Message-ID: <ZXtircr4TllpqyeS@yury-ThinkPad>
Date: Thu, 14 Dec 2023 12:16:45 -0800
From: Yury Norov <yury.norov@...il.com>
To: Alexander Potapenko <glider@...gle.com>
Cc: catalin.marinas@....com, will@...nel.org, pcc@...gle.com,
	andreyknvl@...il.com, andriy.shevchenko@...ux.intel.com,
	aleksander.lobakin@...el.com, linux@...musvillemoes.dk,
	alexandru.elisei@....com, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, eugenis@...gle.com,
	syednwaris@...il.com, william.gray@...aro.org
Subject: Re: [PATCH v10-mte 4/7] arm64: mte: implement CONFIG_ARM64_MTE_COMP

On Thu, Dec 14, 2023 at 12:06:36PM +0100, Alexander Potapenko wrote:
> The config implements the algorithm compressing memory tags for ARM MTE
> during swapping.
> 
> The algorithm is based on RLE and specifically targets buffers of tags
> corresponding to a single page. In many cases a buffer can be compressed
> into 63 bits, making it possible to store it without additional memory
> allocation.
> 
> Suggested-by: Evgenii Stepanov <eugenis@...gle.com>
> Signed-off-by: Alexander Potapenko <glider@...gle.com>
> Acked-by: Catalin Marinas <catalin.marinas@....com>
> 
> ---
>  v10-mte:
>   - added Catalin's Acked-by:
> 
>  v8:
>   - As suggested by Catalin Marinas, only compress tags if they can be
>     stored inline. This simplifies the code drastically.
>   - Update the documentation.
>   - Split off patches introducing bitmap_read()/bitmap_write().
> 
>  v6:
>   - shuffle bits in inline handles so that they can't be confused with
>     canonical pointers;
>   - use kmem_cache_zalloc() to allocate compressed storage
>   - correctly handle range size overflow
>   - minor documentation fixes, clarify the special cases
> 
>  v5:
>   - make code PAGE_SIZE-agnostic, remove hardcoded constants, updated
>     the docs
>   - implement debugfs interface
>   - Address comments by Andy Shevchenko:
>     - update description of mtecomp.c
>     - remove redundant assignments, simplify mte_tags_to_ranges()
>     - various code simplifications
>     - introduce mtecomp.h
>     - add :export: to Documentation/arch/arm64/mte-tag-compression.rst
> 
>  v4:
>   - Addressed comments by Andy Shevchenko:
>     - expanded "MTE" to "Memory Tagging Extension" in Kconfig
>     - fixed kernel-doc comments, moved them to C source
>     - changed variables to unsigned where applicable
>     - some code simplifications, fewer unnecessary assignments
>     - added the mte_largest_idx_bits() helper
>     - added namespace prefixes to all functions
>     - added missing headers (but removed bits.h)
>   - Addressed comments by Yury Norov:
>     - removed test-only functions from mtecomp.h
>     - dropped the algoritm name (all functions are now prefixed with
>       "mte")
>     - added more comments
>     - got rid of MTE_RANGES_INLINE
>     - renamed bitmap_{get,set}_value() to bitmap_{read,write}()
>     - moved the big comment explaining the algorithm to
>       Documentation/arch/arm64/mte-tag-compression.rst, expanded it,
>       add a link to it from Documentation/arch/arm64/index.rst
>     - removed hardcoded ranges from mte_alloc_size()/mte_size_to_ranges()
> 
>  v3:
>   - Addressed comments by Andy Shevchenko:
>    - use bitmap_{set,get}_value() writte by Syed Nayyar Waris
>    - switched to unsigned long everywhere (fewer casts)
>    - simplified the code, removed redundant checks
>    - dropped ea0_compress_inline()
>  - added bit size constants and helpers to access the bitmap
>  - explicitly initialize all compressed sizes in ea0_compress_to_buf()
>  - initialize all handle bits
> 
>  v2:
>   - as suggested by Yury Norov, switched from struct bitq (which is
>     not needed anymore) to <linux/bitmap.h>
>   - add missing symbol exports
> ---
>  Documentation/arch/arm64/index.rst            |   1 +
