[PATCH] vm: writeout watermark tuning

Slightly change the writeout watermark calculations so we keep background
and synchronous writeout watermarks in the same ratios after adjusting them
for the amout of mapped memory.  This ensures we should always attempt to
start background writeout before synchronous writeout and preserves the
admin's desired background-versus-forground ratios after we've
auto-adjusted one of them.
Signed-off-by: Nick Piggin <nickpiggin@cyberone.com.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/page-writeback.c

@@ -153,9 +153,11 @@ get_dirty_limits(struct writeback_state *wbs, long *pbackground, long *pdirty)
 	if (dirty_ratio < 5)
 		dirty_ratio = 5;
 
-	background_ratio = dirty_background_ratio;
-	if (background_ratio >= dirty_ratio)
-		background_ratio = dirty_ratio / 2;
+	/*
+	 * Keep the ratio between dirty_ratio and background_ratio roughly
+	 * what the sysctls are after dirty_ratio has been scaled (above).
+	 */
+	background_ratio = dirty_background_ratio * dirty_ratio/vm_dirty_ratio;
 
 	background = (background_ratio * total_pages) / 100;
 	dirty = (dirty_ratio * total_pages) / 100;