[RFC][PATCH 2/3] VM throttling: Calc dirty limit based on vm.dirty_limit_ratio

[Tomoki Sekiyama <tomoki.sekiyama.qu@hitachi.com>]

This patch modifies get_dirty_limits() to calculate the limit of dirty
pages based on vm.dirty_limit_ratio. It also changes the interface of the
function to return it.

If mapped memory become large and limit_ratio (calculated based on
vm.dirty_limit_ratio) is decreased, dirty_ratio is also decreased to
keep writeback independently among disks.

Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama.qu@hitachi.com>
Signed-off-by: Yuji Kakutani <yuji.kakutani.uw@hitachi.com>
---
 mm/page-writeback.c |   37 +++++++++++++++++++++++++++++--------
 1 file changed, 29 insertions(+), 8 deletions(-)

Index: linux-2.6.20-mm2-bdp/mm/page-writeback.c
===================================================================
--- linux-2.6.20-mm2-bdp.orig/mm/page-writeback.c
+++ linux-2.6.20-mm2-bdp/mm/page-writeback.c
@@ -117,6 +117,9 @@ static void background_writeout(unsigned
  * performing lots of scanning.
  *
  * We only allow 1/2 of the currently-unmapped memory to be dirtied.
+ * `dirty_limit_ratio' is ignored if it is larger than that.
+ * In this case, `dirty_ratio' is decreased to keep writeback independently
+ * among disks.
  *
  * We don't permit the clamping level to fall below 5% - that is getting rather
  * excessive.
@@ -163,14 +166,16 @@ static unsigned long determine_dirtyable
 }

 static void
-get_dirty_limits(long *pbackground, long *pdirty,
+get_dirty_limits(long *pbackground, long *pdirty, long *plimit,
 					struct address_space *mapping)
 {
 	int background_ratio;		/* Percentages */
 	int dirty_ratio;
+	int limit_ratio;
 	int unmapped_ratio;
 	long background;
 	long dirty;
+	long limit;
 	unsigned long available_memory = determine_dirtyable_memory();
 	struct task_struct *tsk;

@@ -178,10 +183,17 @@ get_dirty_limits(long *pbackground, long
 				global_page_state(NR_ANON_PAGES)) * 100) /
 					available_memory;

+	limit_ratio = dirty_limit_ratio;
+	if (limit_ratio > unmapped_ratio / 2)
+		limit_ratio = unmapped_ratio / 2;
+
 	dirty_ratio = vm_dirty_ratio;
-	if (dirty_ratio > unmapped_ratio / 2)
-		dirty_ratio = unmapped_ratio / 2;
+	if (dirty_ratio > dirty_limit_ratio)
+		dirty_ratio = dirty_limit_ratio;
+	dirty_ratio -= dirty_limit_ratio - limit_ratio;

+	if (dirty_limit_ratio < 5)
+		dirty_limit_ratio = 5;
 	if (dirty_ratio < 5)
 		dirty_ratio = 5;

@@ -191,13 +203,16 @@ get_dirty_limits(long *pbackground, long

 	background = (background_ratio * available_memory) / 100;
 	dirty = (dirty_ratio * available_memory) / 100;
+	limit = (limit_ratio * available_memory) / 100;
 	tsk = current;
 	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
 		background += background / 4;
 		dirty += dirty / 4;
+		limit += limit / 4;
 	}
 	*pbackground = background;
 	*pdirty = dirty;
+	*plimit = limit;
 }

 /*
@@ -212,6 +227,7 @@ static void balance_dirty_pages(struct a
 	long nr_reclaimable;
 	long background_thresh;
 	long dirty_thresh;
+	long dirty_limit;
 	unsigned long pages_written = 0;
 	unsigned long write_chunk = sync_writeback_pages();

@@ -226,7 +242,8 @@ static void balance_dirty_pages(struct a
 			.range_cyclic	= 1,
 		};

-		get_dirty_limits(&background_thresh, &dirty_thresh, mapping);
+		get_dirty_limits(&background_thresh, &dirty_thresh,
+							&dirty_limit, mapping);
 		nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
 					global_page_state(NR_UNSTABLE_NFS);
 		if (nr_reclaimable + global_page_state(NR_WRITEBACK) <=
@@ -244,8 +261,8 @@ static void balance_dirty_pages(struct a
 		 */
 		if (nr_reclaimable) {
 			writeback_inodes(&wbc);
-			get_dirty_limits(&background_thresh,
-					 	&dirty_thresh, mapping);
+			get_dirty_limits(&background_thresh, &dirty_thresh,
+					 		&dirty_limit, mapping);
 			nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
 					global_page_state(NR_UNSTABLE_NFS);
 			if (nr_reclaimable +
@@ -335,9 +352,11 @@ void throttle_vm_writeout(void)
 {
 	long background_thresh;
 	long dirty_thresh;
+	long dirty_limit;

         for ( ; ; ) {
-		get_dirty_limits(&background_thresh, &dirty_thresh, NULL);
+		get_dirty_limits(&background_thresh, &dirty_thresh,
+							&dirty_limit, NULL);

                 /*
                  * Boost the allowable dirty threshold a bit for page
@@ -372,8 +391,10 @@ static void background_writeout(unsigned
 	for ( ; ; ) {
 		long background_thresh;
 		long dirty_thresh;
+		long dirty_limit;

-		get_dirty_limits(&background_thresh, &dirty_thresh, NULL);
+		get_dirty_limits(&background_thresh, &dirty_thresh,
+							&dirty_limit,  NULL);
 		if (global_page_state(NR_FILE_DIRTY) +
 			global_page_state(NR_UNSTABLE_NFS) < background_thresh
 				&& min_pages <= 0)