From: Larry Woodman <lwoodman@redhat.com>
Date: Wed, 15 Apr 2009 16:00:43 -0400
Subject: [mm] msync does not sync data for a long time
Message-id: 1239825643.3749.1.camel@dhcp-100-19-198.bos.redhat.com
O-Subject: [Rhel5-U4 patch] msync(..., 0) does not sync data for a long time
Bugzilla: 479079
RH-Acked-by: Rik van Riel <riel@redhat.com>

If you do not pass MS_SYNC to msync() it does nothing and relies on
kupdate to flush out the file's dirty pages.  kupdate has a problem
where if you extend a file and dirty at lease one page in that extent
every 30 seconds it will not start at the beginning and write out dirty
pages in that file at lower offsets until it looks at that file once and
no extensions and modifications were made.

The reason for this is wb_kupdate sets wbc.range_cyclic = 1 and
eventually calls mpage_writepages().  mpage_writepages() does not reset
mapping->writeback_index = 0 unless it did not find any modified
pagecache pages for that file the last time it ran.  This causes it to
start looking at the last offset it used on the previous kupdate run
rather than zero.  If an application extends and modifies a file's pages
at a rate of at least 1 per 30 seconds kupdate will not write modified
pages in that file at lower indexes.

The fix for this is to initialize and pass the wbc range_start and
range_end down to mpage_writepages() and change mpage_writepages() to
not use range_cyclic for kupdate.  The performance consequence of this
is no worse than calling mpage_writepages() from kupdate if pages were
modified the last time it ran.

Fixes BZ479079

diff --git a/fs/mpage.c b/fs/mpage.c
index 2d11334..44215a9 100644
--- a/fs/mpage.c
+++ b/fs/mpage.c
@@ -862,7 +862,7 @@ mpage_writepages(struct address_space *mapping,
 		writepage = mapping->a_ops->writepage;
 
 	pagevec_init(&pvec, 0);
-	if (wbc->range_cyclic) {
+	if (wbc->range_cyclic && !wbc->for_kupdate) {
 		index = mapping->writeback_index; /* Start from prev offset */
 		end = -1;
 	} else {
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index e9fb402..cb415a8 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -405,6 +405,8 @@ static void wb_kupdate(unsigned long arg)
 		.nonblocking	= 1,
 		.for_kupdate	= 1,
 		.range_cyclic	= 1,
+		.range_start	= 0,
+		.range_end 	= LLONG_MAX,
 	};
 
 	sync_supers();