[PATCHv4 0/9] zram: Support multiple compression streams

[Sergey Senozhatsky]

Hello,

	This series 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 deflate 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.

v4:
-- added IS_ERR_VALUE patch (Andrew)
-- documented SIZE units (bytes) (Andrew)
-- re-phrased writeback BIO error comment (Andrew)
-- return zs_malloc() error code from zram_recompress()
-- do not lose zram_recompress() error in recompress_store()
-- corrected a typo
-- fixed previous rebase errors
-- rebased the series

v3:
-- conditionally reschedule during recompression loop so that
   we don't stall RCU grace periods
-- fixed a false-positive WARN_ON

v2:
-- rebased
-- mark completely incompressible pages (neither default nor secondary
   algorithm can compress them) with a new flag so that we don't attempt
   to recompress them all the time

Sergey Senozhatsky (9):
  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()
  zram: Clarify writeback_store() comment
  zram: Use IS_ERR_VALUE() to check for zs_malloc() errors

 Documentation/admin-guide/blockdev/zram.rst |  64 ++-
 drivers/block/zram/Kconfig                  |  55 +++
 drivers/block/zram/zcomp.c                  |   6 +-
 drivers/block/zram/zcomp.h                  |   2 +-
 drivers/block/zram/zram_drv.c               | 458 +++++++++++++++++---
 drivers/block/zram/zram_drv.h               |  16 +-
 6 files changed, 526 insertions(+), 75 deletions(-)

-- 
2.38.0.413.g74048e4d9e-goog