[PATCH 0/3] remove rw_page() from brd, pmem and btt

[Ross Zwisler]

Dan Williams and Christoph Hellwig have recently expressed doubt about
whether the rw_page() interface made sense for synchronous memory drivers
[1][2].  It's unclear whether this interface has any performance benefit
for these drivers, but as we continue to fix bugs it is clear that it does
have a maintenance burden.  This series removes the rw_page()
implementations in brd, pmem and btt to relieve this burden.

The last existing user of the rw_page interface is the zram driver, and
according to the changelog for the patch that added zram_rw_page() that
driver does see a clear performance gain:

  I implemented the feature in zram and tested it.  Test bed was the G2, LG
  electronic mobile device, whtich has msm8974 processor and 2GB memory.

  With a memory allocation test program consuming memory, the system
  generates swap.

  Operating time of swap_write_page() was measured.

  --------------------------------------------------
  |             |   operating time   | improvement |
  |             |  (20 runs average) |             |
  --------------------------------------------------
  |with patch   |    1061.15 us      |    +2.4%    |
  --------------------------------------------------
  |without patch|    1087.35 us      |             |
  --------------------------------------------------

  Each test(with paged_io,with BIO) result set shows normal distribution
  and has equal variance.  I mean the two values are valid result to
  compare.  I can say operation with paged I/O(without BIO) is faster 2.4%
  with confidence level 95%.

These patches have passed ext4 and XFS xfstest regression testing with
a memory mode pmem driver (without DAX), with pmem + btt and with brd.

These patches apply cleanly to the current v4.13-rc2 based linux/master.

[1] https://lists.01.org/pipermail/linux-nvdimm/2017-July/011389.html
[2] https://www.mail-archive.com/linux-block@xxxxxxxxxxxxxxx/msg11170.html

Ross Zwisler (3):
  btt: remove btt_rw_page()
  pmem: remove pmem_rw_page()
  brd: remove brd_rw_page()

 drivers/block/brd.c   | 10 ----------
 drivers/nvdimm/btt.c  | 15 ---------------
 drivers/nvdimm/pmem.c | 21 ---------------------
 3 files changed, 46 deletions(-)

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2.9.4