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From:   Alexander Potapenko <glider@google.com>
Date:   Fri, 22 Sep 2023 10:04:45 +0200
Message-ID: <CAG_fn=W7VqQukmVqmFiuDyi-osHpbXiRJR0HH0nrDU=GApyBPA@mail.gmail.com>
Subject: Re: [PATCH v4 3/5] arm64: mte: implement CONFIG_ARM64_MTE_COMP
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     will@kernel.org, pcc@google.com, andreyknvl@gmail.com,
        andriy.shevchenko@linux.intel.com, linux@rasmusvillemoes.dk,
        yury.norov@gmail.com, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, eugenis@google.com,
        syednwaris@gmail.com, william.gray@linaro.org
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> > +MTE assigns tags to memory pages: for 4K pages those tags occupy 128 b=
ytes
> > +(256 4-bit tags each corresponding to a 16-byte MTE granule). By defau=
lt, 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 subs=
tantial
> > +portion of the physical memory, which in the case of a zram-backed swa=
p 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)?

For trivial data, like zero pages, this is possible in theory, but I
am not sure whether zram takes advantage of this. I removed the claim
:)

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

Good question. What I've observed is that soon after boot roughly 50%
of tag buffers can be compressed into 63 bits.


> > +void mte_tags_to_ranges(u8 *tags, u8 *out_tags, unsigned short *out_si=
zes,
> > +                     size_t *out_len)
> > +{
...
> > +}
> > +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?

Yes, this is for the KUnit test module

>
> --
> Catalin


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