[PATCH] mm: Document improved handling of swappiness==0

[Aaron Tomlin <atomlin@redhat.com>]

Prior to upstream commit fe35004f ("[mm] avoid swapping out
with swappiness==0") setting swappiness to 0, reclaim code
could still evict recently used user anonymous memory to swap
even though there is a significant amount of RAM used for page
cache.

The behaviour of setting swappiness to 0 has since changed.
When set, the reclaim code does not initiate swap until the
amount of free pages and file-backed pages, is less than the
high water mark in a zone.

Let's update the documentation to reflect this.

Signed-off-by: Aaron Tomlin <atomlin@redhat.com>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Bryn M. Reeves <bmr@redhat.com>
---
 Documentation/sysctl/vm.txt | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index 9f5481b..38388ee 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -696,7 +696,9 @@ swappiness
 
 This control is used to define how aggressive the kernel will swap
 memory pages.  Higher values will increase agressiveness, lower values
-decrease the amount of swap.
+decrease the amount of swap.  A value of 0 instructs the kernel not to
+initiate swap until the amount of free and file-backed pages, is less
+than the high water mark in a zone.
 
 The default value is 60.
 
-- 
1.8.4.2