block_dev: implement readpages() to optimize sequential read

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 and 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/$DEV
	...
(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 <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Jeff Moyer <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>