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Date:	Thu, 17 Jul 2014 23:17:39 +0900
From:	Akinobu Mita <>
Cc:	Akinobu Mita <>,
	Andrew Morton <>,
	Jens Axboe <>,
	Alexander Viro <>,
Subject: [PATCH] block_dev: implement readpages() to optimize sequential read

Sequential read from a block device is expected to be equal or faster
than from the file on a filesystem.  But it is not correct due to the
lack of effective readpages() in the address space operations for
block device.

This implements readpages() operation for block device by using
mpage_readpages() which can create multipage BIOs instead of BIOs for
each page.  It enables to reduce system CPU time consumption.

Install 1GB of RAM disk storage:

	# modprobe scsi_debug dev_size_mb=1024 delay=0

Sequential read from file on a filesystem:

	# mkfs.ext4 /dev/$DEV
	# mount /dev/$DEV /mnt
	# fio --name=t --size=512m --rw=read --filename=/mnt/file
	  read : io=524288KB, bw=2133.4MB/s, iops=546133, runt=   240msec

Sequential read from a block device:
	# fio --name=t --size=512m --rw=read --filename=/dev/sdb
(Without this commit)
	  read : io=524288KB, bw=1700.2MB/s, iops=435455, runt=   301msec

(With this commit)
	  read : io=524288KB, bw=2160.4MB/s, iops=553046, runt=   237msec

Signed-off-by: Akinobu Mita <>
Cc: Andrew Morton <>
Cc: Jens Axboe <>
Cc: Alexander Viro <>
 fs/block_dev.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 6d72746..e2f3ad08 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -304,6 +304,12 @@ static int blkdev_readpage(struct file * file, struct page * page)
 	return block_read_full_page(page, blkdev_get_block);
+static int blkdev_readpages(struct file *file, struct address_space *mapping,
+			struct list_head *pages, unsigned nr_pages)
+	return mpage_readpages(mapping, pages, nr_pages, blkdev_get_block);
 static int blkdev_write_begin(struct file *file, struct address_space *mapping,
 			loff_t pos, unsigned len, unsigned flags,
 			struct page **pagep, void **fsdata)
@@ -1622,6 +1628,7 @@ static int blkdev_releasepage(struct page *page, gfp_t wait)
 static const struct address_space_operations def_blk_aops = {
 	.readpage	= blkdev_readpage,
+	.readpages	= blkdev_readpages,
 	.writepage	= blkdev_writepage,
 	.write_begin	= blkdev_write_begin,
 	.write_end	= blkdev_write_end,

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