zram memory control enhance

Currently, zram has no feature to limit memory so theoretically zram can deplete system memory.
Users have asked for a limit several times as even without exhaustion zram makes it hard to control memory usage of the platform. This patchset adds the feature.

Patch 1 makes zs_get_total_size_bytes faster because it would be used frequently in later patches for the new feature.

Patch 2 changes zs_get_total_size_bytes's return unit from bytes to page so that zsmalloc doesn't need unnecessary operation(ie, << PAGE_SHIFT).

Patch 3 adds new feature. I added the feature into zram layer, not zsmalloc because limiation is zram's requirement, not zsmalloc so any other user using zsmalloc(ie, zpool) shouldn't affected by unnecessary branch of zsmalloc. In future, if every users of zsmalloc want the feature, then, we could move the feature from client side to zsmalloc easily but vice versa would be painful.

Patch 4 adds news facility to report maximum memory usage of zram so that this avoids user polling frequently via /sys/block/zram0/ mem_used_total and ensures transient max are not missed.

* From v4
* Add Reviewed-by - Dan
* Clean up document of mem_limit - David

* From v3
* get_zs_total_size_byte function name change - Dan
* clarifiction of the document - Dan
* atomic account instead of introducing new lock in zsmalloc - David
* remove unnecessary atomic instruction in updating max - David

* From v2
* introduce helper funcntion to update max_used_pages for readability - David
* avoid unncessary zs_get_total_size call in updating loop for max_used_pages - David

* From v1
* rebased on next-20140815
* fix up race problem - David, Dan
* reset mem_used_max as current total_bytes, rather than 0 - David
* resetting works with only "0" write for extensiblilty - David, Dan

Minchan Kim (4):
zsmalloc: move pages_allocated to zs_pool
zsmalloc: change return value unit of zs_get_total_size_bytes
zram: zram memory size limitation
zram: report maximum used memory

 Documentation/ABI/testing/sysfs-block-zram |  20 ++++++
 Documentation/blockdev/zram.txt            |  25 +++++--
 drivers/block/zram/zram_drv.c              | 101 ++++++++++++++++++++++++++++-
 drivers/block/zram/zram_drv.h              |   6 ++
 include/linux/zsmalloc.h                   |   2 +-
 mm/zsmalloc.c                              |  30 ++++-----
 6 files changed, 158 insertions(+), 26 deletions(-)

2.0.0