>  .../arch/arm64/mte-tag-compression.rst        | 154 +++++++++++
>  arch/arm64/Kconfig                            |  11 +
>  arch/arm64/include/asm/mtecomp.h              |  39 +++
>  arch/arm64/mm/Makefile                        |   1 +
>  arch/arm64/mm/mtecomp.c                       | 257 ++++++++++++++++++
>  arch/arm64/mm/mtecomp.h                       |  12 +
>  7 files changed, 475 insertions(+)
>  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
> 
> diff --git a/Documentation/arch/arm64/index.rst b/Documentation/arch/arm64/index.rst
> index d08e924204bf1..bf6c1583233a9 100644
> --- a/Documentation/arch/arm64/index.rst
> +++ b/Documentation/arch/arm64/index.rst
> @@ -19,6 +19,7 @@ ARM64 Architecture
>      legacy_instructions
>      memory
>      memory-tagging-extension
> +    mte-tag-compression
>      perf
>      pointer-authentication
>      ptdump
> diff --git a/Documentation/arch/arm64/mte-tag-compression.rst b/Documentation/arch/arm64/mte-tag-compression.rst
> new file mode 100644
> index 0000000000000..8fe6b51a9db6d
> --- /dev/null
> +++ b/Documentation/arch/arm64/mte-tag-compression.rst
> @@ -0,0 +1,154 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +==================================================
> +Tag Compression for Memory Tagging Extension (MTE)
> +==================================================
> +
> +This document describes the algorithm used to compress memory tags used by the
> +ARM Memory Tagging Extension (MTE).
> +
> +Introduction
> +============
> +
> +MTE assigns tags to memory pages: for 4K pages those tags occupy 128 bytes
> +(256 4-bit tags each corresponding to a 16-byte MTE granule), for 16K pages -
> +512 bytes, for 64K pages - 2048 bytes. By default, MTE carves out 3.125% (1/16)
> +of the available physical memory to store the tags.
> +
> +When MTE pages are saved to swap, their tags need to be stored in the kernel
> +memory. If the system swap is used heavily, these tags may take a substantial
> +portion of the physical memory. To reduce memory waste, ``CONFIG_ARM64_MTE_COMP``
> +allows the kernel to store the tags in compressed form.
> +
> +Implementation details
> +======================
> +
> +The algorithm attempts to compress an array of ``MTE_PAGE_TAG_STORAGE``
> +tag bytes into a byte sequence that can be stored in an 8-byte pointer. If that
> +is not possible, the data is stored uncompressed.
> +
> +Tag manipulation and storage
> +----------------------------
> +
> +Tags for swapped pages are stored in an XArray that maps swap entries to 63-bit
> +values (see ``arch/arm64/mm/mteswap.c``). Bit 0 of these values indicates how
> +their contents should be treated:
> +
> + - 0: value is a pointer to an uncompressed buffer allocated with kmalloc()
> +   (always the case if ``CONFIG_ARM64_MTE_COMP=n``) with the highest bit set
> +   to 0;
> + - 1: value contains compressed data.
> +
> +``arch/arm64/include/asm/mtecomp.h`` declares the following functions that
> +manipulate with tags:
> +
> +- mte_compress() - compresses the given ``MTE_PAGE_TAG_STORAGE``-byte ``tags``
> +  buffer into a pointer;
> +- mte_decompress() - decompresses the tags from a pointer;
> +- mte_is_compressed() - returns ``true`` iff the pointer passed to it should be
> +  treated as compressed data.
> +
> +Tag compression
> +---------------
> +
> +The compression algorithm is a variation of RLE (run-length encoding) and works
> +as follows (we will be considering 4K pages and 128-byte tag buffers, but the
> +same approach scales to 16K and 64K pages):
> +
> +1. The input array of 128 (``MTE_PAGE_TAG_STORAGE``) bytes is transformed into
> +   tag ranges (two arrays: ``r_tags[]`` containing tag values and ``r_sizes[]``
> +   containing range lengths) by mte_tags_to_ranges(). Note that
> +   ``r_sizes[]`` sums up to 256 (``MTE_GRANULES_PER_PAGE``).
