page-writeback: fix the calculation of the oldest_jif in wb_kupdate()

wb_kupdate() function has a bug on linux-2.6.30-rc5.  This bug causes
generic_sync_sb_inodes() to start to write inodes back much earlier than
our expectations because it miscalculates oldest_jif in wb_kupdate().

This bug was introduced in 704503d
('mm: fix proc_dointvec_userhz_jiffies "breakage"').

Signed-off-by: Toshiyuki Okajima <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Nick Piggin <[email protected]>
Cc: Al Viro <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Toshiyuki Okajima

mm/page-writeback.c

@@ -94,12 +94,12 @@ unsigned long vm_dirty_bytes;
 /*
  * The interval between `kupdate'-style writebacks
  */
-unsigned int dirty_writeback_interval = 5 * 100; /* sentiseconds */
+unsigned int dirty_writeback_interval = 5 * 100; /* centiseconds */
 
 /*
  * The longest time for which data is allowed to remain dirty
  */
-unsigned int dirty_expire_interval = 30 * 100; /* sentiseconds */
+unsigned int dirty_expire_interval = 30 * 100; /* centiseconds */
 
 /*
  * Flag that makes the machine dump writes/reads and block dirtyings.
@@ -770,7 +770,7 @@ static void wb_kupdate(unsigned long arg)
 
 	sync_supers();
 
-	oldest_jif = jiffies - msecs_to_jiffies(dirty_expire_interval);
+	oldest_jif = jiffies - msecs_to_jiffies(dirty_expire_interval * 10);
 	start_jif = jiffies;
 	next_jif = start_jif + msecs_to_jiffies(dirty_writeback_interval * 10);
 	nr_to_write = global_page_state(NR_FILE_DIRTY) +