writeback: stop background writeback when below background threshold

Treat bdi_start_writeback(0) as a special request to do background write,
and stop such work when we are below the background dirty threshold.

Also simplify the (nr_pages <= 0) checks. Since we already pass in
nr_pages=LONG_MAX for WB_SYNC_ALL and background writes, we don't
need to worry about it being decreased to zero.

Reported-by: Richard Kennedy <richard@rsk.demon.co.uk>
CC: Jan Kara <jack@suse.cz>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index cbd4cba468bd..3c78fc316202 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -593,10 +593,10 @@ static void balance_dirty_pages(struct address_space *mapping,
          * background_thresh, to keep the amount of dirty memory low.
          */
         if ((laptop_mode && pages_written) ||
-            (!laptop_mode && ((nr_writeback = global_page_state(NR_FILE_DIRTY)
-                                          + global_page_state(NR_UNSTABLE_NFS))
+            (!laptop_mode && ((global_page_state(NR_FILE_DIRTY)
+                               + global_page_state(NR_UNSTABLE_NFS))
                                           > background_thresh)))
-                bdi_start_writeback(bdi, nr_writeback);
+                bdi_start_writeback(bdi, 0);
 }
 
 void set_page_dirty_balance(struct page *page, int page_mkwrite)