[PATCH v9 0/4] Implement MTE tag compression for swapped pages

[Alexander Potapenko]

Currently, when MTE pages are swapped out, the tags are kept in the
memory, occupying PAGE_SIZE/32 bytes per page. This is especially
problematic for devices that use zram-backed in-memory swap, because
tags stored uncompressed in the heap effectively reduce the available
amount of swap memory.

The RLE-based algorithm suggested by Evgenii Stepanov and implemented in
this patch series is able to efficiently compress fixed-size tag buffers,
resulting in practical compression ratio of 2x. In many cases it is
possible to store the compressed data in 63-bit Xarray values, resulting
in no extra memory allocations.

This patch series depends on "lib/bitmap: add bitmap_{read,write}()"
(https://lore.kernel.org/linux-arm-kernel/20231030153210.139512-1-glider@xxxxxxxxxx/T/)
that is mailed separately.

v9:
 - split off the stats collection code into a separate patch in the
   series (as suggested by Yury Norov)

v8:
 - split off the bitmap_read()/bitmap_write() series
 - simplified the compression logic (only compress data if it fits into
   a pointer)

v7:
 - fixed comments by Yury Norov, Andy Shevchenko, Rasmus Villemoes
 - added perf tests for bitmap_read()/bitmap_write()
 - more efficient bitmap_write() implementation (meant to be sent in v5)

v6:
 - fixed comments by Yury Norov
 - fixed handling of sizes divisible by MTE_GRANULES_PER_PAGE / 2
   (caught while testing on a real device)

v5:
 - fixed comments by Andy Shevchenko, Catalin Marinas, and Yury Norov
 - added support for 16K- and 64K pages
 - more efficient bitmap_write() implementation

v4:
 - fixed a bunch of comments by Andy Shevchenko and Yury Norov
 - added Documentation/arch/arm64/mte-tag-compression.rst

v3:
 - as suggested by Andy Shevchenko, use
   bitmap_get_value()/bitmap_set_value() written by Syed Nayyar Waris
 - switched to unsigned long to reduce typecasts
 - simplified the compression code

v2:
 - as suggested by Yuri Norov, replace the poorly implemented struct
   bitq with <linux/bitmap.h>


Alexander Potapenko (4):
  arm64: mte: implement CONFIG_ARM64_MTE_COMP
  arm64: mte: add a test for MTE tags compression
  arm64: mte: add compression support to mteswap.c
  arm64: mte: implement CONFIG_ARM64_MTE_SWAP_STATS

 Documentation/arch/arm64/index.rst            |   1 +
 .../arch/arm64/mte-tag-compression.rst        | 166 ++++++++
 arch/arm64/Kconfig                            |  37 ++
 arch/arm64/include/asm/mtecomp.h              |  39 ++
 arch/arm64/mm/Makefile                        |   2 +
 arch/arm64/mm/mtecomp.c                       | 257 +++++++++++++
 arch/arm64/mm/mtecomp.h                       |  12 +
 arch/arm64/mm/mteswap.c                       | 110 +++++-
 arch/arm64/mm/test_mtecomp.c                  | 364 ++++++++++++++++++
 9 files changed, 985 insertions(+), 3 deletions(-)
 create mode 100644 Documentation/arch/arm64/mte-tag-compression.rst
 create mode 100644 arch/arm64/include/asm/mtecomp.h
 create mode 100644 arch/arm64/mm/mtecomp.c
 create mode 100644 arch/arm64/mm/mtecomp.h
 create mode 100644 arch/arm64/mm/test_mtecomp.c

-- 
2.42.0.869.gea05f2083d-goog