writeback: Generalize and expose wb_completion

wb_completion is used to track writeback completions.  We want to use
it from memcg side for foreign inode flushes.  This patch updates it
to remember the target waitq instead of assuming bdi->wb_waitq and
expose it outside of fs-writeback.c.

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

1 files changed, 20 insertions, 0 deletions

diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index 6a1a8a314d85..8fb740178d5d 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -67,6 +67,26 @@ enum wb_reason {
 	WB_REASON_MAX,
 };
 
+struct wb_completion {
+	atomic_t		cnt;
+	wait_queue_head_t	*waitq;
+};
+
+#define __WB_COMPLETION_INIT(_waitq)	\
+	(struct wb_completion){ .cnt = ATOMIC_INIT(1), .waitq = (_waitq) }
+
+/*
+ * If one wants to wait for one or more wb_writeback_works, each work's
+ * ->done should be set to a wb_completion defined using the following
+ * macro.  Once all work items are issued with wb_queue_work(), the caller
+ * can wait for the completion of all using wb_wait_for_completion().  Work
+ * items which are waited upon aren't freed automatically on completion.
+ */
+#define WB_COMPLETION_INIT(bdi)		__WB_COMPLETION_INIT(&(bdi)->wb_waitq)
+
+#define DEFINE_WB_COMPLETION(cmpl, bdi)	\
+	struct wb_completion cmpl = WB_COMPLETION_INIT(bdi)
+
 /*
  * For cgroup writeback, multiple wb's may map to the same blkcg.  Those
  * wb's can operate mostly independently but should share the congested