linux-kernel - Re: [PATCH v5 1/4] riscv: Move kernel mapping to vmalloc zone

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Date:   Fri, 24 Jul 2020 08:33:12 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Alex Ghiti <alex@...ti.fr>, Palmer Dabbelt <palmer@...belt.com>
Cc:     mpe@...erman.id.au, paulus@...ba.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        aou@...s.berkeley.edu, Anup Patel <Anup.Patel@....com>,
        Atish Patra <Atish.Patra@....com>, zong.li@...ive.com,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-riscv@...ts.infradead.org, linux-mm@...ck.org
Subject: Re: [PATCH v5 1/4] riscv: Move kernel mapping to vmalloc zone

On Thu, 2020-07-23 at 01:21 -0400, Alex Ghiti wrote:
> > works fine with huge pages, what is your problem there ? You rely on
> > punching small-page size holes in there ?
> > 
> 
> ARCH_HAS_STRICT_KERNEL_RWX prevents the use of a hugepage for the kernel 
> mapping in the direct mapping as it sets different permissions to 
> different part of the kernel (data, text..etc).

Ah ok, that can be solved in a couple of ways...

One is to use the linker script to ensure those sections are linked
HUGE_PAGE_SIZE appart and moved appropriately by early boot code. One
is to selectively degrade just those huge pages.

I'm not familiar with the RiscV MMU (I should probably go have a look)
but if it's a classic radix tree with huge pages at PUD/PMD level, then
you could just degrade the one(s) that cross those boundaries.

Cheers,
Ben.