[PATCH v2 0/4] skip swapcache for super fast device

[Minchan Kim]

With fast swap storage, platform want to use swap more aggressively
and swap-in is crucial to application latency.

The rw_page based synchronous devices like zram, pmem and btt are such
fast storage. When I profile swapin performance with zram lz4 decompress
test, S/W overhead is more than 70%. Maybe, it would be bigger in nvdimm.

This patch aims for reducing swap-in latency via skipping swapcache
if swap device is synchronous device like rw_page based device.

It enhances 45% my swapin test(5G sequential swapin, no readahead,
from 2.41sec to 1.64sec).

Andrew, [1] is zram specific patch so could be applied separately
but this patch is based on that so I include it in this series.

* From v1
  * style fix
  * a bug fix
  * drop page-cluster based readahead off
    * This regression could be solved by other patch from Huang.
      http://lkml.kernel.org/r/87tw04in60.fsf@xxxxxxxxxxxxxxxxxxxx
  
Minchan Kim (4):
  [1] zram: set BDI_CAP_STABLE_WRITES once
  [2] bdi: introduce BDI_CAP_SYNCHRONOUS_IO
  [3] mm:swap: introduce SWP_SYNCHRONOUS_IO
  [4] mm:swap: skip swapcache for swapin of synchronous device

 drivers/block/brd.c           |  2 ++
 drivers/block/zram/zram_drv.c | 16 +++++--------
 drivers/nvdimm/btt.c          |  3 +++
 drivers/nvdimm/pmem.c         |  2 ++
 include/linux/backing-dev.h   |  8 +++++++
 include/linux/swap.h          | 14 +++++++++++-
 mm/memory.c                   | 52 ++++++++++++++++++++++++++++++-------------
 mm/page_io.c                  |  6 ++---
 mm/swapfile.c                 | 14 ++++++++----
 9 files changed, 83 insertions(+), 34 deletions(-)

-- 
2.7.4