare.org>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Bob Liu <bob.liu@oracle.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Luigi Semenzato <semenzato@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Seth Jennings <sjenning@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Conflicts:
	drivers/staging/Kconfig
	drivers/staging/Makefile
	drivers/staging/zram/zram_drv.h
	drivers/staging/zsmalloc/Kconfig
	mm/Kconfig
	mm/Makefile