writeback: disable periodic old data writeback for !dirty_writeback_centisecs

Prior to 2.6.32, setting /proc/sys/vm/dirty_writeback_centisecs disabled
periodic dirty writeback from kupdate. This got broken and now causes
excessive sys CPU usage if set to zero, as we'll keep beating on
schedule().

Cc: stable@kernel.org
Reported-by: Justin Maggard <jmaggard10@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 12 insertions, 2 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 4b37f7cea4dd..760dc8d0b4ff 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -852,6 +852,12 @@ static long wb_check_old_data_flush(struct bdi_writeback *wb)
         unsigned long expired;
         long nr_pages;
 
+        /*
+         * When set to zero, disable periodic writeback
+         */
+        if (!dirty_writeback_interval)
+                return 0;
+
         expired = wb->last_old_flush +
                         msecs_to_jiffies(dirty_writeback_interval * 10);
         if (time_before(jiffies, expired))
@@ -947,8 +953,12 @@ int bdi_writeback_task(struct bdi_writeback *wb)
                                 break;
                 }
 
-                wait_jiffies = msecs_to_jiffies(dirty_writeback_interval * 10);
-                schedule_timeout_interruptible(wait_jiffies);
+                if (dirty_writeback_interval) {
+                        wait_jiffies = msecs_to_jiffies(dirty_writeback_interval * 10);
+                        schedule_timeout_interruptible(wait_jiffies);
+                } else
+                        schedule();
+
                 try_to_freeze();
         }