zram: Support multiple compression streams

Hello,

	RFC series that adds support for multiple (per-CPU)
compression streams (at point only 2). The main idea is that
different compression algorithms have different characteristics
and zram may benefit when it uses a combination of algorithms:
a default algorithm that is faster but have lower compression
rate and a secondary algorithm that can use higher compression
rate at a price of slower compression/decompression.

	There are several use-case for this functionality:

- huge pages re-compression: zstd or defalte can successfully
compress huge pages (~50% of huge pages on my synthetic ChromeOS
tests), IOW pages that lzo was not able to compress.

- idle pages re-compression: idle/cold pages sit in the memory
and we may reduce zsmalloc memory usage if we recompress those
idle pages.

	User-space has a number of ways to control the behavior
and impact of zram recompression: what type of pages should be
recompressed, size watermarks, etc. Please refer to documentation
patch.

Sergey Senozhatsky (7):
  zram: Preparation for multi-zcomp support
  zram: Add recompression algorithm sysfs knob
  zram: Factor out WB and non-WB zram read functions
  zram: Introduce recompress sysfs knob
  documentation: Add recompression documentation
  zram: Add recompression algorithm choice to Kconfig
  zram: Add recompress flag to read_block_state()

 Documentation/admin-guide/blockdev/zram.rst |  64 ++-
 drivers/block/zram/Kconfig                  |  51 +++
 drivers/block/zram/zcomp.c                  |   6 +-
 drivers/block/zram/zcomp.h                  |   2 +-
 drivers/block/zram/zram_drv.c               | 432 +++++++++++++++++---
 drivers/block/zram/zram_drv.h               |  15 +-
 6 files changed, 504 insertions(+), 66 deletions(-)

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2.37.2.789.g6183377224-goog