> +
> +   If ``r_sizes[]`` consists of a single element
> +   (``{ MTE_GRANULES_PER_PAGE }``), the corresponding range is split into two
> +   halves, i.e.::
> +
> +     r_sizes_new[2] = { MTE_GRANULES_PER_PAGE/2, MTE_GRANULES_PER_PAGE/2 };
> +     r_tags_new[2] = { r_tags[0], r_tags[0] };
> +
> +2. The number of the largest element of ``r_sizes[]`` is stored in
> +   ``largest_idx``. The element itself is thrown away from ``r_sizes[]``,
> +   because it can be reconstructed from the sum of the remaining elements. Note
> +   that now none of the remaining ``r_sizes[]`` elements exceeds
> +   ``MTE_GRANULES_PER_PAGE/2``.
> +
> +3. If the number ``N`` of ranges does not exceed ``6``, the ranges can be
> +   compressed into 64 bits. This is done by storing the following values packed
> +   into the pointer (``i<size>`` means a ``<size>``-bit unsigned integer)
> +   treated as a bitmap (see ``include/linux/bitmap.h``)::
> +
> +    bit 0      :      (always 1) : i1
> +    bits 1-3   :     largest_idx : i3
> +    bits 4-27  :    r_tags[0..5] : i4 x 6
> +    bits 28-62 :   r_sizes[0..4] : i7 x 5
> +    bit 63     :      (always 0) : i1
> +
> +   If N is less than 6, ``r_tags`` and ``r_sizes`` are padded up with zero
> +   values. The unused bits in the pointer, including bit 63, are also set to 0,
> +   so the compressed data can be stored in XArray.
> +
> +   Range size of ``MTE_GRANULES_PER_PAGE/2`` (at most one) does not fit into
> +   i7 and will be written as 0. This case is handled separately by the
> +   decompressing procedure.
> +
> +Tag decompression
> +-----------------
> +
> +The decompression algorithm performs the steps below.
> +
> +1. Read the lowest bit of the data from the input buffer and check that it is 1,
> +   otherwise bail out.
> +
> +2. Read ``largest_idx``, ``r_tags[]`` and ``r_sizes[]`` from the
> +   input buffer.
> +
> +   If ``largest_idx`` is zero, and all ``r_sizes[]`` are zero, set
> +   ``r_sizes[0] = MTE_GRANULES_PER_PAGE/2``.
> +
> +   Calculate the removed largest element of ``r_sizes[]`` as
> +   ``largest = 256 - sum(r_sizes)`` and insert it into ``r_sizes`` at
> +   position ``largest_idx``.
> +
> +6. For each ``r_sizes[i] > 0``, add a 4-bit value ``r_tags[i]`` to the output
> +   buffer ``r_sizes[i]`` times.
> +
> +
> +Why these numbers?
> +------------------
> +
> +To be able to reconstruct ``N`` tag ranges from the compressed data, we need to
> +store the indicator bit together with ``largest_idx``, ``r_tags[N]``, and
> +``r_sizes[N-1]`` in 63 bits.
> +Knowing that the sizes do not exceed ``MTE_PAGE_TAG_STORAGE``, each of them can be
> +packed into ``S = ilog2(MTE_PAGE_TAG_STORAGE)`` bits, whereas a single tag occupies
> +4 bits.
> +
> +It is evident that the number of ranges that can be stored in 63 bits is
> +strictly less than 8, therefore we only need 3 bits to store ``largest_idx``.
> +
> +The maximum values of ``N`` so that the number ``1 + 3 + N * 4 + (N-1) * S`` of
> +storage bits does not exceed 63, are shown in the table below::
> +
> + +-----------+-----------------+----+---+-------------------+
> + | Page size | Tag buffer size |  S | N |    Storage bits   |
> + +-----------+-----------------+----+---+-------------------+
> + |      4 KB |           128 B |  7 | 6 | 63 = 1+3+6*4+5*7  |
> + |     16 KB |           512 B |  9 | 5 | 60 = 1+3+5*4+4*9  |
> + |     64 KB |          2048 B | 11 | 4 | 53 = 1+3+4*4+3*11 |
> + +-----------+-----------------+----+---+-------------------+
> +
> +Note
> +----
> +
> +Tag compression and decompression implicitly rely on the fixed MTE tag size
> +(4 bits) and number of tags per page. Should these values change, the algorithm
> +may need to be revised.
