[PATCH] doc: add description to dirtytime_expire_seconds

[Yang Shi <yang.shi@linux.alibaba.com>]

commit 1efff914afac8a965ad63817ecf8861a927c2ace ("fs: add
dirtytime_expire_seconds sysctl") introduced dirtytime_expire_seconds
knob, but there is not description about it in
Documentation/sysctl/vm.txt.

Add the description for it.

Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
---
I didn't dig into the old review discussion about why the description
was not added at the first place. I'm supposed every knob under /proc/sys
should have a brief description.

 Documentation/sysctl/vm.txt | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index 17256f2..f4f4f9c 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -27,6 +27,7 @@ Currently, these files are in /proc/sys/vm:
 - dirty_bytes
 - dirty_expire_centisecs
 - dirty_ratio
+- dirtytime_expire_seconds
 - dirty_writeback_centisecs
 - drop_caches
 - extfrag_threshold
@@ -178,6 +179,16 @@ The total available memory is not equal to total system memory.
 
 ==============================================================
 
+dirtytime_expire_seconds
+
+When a lazytime inode is constantly having its pages dirtied, it with an
+updated timestamp will never get chance to be written out.  This tunable
+is used to define when dirty inode is old enough to be eligible for
+writeback by the kernel flusher threads. And, it is also used as the
+interval to wakeup dirtytime_writeback thread. It is expressed in seconds.
+
+==============================================================
+
 dirty_writeback_centisecs
 
 The kernel flusher threads will periodically wake up and write `old' data
-- 
1.8.3.1