Re: [PATCH v5 0/5] Implement MTE tag compression for swapped pages

[Andy Shevchenko]

+Cc: Olek, who internally is being developed something similar to your first
patch here.

On Fri, Sep 22, 2023 at 10:08:42AM +0200, Alexander Potapenko wrote:
> 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.
> 
> 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        | 245 +++++++++
>  arch/arm64/Kconfig                            |  21 +
>  arch/arm64/include/asm/mtecomp.h              |  13 +
>  arch/arm64/mm/Makefile                        |   7 +
>  arch/arm64/mm/mtecomp.c                       | 467 ++++++++++++++++++
>  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                  | 287 +++++++++++
>  include/linux/bitmap.h                        |  68 +++
>  lib/test_bitmap.c                             | 115 +++++
>  14 files changed, 1355 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
> 
> -- 
> 2.42.0.515.g380fc7ccd1-goog
> 

-- 
With Best Regards,
Andy Shevchenko