[RFC][PATCH 2/2] Make watermarks tunable separately

[Satoru Moriya <satoru.moriya@hds.com>]

This patch introduces three new sysctls to /proc/sys/vm:
wmark_min_kbytes, wmark_low_kbytes and wmark_high_kbytes.

Each entry is used to compute watermark[min], watermark[low]
and watermark[high] for each zone.

These parameters are also updated when min_free_kbytes are
changed because originally they are set based on min_free_kbytes.
On the other hand, min_free_kbytes is updated when wmark_free_kbytes
changes.

By using the parameters one can adjust the difference among
watermark[min], watermark[low] and watermark[high] and as a result
one can tune the kernel reclaim behaviour to fit their requirement.

Signed-off-by: Satoru Moriya <satoru.moriya@hds.com>
---
 Documentation/sysctl/vm.txt |   37 +++++++++++++++
 include/linux/mmzone.h      |    6 ++
 kernel/sysctl.c             |   28 +++++++++++-
 mm/page_alloc.c             |  109 +++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 179 insertions(+), 1 deletions(-)

diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index e10b279..674681d 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -55,6 +55,9 @@ Currently, these files are in /proc/sys/vm:
 - stat_interval
 - swappiness
 - vfs_cache_pressure
+- wmark_high_kbytes
+- wmark_low_kbytes
+- wmark_min_kbytes
 - zone_reclaim_mode
 
 ==============================================================
@@ -360,6 +363,8 @@ become subtly broken, and prone to deadlock under high loads.
 
 Setting this too high will OOM your machine instantly.
 
+This is also updated when wmark_min_free_kbytes changes.
+
 =============================================================
 
 min_slab_ratio:
@@ -664,6 +669,38 @@ causes the kernel to prefer to reclaim dentries and inodes.
 
 ==============================================================
 
+wmark_high_kbytes
+
+Contains the amount of free memory above which kswapd stops reclaiming pages.
+
+The Linux VM uses this number to compute a watermark[WMARK_HIGH] value for
+each zone in the system. This is also updated when min_free_kbytes is updated.
+The minimum is wmark_low_kbytes.
+
+==============================================================
+
+wmark_low_kbytes
+
+Contains the amount of free memory below which kswapd starts to reclaim pages.
+
+The Linux VM uses this number to compute a watermark[WMARK_LOW] value for
+each zone in the system. This is also updated when min_free_kbytes changes