> +
> +
> +Programming Interface
> +=====================
> +
> + .. kernel-doc:: arch/arm64/include/asm/mtecomp.h
> + .. kernel-doc:: arch/arm64/mm/mtecomp.c
> +   :export:
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 7b071a00425d2..5f4d4b49a512e 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -2078,6 +2078,17 @@ config ARM64_EPAN
>  	  if the cpu does not implement the feature.
>  endmenu # "ARMv8.7 architectural features"
>  
> +config ARM64_MTE_COMP
> +	bool "Tag compression for ARM64 Memory Tagging Extension"
> +	default y
> +	depends on ARM64_MTE
> +	help
> +	  Enable tag compression support for ARM64 Memory Tagging Extension.
> +
> +	  Tag buffers corresponding to swapped RAM pages are compressed using
> +	  RLE to conserve heap memory. In the common case compressed tags
> +	  occupy 2.5x less memory.
> +
>  config ARM64_SVE
>  	bool "ARM Scalable Vector Extension support"
>  	default y
> diff --git a/arch/arm64/include/asm/mtecomp.h b/arch/arm64/include/asm/mtecomp.h
> new file mode 100644
> index 0000000000000..b9a3a921a38d4
> --- /dev/null
> +++ b/arch/arm64/include/asm/mtecomp.h
> @@ -0,0 +1,39 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef __ASM_MTECOMP_H
> +#define __ASM_MTECOMP_H
> +
> +#include <linux/types.h>
> +
> +/**
> + * mte_is_compressed() - check if the supplied pointer contains compressed tags.
> + * @ptr: pointer returned by kmalloc() or mte_compress().
> + *
> + * Returns: true iff bit 0 of @ptr is 1, which is only possible if @ptr was
> + * returned by mte_is_compressed().
> + */
> +static inline bool mte_is_compressed(void *ptr)
> +{
> +	return ((unsigned long)ptr & 1);
> +}
> +
> +#if defined(CONFIG_ARM64_MTE_COMP)
> +
> +void *mte_compress(u8 *tags);
> +bool mte_decompress(void *handle, u8 *tags);
> +
> +#else
> +
> +static inline void *mte_compress(u8 *tags)
> +{
> +	return NULL;
> +}
> +
> +static inline bool mte_decompress(void *data, u8 *tags)
> +{
> +	return false;
> +}
> +
> +#endif // CONFIG_ARM64_MTE_COMP
> +
> +#endif // __ASM_MTECOMP_H
> diff --git a/arch/arm64/mm/Makefile b/arch/arm64/mm/Makefile
> index dbd1bc95967d0..46778f6dd83c2 100644
> --- a/arch/arm64/mm/Makefile
> +++ b/arch/arm64/mm/Makefile
> @@ -10,6 +10,7 @@ obj-$(CONFIG_TRANS_TABLE)	+= trans_pgd.o
>  obj-$(CONFIG_TRANS_TABLE)	+= trans_pgd-asm.o
>  obj-$(CONFIG_DEBUG_VIRTUAL)	+= physaddr.o
>  obj-$(CONFIG_ARM64_MTE)		+= mteswap.o
> +obj-$(CONFIG_ARM64_MTE_COMP)	+= mtecomp.o
>  KASAN_SANITIZE_physaddr.o	+= n
>  
>  obj-$(CONFIG_KASAN)		+= kasan_init.o
> diff --git a/arch/arm64/mm/mtecomp.c b/arch/arm64/mm/mtecomp.c
> new file mode 100644
> index 0000000000000..c948921525030
> --- /dev/null
> +++ b/arch/arm64/mm/mtecomp.c
> @@ -0,0 +1,257 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +
> +/*
> + * MTE tag compression algorithm.
