[PATCH 17/45] writeback: only allow two background writeback works

[Wu Fengguang]

balance_dirty_pages() need a reliable way to ensure some background work
is scheduled for running. We cannot simply run bdi_start_writeback()
because that would queue up huge number of works (which takes memory and
time).

CC: Jens Axboe <jens.axboe@xxxxxxxxxx> 
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 fs/fs-writeback.c           |   14 ++------------
 include/linux/backing-dev.h |   26 +++++++++++++++++++++++++-
 2 files changed, 27 insertions(+), 13 deletions(-)

--- linux.orig/fs/fs-writeback.c	2009-10-06 23:38:42.000000000 +0800
+++ linux/fs/fs-writeback.c	2009-10-06 23:38:43.000000000 +0800
@@ -85,18 +85,6 @@ static inline void bdi_work_init(struct 
 int sysctl_dirty_debug __read_mostly;
 
 
-/**
- * writeback_in_progress - determine whether there is writeback in progress
- * @bdi: the device's backing_dev_info structure.
- *
- * Determine whether there is writeback waiting to be handled against a
- * backing device.
- */
-int writeback_in_progress(struct backing_dev_info *bdi)
-{
-	return !list_empty(&bdi->work_list);
-}
-
 static void bdi_work_clear(struct bdi_work *work)
 {
 	clear_bit(WS_USED_B, &work->state);
@@ -147,6 +135,8 @@ static void wb_clear_pending(struct bdi_
 
 		spin_lock(&bdi->wb_lock);
 		list_del_rcu(&work->list);
+		if (work->args.for_background)
+			clear_bit(WB_FLAG_BACKGROUND_WORK, &bdi->wb_mask);
 		spin_unlock(&bdi->wb_lock);
 
 		wb_work_complete(work);
--- linux.orig/include/linux/backing-dev.h	2009-10-06 23:37:47.000000000 +0800
+++ linux/include/linux/backing-dev.h	2009-10-06 23:38:43.000000000 +0800
@@ -94,6 +94,11 @@ struct backing_dev_info {
 #endif
 };
 
+/*
+ * background work queued, set to avoid redundant background works
+ */
+#define WB_FLAG_BACKGROUND_WORK		30
+
 int bdi_init(struct backing_dev_info *bdi);
 void bdi_destroy(struct backing_dev_info *bdi);
 
@@ -248,7 +253,26 @@ int bdi_set_max_ratio(struct backing_dev
 extern struct backing_dev_info default_backing_dev_info;
 void default_unplug_io_fn(struct backing_dev_info *bdi, struct page *page);
 
-int writeback_in_progress(struct backing_dev_info *bdi);
+/**
+ * writeback_in_progress - determine whether there is writeback in progress
+ * @bdi: the device's backing_dev_info structure.
+ *
+ * Determine whether there is writeback waiting to be handled against a
+ * backing device.
+ */
+static inline int writeback_in_progress(struct backing_dev_info *bdi)
+{
+	return !list_empty(&bdi->work_list);
+}
+
+/*
+ * Helper to limit # of background writeback works in circulation to 2.
+ * (one running and another queued)
+ */
+static inline int can_submit_background_writeback(struct backing_dev_info *bdi)
+{
+       return !test_and_set_bit(WB_FLAG_BACKGROUND_WORK, &bdi->wb_mask);
+}
 
 static inline int bdi_congested(struct backing_dev_info *bdi, int bdi_bits)
 {


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