There was some confusion about what these functions did. Make it clear
that this is a hint for upper layers to pass to the block layer, and
that it does not guarantee that I/O will not be submitted between a
start and finish plug.
Reported-by: "Darrick J. Wong" <darrick.wong@xxxxxxxxxx>
Signed-off-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
diff --git a/block/blk-core.c b/block/blk-core.c
index c78042975737..f2732f106a2e 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1683,6 +1683,15 @@ EXPORT_SYMBOL(kblockd_mod_delayed_work_on);
* @plug: The &struct blk_plug that needs to be initialized
*
* Description:
+ * blk_start_plug() indicates to the block layer an intent by the caller
+ * to submit multiple I/O requests in a batch. The block layer may use
+ * this hint to defer submitting I/Os from the caller until blk_finish_plug()
+ * is called. However, the block layer may choose to submit requests
+ * before a call to blk_finish_plug() if the number of queued I/Os
+ * exceeds %BLK_MAX_REQUEST_COUNT, or if the size of the I/O is larger than
+ * %BLK_PLUG_FLUSH_SIZE. The queued I/Os may also be submitted early if
+ * the task schedules (see below).
+ *
* Tracking blk_plug inside the task_struct will help with auto-flushing the
* pending I/O should the task end up blocking between blk_start_plug() and
* blk_finish_plug(). This is important from a performance perspective, but
@@ -1765,6 +1774,16 @@ void blk_flush_plug_list(struct blk_plug *plug, bool from_schedule)
blk_mq_flush_plug_list(plug, from_schedule);
}
+/**
+ * blk_finish_plug - mark the end of a batch of submitted I/O
+ * @plug: The &struct blk_plug passed to blk_start_plug()
+ *
+ * Description:
+ * Indicate that a batch of I/O submissions is complete. This function
+ * must be paired with an initial call to blk_start_plug(). The intent
+ * is to allow the block layer to optimize I/O submission. See the
+ * documentation for blk_start_plug() for more information.
+ */
void blk_finish_plug(struct blk_plug *plug)
{
if (plug != current->plug)
