[PATCH 2/6] MM: annotate congestion_wait() and wait_iff_congested() as ineffective.

[NeilBrown <neilb@xxxxxxx>]

Only 4 subsystems call set_bdi_congested() or clear_bdi_congested():
 block/pktcdvd, fs/ceph fs/fuse fs/nfs

It may make sense to use congestion_wait() or wait_iff_congested()
within these subsystems, but they have no value outside of these.

Add documentation comments to these functions to discourage further use.

Signed-off-by: NeilBrown <neilb@xxxxxxx>
---
 include/linux/backing-dev.h |    7 +++++++
 mm/backing-dev.c            |    9 +++++++++
 2 files changed, 16 insertions(+)

diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index ac7f231b8825..cc9513840351 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -153,6 +153,13 @@ static inline int wb_congested(struct bdi_writeback *wb, int cong_bits)
 	return wb->congested & cong_bits;
 }
 
+/* NOTE congestion_wait() and wait_iff_congested() are
+ * largely useless except as documentation.
+ * congestion_wait() will (almost) always wait for the given timeout.
+ * wait_iff_congested() will (almost) never wait, but will call
+ * cond_resched().
+ * Were possible an alternative waiting strategy should be found.
+ */
 long congestion_wait(int sync, long timeout);
 long wait_iff_congested(int sync, long timeout);
 
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 4a9d4e27d0d9..53472ab38796 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -1023,6 +1023,11 @@ EXPORT_SYMBOL(set_bdi_congested);
  * Waits for up to @timeout jiffies for a backing_dev (any backing_dev) to exit
  * write congestion.  If no backing_devs are congested then just wait for the
  * next write to be completed.
+ *
+ * NOTE: in the current implementation, hardly any backing_devs are ever
+ * marked as congested, and write-completion is rarely reported (see calls
+ * to clear_bdi_congested).  So this should not be assumed to ever wake before
+ * the timeout.
  */
 long congestion_wait(int sync, long timeout)
 {
@@ -1054,6 +1059,10 @@ EXPORT_SYMBOL(congestion_wait);
  * The return value is 0 if the sleep is for the full timeout. Otherwise,
  * it is the number of jiffies that were still remaining when the function
  * returned. return_value == timeout implies the function did not sleep.
+ *
+ * NOTE: in the current implementation, hardly any backing_devs are ever
+ * marked as congested, and write-completion is rarely reported (see calls
+ * to clear_bdi_congested).  So this should not be assumed to sleep at all.
  */
 long wait_iff_congested(int sync, long timeout)
 {