From 40405851af73c59678ffd8f490e6b288c7fbaf29 Mon Sep 17 00:00:00 2001 From: Jeff Moyer Date: Tue, 8 Jan 2019 16:57:34 -0500 Subject: [PATCH] block: clarify documentation for blk_{start|finish}_plug 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" Reviewed-by: Darrick J. Wong Reviewed-by: Ming Lei Signed-off-by: Jeff Moyer Signed-off-by: Jens Axboe --- block/blk-core.c | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) 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) -- 2.11.0