[PATCH 00/10] zram: redesign zcomp and rework backends

[Sergey Senozhatsky]

The series has a number of memory usage optimisations and fixes
that result in savings from double digits KB to triple digits
KB per-context/per-CPU, depending on backend and its configuration.
More details are in the corresponding commit messages.

The zcomp redesign and backend rework happen in the final two
patches.  zcomp part, basically, attempts to address the priority
inversion problem, that is seen on some setups.  In zcomp, we have
always kept Read and Write contexts together in per-CPU streams.
With preemptible zram this becomes a bit of a problem, as a higher
priority Reader can preempt a Writer, which may hold a stream
lock, but since the compression stream is locked by the preempted
Writer, the Reader gets blocked on the very same stream lock.
However, we don't have any reasons to have Read and Write contexts
share and compete for the stream, because Read (decompression) and
Write (compression) paths are completely independent, they never
touch each other's buffers.  Thus, we can split compression streams
into per-CPU Read streams and per-CPU Write streams, so that Readers
and Writers don't block each other anymore.  Kudos to Barry Song
for the idea.  Decoupling Reads and Writes results in noticeable
performance improvements on synthetic tests.

The last patch in the series builds atop of decoupled Read and
Write streams and reworks the way secondary streams are
handled.  Secondary compression streams are only used from
recompression, which is serialized by device lock, IOW it's
single-threaded.  However, we still allocated secondary streams
per-CPU (we needed to permit concurrent Reads of recompressed
objects).  Because we now have dedicated R/W streams, we can
allocate a singleton compression context (for recompression) yet
still have per-CPU decompression contexts.  This saves a notable
amount of memory with some backends and configurations (e.g. zstd,
deflate, lz4hc).

Sergey Senozhatsky (10):
  zram: prefix backends printk-s
  zram: remove debugfs entry on init failure
  zram: zstd: do not allocate empty C/D-dictionaries
  zram: lz4hc: pre-initialize dictionary stream in setup_params
  zram: lz4: use LZ4_decompress_safe_usingDict()
  zram: lz4hc: use LZ4_decompress_safe_usingDict()
  zram: zstd: do not use kvzalloc() for zstd allocations
  zram: reset writeback state in zram_reset_device()
  zram: split zcomp into separate R/W streams
  zram: use a singleton compression context for recompression

 drivers/block/zram/backend_842.c     |   6 +-
 drivers/block/zram/backend_deflate.c |  98 ++++++++------
 drivers/block/zram/backend_lz4.c     |  85 +++---------
 drivers/block/zram/backend_lz4hc.c   | 105 ++++++---------
 drivers/block/zram/backend_lzo.c     |   6 +-
 drivers/block/zram/backend_lzorle.c  |   6 +-
 drivers/block/zram/backend_zstd.c    | 133 ++++++++++++-------
 drivers/block/zram/zcomp.c           | 189 ++++++++++++++++++++++-----
 drivers/block/zram/zcomp.h           |  35 +++--
 drivers/block/zram/zram_drv.c        |  51 +++++---
 10 files changed, 421 insertions(+), 293 deletions(-)

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2.56.0.rc1.315.gc6ed9934b7-goog