[patch 1/1] Writeback fix for concurrent large and small file writes.

[mrubin@google.com (Michael Rubin)]

From: Michael Rubin <mrubin@google.com>

Fixing a bug where writing to large files while concurrently writing to
smaller ones creates a situation where writeback cannot keep up with the
traffic and memory baloons until the we hit the threshold watermark. This
can result in surprising latency spikes when syncing. This latency
can take minutes on large memory systems. Upon request I can provide
a test to reproduce this situation.

The only concern I have is that this makes the wb_kupdate slightly more
agressive. I am not sure it is enough to cause any problems. I think
there is enough checks to throttle the background activity.

Feng also the one line change that you recommended here 
http://marc.info/?l=linux-kernel&m=119629655402153&w=2 had no effect.

Signed-off-by: Michael Rubin <mrubin@google.com>
---
Index: 2624rc3_feng/fs/fs-writeback.c
===================================================================
--- 2624rc3_feng.orig/fs/fs-writeback.c	2007-11-29 14:44:24.000000000 -0800
+++ 2624rc3_feng/fs/fs-writeback.c	2007-12-10 17:21:45.000000000 -0800
@@ -408,8 +408,7 @@ sync_sb_inodes(struct super_block *sb, s
 {
 	const unsigned long start = jiffies;	/* livelock avoidance */
 
-	if (!wbc->for_kupdate || list_empty(&sb->s_io))
-		queue_io(sb, wbc->older_than_this);
+	queue_io(sb, wbc->older_than_this);
 
 	while (!list_empty(&sb->s_io)) {
 		struct inode *inode = list_entry(sb->s_io.prev,
Index: 2624rc3_feng/mm/page-writeback.c
===================================================================
--- 2624rc3_feng.orig/mm/page-writeback.c	2007-11-16 21:16:36.000000000 -0800
+++ 2624rc3_feng/mm/page-writeback.c	2007-12-10 17:37:17.000000000 -0800
@@ -638,7 +638,7 @@ static void wb_kupdate(unsigned long arg
 		wbc.nr_to_write = MAX_WRITEBACK_PAGES;
 		writeback_inodes(&wbc);
 		if (wbc.nr_to_write > 0) {
-			if (wbc.encountered_congestion || wbc.more_io)
+			if (wbc.encountered_congestion)
 				congestion_wait(WRITE, HZ/10);
 			else
 				break;	/* All the old data is written */
--
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