Re: [PATCH v4 3/5] arm64: mte: implement CONFIG_ARM64_MTE_COMP

[Catalin Marinas <catalin.marinas@arm.com>]

Just some random comments, I haven't reviewed the series properly.

On Thu, Jul 20, 2023 at 07:39:54PM +0200, Alexander Potapenko wrote:
> diff --git a/Documentation/arch/arm64/mte-tag-compression.rst b/Documentation/arch/arm64/mte-tag-compression.rst
> new file mode 100644
> index 0000000000000..af6716d53c1a8
> --- /dev/null
> +++ b/Documentation/arch/arm64/mte-tag-compression.rst
> @@ -0,0 +1,212 @@
> +.. 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). 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, which in the case of a zram-backed swap may
> +even exceed the memory used to store the swapped pages themselves.

Hmm, I'm not sure about this claim ;). Is the zram so good that it
manages a 32x compression (4096/128)?

How much would we save if we only do the compression when it can fit in
63 bits?

> +/**
> + * mte_tags_to_ranges() - break @tags into arrays of tag ranges.
> + * @tags: 128-byte array containing 256 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[]).
> + */
> +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[0] = prev_tag;
> +	for (i = 0; i < MTE_PAGE_TAG_STORAGE; i++) {
> +		for (j = 0; j < 2; j++) {
> +			cur_tag = j ? (tags[i] % 16) : (tags[i] / 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);

What's with the exports here? Are we expecting these functions to be
called from loadable modules?

-- 
Catalin