[PATCH v6 0/5] 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 between 2.5x and 4x. In most
cases it is possible to store the compressed data in 63-bit Xarray values,
resulting in no extra memory allocations.

Our measurements show that the proposed algorithm provides better
compression than existing kernel compression algorithms (LZ4, LZO,
LZ4HC, ZSTD) can offer.

To implement compression/decompression, we also extend <linux/bitmap.h>
with methods to read/write bit values at arbitrary places in the map.

We refactor arch/arm64/mm/mteswap.c to support both the compressed
(CONFIG_ARM64_MTE_COMP) and non-compressed case. For the former, in
addition to tag compression, we move tag allocation from kmalloc() to
separate kmem caches, providing greater locality and relaxing the
alignment requirements.

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):
  lib/test_bitmap: add tests for bitmap_{read,write}()
  arm64: mte: implement CONFIG_ARM64_MTE_COMP
  arm64: mte: add a test for MTE tags compression
  arm64: mte: add compression support to mteswap.c

Syed Nayyar Waris (1):
  lib/bitmap: add bitmap_{read,write}()

 Documentation/arch/arm64/index.rst            |   1 +
 .../arch/arm64/mte-tag-compression.rst        | 266 +++++++++
 arch/arm64/Kconfig                            |  21 +
 arch/arm64/include/asm/mtecomp.h              |  13 +
 arch/arm64/mm/Makefile                        |   7 +
 arch/arm64/mm/mtecomp.c                       | 524 ++++++++++++++++++
 arch/arm64/mm/mtecomp.h                       |  12 +
 arch/arm64/mm/mteswap.c                       |  20 +-
 arch/arm64/mm/mteswap.h                       |  12 +
 arch/arm64/mm/mteswap_comp.c                  |  60 ++
 arch/arm64/mm/mteswap_nocomp.c                |  38 ++
 arch/arm64/mm/test_mtecomp.c                  | 377 +++++++++++++
 include/linux/bitmap.h                        |  68 +++
 lib/test_bitmap.c                             | 118 ++++
 14 files changed, 1526 insertions(+), 11 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/mteswap.h
 create mode 100644 arch/arm64/mm/mteswap_comp.c
 create mode 100644 arch/arm64/mm/mteswap_nocomp.c
 create mode 100644 arch/arm64/mm/test_mtecomp.c

-- 
2.42.0.609.gbb76f46606-goog