linux-kernel - Re: [PATCH 2/3] Documentation: riscv: Add documentation that describes the VM layout

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Date:   Wed, 10 Mar 2021 12:42:56 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Alex Ghiti <alex@...ti.fr>
Cc:     David Hildenbrand <david@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH 2/3] Documentation: riscv: Add documentation that
 describes the VM layout

On Thu, Feb 25, 2021 at 12:56 PM Alex Ghiti <alex@...ti.fr> wrote:
>
> Le 2/25/21 à 5:34 AM, David Hildenbrand a écrit :
> >                   |            |                  |         |> +
> > ffffffc000000000 | -256    GB | ffffffc7ffffffff |   32 GB | kasan
> >> +   ffffffcefee00000 | -196    GB | ffffffcefeffffff |    2 MB | fixmap
> >> +   ffffffceff000000 | -196    GB | ffffffceffffffff |   16 MB | PCI io
> >> +   ffffffcf00000000 | -196    GB | ffffffcfffffffff |    4 GB | vmemmap
> >> +   ffffffd000000000 | -192    GB | ffffffdfffffffff |   64 GB |
> >> vmalloc/ioremap space
> >> +   ffffffe000000000 | -128    GB | ffffffff7fffffff |  126 GB |
> >> direct mapping of all physical memory
> >
> > ^ So you could never ever have more than 126 GB, correct?
> >
> > I assume that's nothing new.
> >
>
> Before this patch, the limit was 128GB, so in my sense, there is nothing
> new. If ever we want to increase that limit, we'll just have to lower
> PAGE_OFFSET, there is still some unused virtual addresses after kasan
> for example.

Linus Walleij is looking into changing the arm32 code to have the kernel
direct map inside of the vmalloc area, which would be another place
that you could use here. It would be nice to not have too many different
ways of doing this, but I'm not sure how hard it would be to rework your
code, or if there are any downsides of doing this.

        Arnd