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>

2 files changed, 20 insertions, 14 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index c59d6737036c..476be9b10881 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -41,8 +41,9 @@ struct wb_writeback_args {
         long nr_pages;
         struct super_block *sb;
         enum writeback_sync_modes sync_mode;
-        int for_kupdate;
-        int range_cyclic;
+        int for_kupdate:1;
+        int range_cyclic:1;
+        int for_background:1;
 };
 
 /*
@@ -257,6 +258,15 @@ void bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages)
                 .range_cyclic   = 1,
         };
 
+        /*
+         * We treat @nr_pages=0 as the special case to do background writeback,
+         * ie. to sync pages until the background dirty threshold is reached.
+         */
+        if (!nr_pages) {
+                args.nr_pages = LONG_MAX;
+                args.for_background = 1;
+        }
+
         bdi_alloc_queue_work(bdi, &args);
 }
 
@@ -720,20 +730,16 @@ static long wb_writeback(struct bdi_writeback *wb,
 
         for (;;) {
                 /*
-                 * Don't flush anything for non-integrity writeback where
-                 * no nr_pages was given
+                 * Stop writeback when nr_pages has been consumed
                  */
-                if (!args->for_kupdate && args->nr_pages <= 0 &&
-                     args->sync_mode == WB_SYNC_NONE)
+                if (args->nr_pages <= 0)
                         break;
 
                 /*
-                 * If no specific pages were given and this is just a
-                 * periodic background writeout and we are below the
-                 * background dirty threshold, don't do anything
+                 * For background writeout, stop when we are below the
+                 * background dirty threshold
                  */
-                if (args->for_kupdate && args->nr_pages <= 0 &&
-                    !over_bground_thresh())
+                if (args->for_background && !over_bground_thresh())
                         break;
 
                 wbc.more_io = 0;
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)