[RFC][PATCH 3/7] writeback: merge for_kupdate and !for_kupdate requeue io logics

[Wu Fengguang <fengguang.wu@intel.com>]

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Unify the logic for kupdate and non-kupdate cases.
There won't be starvation because the inodes requeued into b_more_io or
b_more_io_wait will later be spliced _after_ the remaining inodes in b_io,
hence won't stand in the way of other inodes in the next run.

CC: Dave Chinner <david@fromorbit.com>
Cc: Martin Bligh <mbligh@google.com>
Cc: Michael Rubin <mrubin@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
---
 fs/fs-writeback.c |   39 ++++++---------------------------------
 1 file changed, 6 insertions(+), 33 deletions(-)

--- linux.orig/fs/fs-writeback.c	2009-09-09 20:47:11.000000000 +0800
+++ linux/fs/fs-writeback.c	2009-09-09 20:48:01.000000000 +0800
@@ -426,45 +426,18 @@ writeback_single_inode(struct inode *ino
 		} else if (mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
 			/*
 			 * We didn't write back all the pages.  nfs_writepages()
-			 * sometimes bales out without doing anything. Redirty
-			 * the inode; Move it from b_io onto b_more_io/b_dirty.
+			 * sometimes bales out without doing anything.
 			 */
-			/*
-			 * akpm: if the caller was the kupdate function we put
-			 * this inode at the head of b_dirty so it gets first
-			 * consideration.  Otherwise, move it to the tail, for
-			 * the reasons described there.  I'm not really sure
-			 * how much sense this makes.  Presumably I had a good
-			 * reasons for doing it this way, and I'd rather not
-			 * muck with it at present.
-			 */
-			if (wbc->for_kupdate) {
+			inode->i_state |= I_DIRTY_PAGES;
+			if (wbc->nr_to_write <= 0) {
 				/*
-				 * For the kupdate function we move the inode
-				 * to b_more_io so it will get more writeout as
-				 * soon as the queue becomes uncongested.
+				 * slice used up: queue for next turn
 				 */
-				inode->i_state |= I_DIRTY_PAGES;
-				if (wbc->nr_to_write <= 0) {
-					/*
-					 * slice used up: queue for next turn
-					 */
-					requeue_io(inode);
-				} else {
-					/*
-					 * somehow blocked: retry later
-					 */
-					redirty_tail(inode);
-				}
+				requeue_io(inode);
 			} else {
 				/*
-				 * Otherwise fully redirty the inode so that
-				 * other inodes on this superblock will get some
-				 * writeout.  Otherwise heavy writing to one
-				 * file would indefinitely suspend writeout of
-				 * all the other files.
+				 * somehow blocked: retry later
 				 */
-				inode->i_state |= I_DIRTY_PAGES;
 				redirty_tail(inode);
 			}
 		} else if (atomic_read(&inode->i_count)) {

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