[PATCH 4/5] block: shrink plug->{nr_ios, rq_count} to unsigned char

[Jens Axboe <axboe@xxxxxxxxx>]

We never use more than 64 max in here, we can change them from unsigned
short to just a byte. Add a BUILD_BUG_ON() check, in case the max plug
count changes in the future.

Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
---
 block/blk-core.c       | 4 ++--
 block/blk-mq.c         | 2 ++
 include/linux/blkdev.h | 8 ++++----
 3 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index cc4db4d92c75..902799f71a59 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1063,7 +1063,7 @@ int kblockd_mod_delayed_work_on(int cpu, struct delayed_work *dwork,
 }
 EXPORT_SYMBOL(kblockd_mod_delayed_work_on);
 
-void blk_start_plug_nr_ios(struct blk_plug *plug, unsigned short nr_ios)
+void blk_start_plug_nr_ios(struct blk_plug *plug, unsigned char nr_ios)
 {
 	struct task_struct *tsk = current;
 
@@ -1076,7 +1076,7 @@ void blk_start_plug_nr_ios(struct blk_plug *plug, unsigned short nr_ios)
 	plug->cur_ktime = 0;
 	plug->mq_list = NULL;
 	plug->cached_rq = NULL;
-	plug->nr_ios = min_t(unsigned short, nr_ios, BLK_MAX_REQUEST_COUNT);
+	plug->nr_ios = min_t(unsigned char, nr_ios, BLK_MAX_REQUEST_COUNT);
 	plug->rq_count = 0;
 	plug->multiple_queues = false;
 	plug->has_elevator = false;
diff --git a/block/blk-mq.c b/block/blk-mq.c
index aff9e9492f59..a9b4a66e1e13 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1283,6 +1283,8 @@ EXPORT_SYMBOL(blk_mq_start_request);
  */
 static inline unsigned short blk_plug_max_rq_count(struct blk_plug *plug)
 {
+	BUILD_BUG_ON(2 * BLK_MAX_REQUEST_COUNT > U8_MAX);
+
 	if (plug->multiple_queues)
 		return BLK_MAX_REQUEST_COUNT * 2;
 	return BLK_MAX_REQUEST_COUNT;
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 81a7fca1b4f7..5b17d0e460e4 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -943,9 +943,9 @@ struct blk_plug {
 	/* if ios_left is > 1, we can batch tag/rq allocations */
 	struct request *cached_rq;
 	u64 cur_ktime;
-	unsigned short nr_ios;
+	unsigned char nr_ios;
 
-	unsigned short rq_count;
+	unsigned char rq_count;
 
 	bool multiple_queues;
 	bool has_elevator;
@@ -963,7 +963,7 @@ struct blk_plug_cb {
 extern struct blk_plug_cb *blk_check_plugged(blk_plug_cb_fn unplug,
 					     void *data, int size);
 extern void blk_start_plug(struct blk_plug *);
-extern void blk_start_plug_nr_ios(struct blk_plug *, unsigned short);
+extern void blk_start_plug_nr_ios(struct blk_plug *, unsigned char);
 extern void blk_finish_plug(struct blk_plug *);
 
 void __blk_flush_plug(struct blk_plug *plug, bool from_schedule);
@@ -1010,7 +1010,7 @@ struct blk_plug {
 };
 
 static inline void blk_start_plug_nr_ios(struct blk_plug *plug,
-					 unsigned short nr_ios)
+					 unsigned char nr_ios)
 {
 }
 
-- 
2.43.0