Re: [PATCH v4 3/5] arm64: mte: implement CONFIG_ARM64_MTE_COMP
From: Catalin Marinas <catalin.marinas@arm.com>
Date: 2023-08-18 17:58:15
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Just some random comments, I haven't reviewed the series properly. On Thu, Jul 20, 2023 at 07:39:54PM +0200, Alexander Potapenko wrote:
quoted hunk ↗ jump to hunk
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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel