writeback: make bdi_has_dirty_io() take multiple bdi_writeback's into account

bdi_has_dirty_io() used to only reflect whether the root wb
(bdi_writeback) has dirty inodes.  For cgroup writeback support, it
needs to take all active wb's into account.  If any wb on the bdi has
dirty inodes, bdi_has_dirty_io() should return true.

To achieve that, as inode_wb_list_{move|del}_locked() now keep track
of the dirty state transition of each wb, the number of dirty wbs can
be counted in the bdi; however, bdi is already aggregating
wb->avg_write_bandwidth which can easily be guaranteed to be > 0 when
there are any dirty inodes by ensuring wb->avg_write_bandwidth can't
dip below 1.  bdi_has_dirty_io() can simply test whether
bdi->tot_write_bandwidth is zero or not.

While this bumps the value of wb->avg_write_bandwidth to one when it
used to be zero, this shouldn't cause any meaningful behavior
difference.

bdi_has_dirty_io() is made an inline function which tests whether
->tot_write_bandwidth is non-zero.  Also, WARN_ON_ONCE()'s on its
value are added to inode_wb_list_{move|del}_locked().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

2 files changed, 7 insertions, 8 deletions

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 161ddf1..d2f16fc9 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -256,11 +256,6 @@ static int __init default_bdi_init(void)
 }
 subsys_initcall(default_bdi_init);
 
-bool bdi_has_dirty_io(struct backing_dev_info *bdi)
-{
-	return wb_has_dirty_io(&bdi->wb);
-}
-
 /*
  * This function is used when the first inode for this wb is marked dirty. It
  * wakes-up the corresponding bdi thread which should then take care of the
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index c95eb24..99b8846 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -881,9 +881,13 @@ static void wb_update_write_bandwidth(struct bdi_writeback *wb,
 		avg += (old - avg) >> 3;
 
 out:
-	if (wb_has_dirty_io(wb))
-		atomic_long_add(avg - wb->avg_write_bandwidth,
-				&wb->bdi->tot_write_bandwidth);
+	/* keep avg > 0 to guarantee that tot > 0 if there are dirty wbs */
+	avg = max(avg, 1LU);
+	if (wb_has_dirty_io(wb)) {
+		long delta = avg - wb->avg_write_bandwidth;
+		WARN_ON_ONCE(atomic_long_add_return(delta,
+					&wb->bdi->tot_write_bandwidth) <= 0);
+	}
 	wb->write_bandwidth = bw;
 	wb->avg_write_bandwidth = avg;
 }