writeback: fixups for !dirty_writeback_centisecs

Commit 69b62d01 fixed up most of the places where we would enter
busy schedule() spins when disabling the periodic background
writeback. This fixes up the sb timer so that it doesn't get
hammered on with the delay disabled, and ensures that it gets
rearmed if needed when /proc/sys/vm/dirty_writeback_centisecs
gets modified.

bdi_forker_task() also needs to check for !dirty_writeback_centisecs
and use schedule() appropriately, fix that up too.
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

include/linux/backing-dev.h

@@ -109,6 +109,7 @@ void bdi_start_writeback(struct backing_dev_info *bdi, struct super_block *sb,
 				long nr_pages, int sb_locked);
 int bdi_writeback_task(struct bdi_writeback *wb);
 int bdi_has_dirty_io(struct backing_dev_info *bdi);
+void bdi_arm_supers_timer(void);
 
 extern spinlock_t bdi_lock;
 extern struct list_head bdi_list;

mm/backing-dev.c

@@ -48,7 +48,6 @@ static struct timer_list sync_supers_timer;
 
 static int bdi_sync_supers(void *);
 static void sync_supers_timer_fn(unsigned long);
-static void arm_supers_timer(void);
 
 static void bdi_add_default_flusher_task(struct backing_dev_info *bdi);
 
@@ -252,7 +251,7 @@ static int __init default_bdi_init(void)
 
 	init_timer(&sync_supers_timer);
 	setup_timer(&sync_supers_timer, sync_supers_timer_fn, 0);
-	arm_supers_timer();
+	bdi_arm_supers_timer();
 
 	err = bdi_init(&default_backing_dev_info);
 	if (!err)
@@ -374,10 +373,13 @@ static int bdi_sync_supers(void *unused)
 	return 0;
 }
 
-static void arm_supers_timer(void)
+void bdi_arm_supers_timer(void)
 {
 	unsigned long next;
 
+	if (!dirty_writeback_interval)
+		return;
+
 	next = msecs_to_jiffies(dirty_writeback_interval * 10) + jiffies;
 	mod_timer(&sync_supers_timer, round_jiffies_up(next));
 }
@@ -385,7 +387,7 @@ static void arm_supers_timer(void)
 static void sync_supers_timer_fn(unsigned long unused)
 {
 	wake_up_process(sync_supers_tsk);
-	arm_supers_timer();
+	bdi_arm_supers_timer();
 }
 
 static int bdi_forker_task(void *ptr)
@@ -428,7 +430,10 @@ static int bdi_forker_task(void *ptr)
 
 			spin_unlock_bh(&bdi_lock);
 			wait = msecs_to_jiffies(dirty_writeback_interval * 10);
+			if (wait)
 				schedule_timeout(wait);
+			else
+				schedule();
 			try_to_freeze();
 			continue;
 		}

mm/page-writeback.c

@@ -690,6 +690,7 @@ int dirty_writeback_centisecs_handler(ctl_table *table, int write,
 	void __user *buffer, size_t *length, loff_t *ppos)
 {
 	proc_dointvec(table, write, buffer, length, ppos);
+	bdi_arm_supers_timer();
 	return 0;
 }