remove ->rw_page

[Christoph Hellwig <hch@lst.de>]

Hi all,

this series removes the ->rw_page block_device_operation, which is an old
and clumsy attempt at a simple read/write fast path for the block layer.
It isn't actually used by the fastest block layer operations that we
support (polled I/O through io_uring), but only used by the mpage buffered
I/O helpers which are some of the slowest I/O we have and do not make any
difference there at all, and zram which is a block device abused to
duplicate the zram functionality.  Given that zram is heavily used we
need to make sure there is a good replacement for synchronous I/O, so
this series adds a new flag for drivers that complete I/O synchronously
and uses that flag to use on-stack bios and synchronous submission for
them in the swap code.

Diffstat:
 block/bdev.c                  |   78 ------------------
 drivers/block/brd.c           |   15 ---
 drivers/block/zram/zram_drv.c |   61 --------------
 drivers/nvdimm/btt.c          |   16 ---
 drivers/nvdimm/pmem.c         |   24 -----
 fs/mpage.c                    |   10 --
 include/linux/blkdev.h        |   12 +-
 mm/page_io.c                  |  182 ++++++++++++++++++++++--------------------
 mm/swap.h                     |    9 --
 mm/swapfile.c                 |    2 
 10 files changed, 114 insertions(+), 295 deletions(-)