Messages in this thread | ![/](/images/icornerl.gif) | | Date | Fri, 18 Aug 2023 18:57:13 +0100 | From | Catalin Marinas <> | Subject | Re: [PATCH v4 3/5] arm64: mte: implement CONFIG_ARM64_MTE_COMP |
| |
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
| ![\](/images/icornerr.gif) |