> + * See Documentation/arch/arm64/mte-tag-compression.rst for more details.
> + */
> +
> +#include <linux/bits.h>
> +#include <linux/bitmap.h>
> +#include <linux/bitops.h>
> +#include <linux/export.h>
> +#include <linux/gfp.h>
> +#include <linux/module.h>
> +#include <linux/slab.h>
> +#include <linux/string.h>
> +#include <linux/types.h>
> +
> +#include <asm/mtecomp.h>
> +
> +#include "mtecomp.h"
> +
> +#define MTE_BITS_PER_LARGEST_IDX 3
> +/* Range size cannot exceed MTE_GRANULES_PER_PAGE / 2. */
> +#define MTE_BITS_PER_SIZE (ilog2(MTE_GRANULES_PER_PAGE) - 1)
> +
> +/*
> + * See Documentation/arch/arm64/mte-tag-compression.rst for details on how the
> + * maximum number of ranges is calculated.
> + */
> +#if defined(CONFIG_ARM64_4K_PAGES)
> +#define MTE_MAX_RANGES 6
> +#elif defined(CONFIG_ARM64_16K_PAGES)
> +#define MTE_MAX_RANGES 5
> +#else
> +#define MTE_MAX_RANGES 4
> +#endif
> +
> +/**
> + * mte_tags_to_ranges() - break @tags into arrays of tag ranges.
> + * @tags: MTE_GRANULES_PER_PAGE-byte array containing MTE tags.
> + * @out_tags: u8 array to store the tag of every range.
> + * @out_sizes: unsigned short array to store the size of every range.
> + * @out_len: length of @out_tags and @out_sizes (output parameter, initially
> + *           equal to lengths of out_tags[] and out_sizes[]).
> + *
> + * This function is exported for testing purposes.
> + */
> +void mte_tags_to_ranges(u8 *tags, u8 *out_tags, unsigned short *out_sizes,
> +			size_t *out_len)
> +{
> +	u8 prev_tag = tags[0] / 16; /* First tag in the array. */
> +	unsigned int cur_idx = 0, i, j;
> +	u8 cur_tag;
> +
> +	memset(out_tags, 0, array_size(*out_len, sizeof(*out_tags)));
> +	memset(out_sizes, 0, array_size(*out_len, sizeof(*out_sizes)));
> +
> +	out_tags[cur_idx] = prev_tag;
> +	for (i = 0; i < MTE_GRANULES_PER_PAGE; i++) {
> +		j = i % 2;
> +		cur_tag = j ? (tags[i / 2] % 16) : (tags[i / 2] / 16);
> +		if (cur_tag == prev_tag) {
> +			out_sizes[cur_idx]++;
> +		} else {
> +			cur_idx++;
> +			prev_tag = cur_tag;
> +			out_tags[cur_idx] = prev_tag;
> +			out_sizes[cur_idx] = 1;
> +		}
> +	}
> +	*out_len = cur_idx + 1;
> +}
> +EXPORT_SYMBOL_NS(mte_tags_to_ranges, MTECOMP);
> +
> +/**
> + * mte_ranges_to_tags() - fill @tags using given tag ranges.
> + * @r_tags: u8[] containing the tag of every range.
> + * @r_sizes: unsigned short[] containing the size of every range.
> + * @r_len: length of @r_tags and @r_sizes.
> + * @tags: MTE_GRANULES_PER_PAGE-byte array to write the tags to.
> + *
> + * This function is exported for testing purposes.
> + */
> +void mte_ranges_to_tags(u8 *r_tags, unsigned short *r_sizes, size_t r_len,
> +			u8 *tags)
> +{
> +	unsigned int i, j, pos = 0;
> +	u8 prev;
> +
> +	for (i = 0; i < r_len; i++) {
> +		for (j = 0; j < r_sizes[i]; j++) {
> +			if (pos % 2)
> +				tags[pos / 2] = (prev << 4) | r_tags[i];
> +			else
> +				prev = r_tags[i];
> +			pos++;
> +		}
> +	}
> +}
> +EXPORT_SYMBOL_NS(mte_ranges_to_tags, MTECOMP);
> +
> +static void mte_bitmap_write(unsigned long *bitmap, unsigned long value,
> +			     unsigned long *pos, unsigned long bits)
> +{
> +	bitmap_write(bitmap, value, *pos, bits);
> +	*pos += bits;
> +}
> +
> +/* Compress ranges into an unsigned long. */
> +static void mte_compress_to_ulong(size_t len, u8 *tags, unsigned short *sizes,
> +				  unsigned long *result)
> +{
> +	unsigned long bit_pos = 0;
> +	unsigned int largest_idx, i;
> +	unsigned short largest = 0;
> +
> +	for (i = 0; i < len; i++) {
> +		if (sizes[i] > largest) {
> +			largest = sizes[i];
> +			largest_idx = i;
> +		}
> +	}
> +	/* Bit 1 in position 0 indicates compressed data. */
> +	mte_bitmap_write(result, 1, &bit_pos, 1);
> +	mte_bitmap_write(result, largest_idx, &bit_pos,
> +			 MTE_BITS_PER_LARGEST_IDX);
> +	for (i = 0; i < len; i++)
> +		mte_bitmap_write(result, tags[i], &bit_pos, MTE_TAG_SIZE);
> +	if (len == 1) {
> +		/*
> +		 * We are compressing MTE_GRANULES_PER_PAGE of identical tags.
> +		 * Split it into two ranges containing
> +		 * MTE_GRANULES_PER_PAGE / 2 tags, so that it falls into the
> +		 * special case described below.
> +		 */
> +		mte_bitmap_write(result, tags[0], &bit_pos, MTE_TAG_SIZE);
> +		i = 2;
> +	} else {
> +		i = len;
> +	}
> +	for (; i < MTE_MAX_RANGES; i++)
> +		mte_bitmap_write(result, 0, &bit_pos, MTE_TAG_SIZE);
> +	/*
> +	 * Most of the time sizes[i] fits into MTE_BITS_PER_SIZE, apart from a
> +	 * special case when:
> +	 *   len = 2;
> +	 *   sizes = { MTE_GRANULES_PER_PAGE / 2, MTE_GRANULES_PER_PAGE / 2};
> +	 * In this case largest_idx will be set to 0, and the size written to
> +	 * the bitmap will be also 0.
> +	 */
> +	for (i = 0; i < len; i++) {
> +		if (i != largest_idx)
> +			mte_bitmap_write(result, sizes[i], &bit_pos,
> +					 MTE_BITS_PER_SIZE);
> +	}
> +	for (i = len; i < MTE_MAX_RANGES; i++)
> +		mte_bitmap_write(result, 0, &bit_pos, MTE_BITS_PER_SIZE);
> +}
> +
> +/**
> + * mte_compress() - compress the given tag array.
> + * @tags: MTE_GRANULES_PER_PAGE-byte array to read the tags from.
> + *
> + * Attempts to compress the user-supplied tag array.
> + *
> + * Returns: compressed data or NULL.
> + */
> +void *mte_compress(u8 *tags)
> +{
> +	unsigned short *r_sizes;
> +	void *result = NULL;
> +	u8 *r_tags;
> +	size_t r_len;
> +
> +	r_sizes = kmalloc_array(MTE_GRANULES_PER_PAGE, sizeof(unsigned short),
> +				GFP_KERNEL);
> +	r_tags = kmalloc(MTE_GRANULES_PER_PAGE, GFP_KERNEL);
> +	if (!r_sizes || !r_tags)
> +		goto ret;
> +	r_len = MTE_GRANULES_PER_PAGE;
> +	mte_tags_to_ranges(tags, r_tags, r_sizes, &r_len);
> +	if (r_len <= MTE_MAX_RANGES)
> +		mte_compress_to_ulong(r_len, r_tags, r_sizes,
> +				      (unsigned long *)&result);
> +ret:
> +	kfree(r_tags);
> +	kfree(r_sizes);
> +	return result;
> +}
> +EXPORT_SYMBOL_NS(mte_compress, MTECOMP);
> +
> +static unsigned long mte_bitmap_read(const unsigned long *bitmap,
> +				     unsigned long *pos, unsigned long bits)
> +{
> +	unsigned long start = *pos;
> +
> +	*pos += bits;
> +	return bitmap_read(bitmap, start, bits);
> +}
> +
> +/**
> + * mte_decompress() - decompress the tag array from the given pointer.
