linux-kernel - [PATCH] null-blk: save memory footprint for struct nullb

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Message-ID: <20220426022133.3999006-1-yukuai3@huawei.com>
Date:   Tue, 26 Apr 2022 10:21:33 +0800
From:   Yu Kuai <yukuai3@...wei.com>
To:     <axboe@...nel.dk>, <shinichiro.kawasaki@....com>,
        <chaitanya.kulkarni@....com>
CC:     <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <yukuai3@...wei.com>, <yi.zhang@...wei.com>
Subject: [PATCH] null-blk: save memory footprint for struct nullb_cmd

Total 16 bytes can be saved in two ways:

1) The field 'bio' will only be used in bio based mode, and the field
   'rq' will only be used in mq mode. Since they won't be used in the
   same time, declare a union for them.
2) The field 'bool fake_timeout' can be placed in the hole after the
   field 'error'.

Signed-off-by: Yu Kuai <yukuai3@...wei.com>
---
 drivers/block/null_blk/null_blk.h | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/block/null_blk/null_blk.h b/drivers/block/null_blk/null_blk.h
index 78eb56b0ca55..4525a65e1b23 100644
--- a/drivers/block/null_blk/null_blk.h
+++ b/drivers/block/null_blk/null_blk.h
@@ -16,13 +16,15 @@
 #include <linux/mutex.h>
 
 struct nullb_cmd {
-	struct request *rq;
-	struct bio *bio;
+	union {
+		struct request *rq;
+		struct bio *bio;
+	};
 	unsigned int tag;
 	blk_status_t error;
+	bool fake_timeout;
 	struct nullb_queue *nq;
 	struct hrtimer timer;
-	bool fake_timeout;
 };
 
 struct nullb_queue {
-- 
2.31.1