Subject: Re: [PATCH v4 5/5] arm64: mte: add compression support to mteswap.c

From: Catalin Marinas <[email-masked]>
Date: Fri, 18 Aug 2023 19:18:06 +0100

On Thu, Jul 20, 2023 at 07:39:56PM +0200, Alexander Potapenko wrote:
> Soon after booting Android, tag compression saves ~2.5x memory previously
> spent by mteswap.c on tag allocations. With the growing uptime, the
> savings reach 20x and even more.

This sounds like a marketing claim ;). The way I read your statement is
that the memory used for tag storage is 2.5x less with your patches and
that's great. It means a 2.5x compression on average. How does the
compression get so much better to 20x with more uptime? The number of
tag storage allocations should be proportional to the pages swapped out
(not equal as not all would be mapped as PROT_MTE). So you can indeed
have a lot more pages swapped out than available in RAM and the tag
storage can take space but not sure which numbers you divided to get
20x.

Anyway, it would be nice to see the full picture of what the savings
relative to the total RAM is. Given that the swap in this instance is
zram, you have an upper bound of how many pages it can store. I'm just
trying to assess whether the complexity added here is worth it.

Maybe not as good as the RLE algorithm here, I was wondering whether we
could use zswap to save the tags together with the page. I looked some
time ago at it seemed slightly easier for zswap than zram. Another
option is to make the swap format more generic to support metadata
storage. Yet another option is for the zram to flag that it can compress
the metadata together with the data (no swap format change needed; when
decompressing the page, it populates the tags as well).

-- 
Catalin

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