> + * @data: pointer returned by @mte_compress()
> + * @tags: MTE_GRANULES_PER_PAGE-byte array to write the tags to.
> + *
> + * Reads the compressed data and writes it into the user-supplied tag array.
> + *
> + * Returns: true on success, false if the passed data is uncompressed.
> + */
> +bool mte_decompress(void *data, u8 *tags)
> +{
> +	unsigned short r_sizes[MTE_MAX_RANGES];
> +	u8 r_tags[MTE_MAX_RANGES];
> +	unsigned int largest_idx, i;
> +	unsigned long bit_pos = 0;
> +	unsigned long *bitmap;
> +	unsigned short sum;
> +	size_t max_ranges;
> +
> +	if (!mte_is_compressed(data))
> +		return false;
> +
> +	bitmap = (unsigned long *)&data;

That looks weird... You're casting address of a 'data' to a bitmap
instead of 'data'. At the 1st glance it makes little sense because
'data' is passed as parameter. Moreover, in mte_is_compressed()
you pass 'data', not '&data'. Can you please comment on your
intention?

> +	max_ranges = MTE_MAX_RANGES;
> +	/* Skip the leading bit indicating the inline case. */
> +	mte_bitmap_read(bitmap, &bit_pos, 1);
> +	largest_idx =
> +		mte_bitmap_read(bitmap, &bit_pos, MTE_BITS_PER_LARGEST_IDX);

Nit: really no need to split the line - we're OK with 100-chars per
line now.

> +	if (largest_idx >= MTE_MAX_RANGES)
> +		return false;
> +
> +	for (i = 0; i < max_ranges; i++)
> +		r_tags[i] = mte_bitmap_read(bitmap, &bit_pos, MTE_TAG_SIZE);
> +	for (i = 0, sum = 0; i < max_ranges; i++) {
> +		if (i == largest_idx)
> +			continue;
> +		r_sizes[i] =
> +			mte_bitmap_read(bitmap, &bit_pos, MTE_BITS_PER_SIZE);
> +		/*
> +		 * Special case: tag array consists of two ranges of
> +		 * `MTE_GRANULES_PER_PAGE / 2` tags.
> +		 */
> +		if ((largest_idx == 0) && (i == 1) && (r_sizes[i] == 0))
> +			r_sizes[i] = MTE_GRANULES_PER_PAGE / 2;
> +		if (!r_sizes[i]) {
> +			max_ranges = i;
> +			break;
> +		}
> +		sum += r_sizes[i];
> +	}
> +	if (sum >= MTE_GRANULES_PER_PAGE)
> +		return false;
> +	r_sizes[largest_idx] = MTE_GRANULES_PER_PAGE - sum;
> +	mte_ranges_to_tags(r_tags, r_sizes, max_ranges, tags);
> +	return true;
> +}
> +EXPORT_SYMBOL_NS(mte_decompress, MTECOMP);
> diff --git a/arch/arm64/mm/mtecomp.h b/arch/arm64/mm/mtecomp.h
> new file mode 100644
> index 0000000000000..b94cf0384f2af
> --- /dev/null
> +++ b/arch/arm64/mm/mtecomp.h
> @@ -0,0 +1,12 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef ARCH_ARM64_MM_MTECOMP_H_
> +#define ARCH_ARM64_MM_MTECOMP_H_
> +
> +/* Functions exported from mtecomp.c for test_mtecomp.c. */
> +void mte_tags_to_ranges(u8 *tags, u8 *out_tags, unsigned short *out_sizes,
> +			size_t *out_len);
> +void mte_ranges_to_tags(u8 *r_tags, unsigned short *r_sizes, size_t r_len,
> +			u8 *tags);
> +
> +#endif  // ARCH_ARM64_MM_TEST_MTECOMP_H_
> -- 
> 2.43.0.472.g3155946c3a-goog

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