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>

1 files changed, 3 insertions, 2 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index bbccf68b38db..c98d3923d9a9 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -99,6 +99,7 @@ static bool wb_io_lists_populated(struct bdi_writeback *wb)
 		return false;
 	} else {
 		set_bit(WB_has_dirty_io, &wb->state);
+		WARN_ON_ONCE(!wb->avg_write_bandwidth);
 		atomic_long_add(wb->avg_write_bandwidth,
 				&wb->bdi->tot_write_bandwidth);
 		return true;
@@ -110,8 +111,8 @@ static void wb_io_lists_depopulated(struct bdi_writeback *wb)
 	if (wb_has_dirty_io(wb) && list_empty(&wb->b_dirty) &&
 	    list_empty(&wb->b_io) && list_empty(&wb->b_more_io)) {
 		clear_bit(WB_has_dirty_io, &wb->state);
-		atomic_long_sub(wb->avg_write_bandwidth,
-				&wb->bdi->tot_write_bandwidth);
+		WARN_ON_ONCE(atomic_long_sub_return(wb->avg_write_bandwidth,
+					&wb->bdi->tot_write_bandwidth) < 0);
 	}
 }