Re: [PATCH] riscv: Only extend kernel reservation if mapped read-only

[Geert Uytterhoeven]

Hi Alex,

On Thu, Apr 29, 2021 at 4:09 PM Alex Ghiti <alex@xxxxxxxx> wrote:
> Le 4/28/21 à 9:45 AM, Geert Uytterhoeven a écrit :
> > When the kernel mapping was moved outside of the linear mapping, the
> > kernel memory reservation was increased, to take into account mapping
> > granularity.  However, this is done unconditionally, regardless of
> > whether the kernel memory is mapped read-only or not.
> >
> > If this extension is not needed, up to 2 MiB may be lost, which has a
> > big impact on e.g. Canaan K210 (64-bit nommu) platforms with only 8 MiB
> > of RAM.
> >
> > Reclaim the lost memory by only extending the reserved region when
> > needed, i.e. matching the conditional logic around the call to
> > protect_kernel_linear_mapping_text_rodata().
> >
> > Fixes: 2bfc6cd81bd17e43 ("riscv: Move kernel mapping outside of linear mapping")
> > Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
> > ---
> > Only tested on K210 (SiPeed MAIX BiT):
> >
> >      -Memory: 5852K/8192K available (1344K kernel code, 147K rwdata, 272K rodata, 106K init, 72K bss, 2340K reserved, 0K cma-reserved)
> >      +Memory: 5948K/8192K available (1344K kernel code, 147K rwdata, 272K rodata, 106K init, 72K bss, 2244K reserved, 0K cma-reserved)
> >
> > Yes, I was lucky, as only 96 KiB was lost ;-)
> > ---
> >   arch/riscv/mm/init.c | 10 ++++++++--
> >   1 file changed, 8 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> > index 788eb222deacf994..3439783f26abc488 100644
> > --- a/arch/riscv/mm/init.c
> > +++ b/arch/riscv/mm/init.c
> > @@ -136,11 +136,17 @@ void __init setup_bootmem(void)
> >
> >       /*
> >        * Reserve from the start of the kernel to the end of the kernel
> > -      * and make sure we align the reservation on PMD_SIZE since we will
> > +      */
> > +#if defined(CONFIG_STRICT_KERNEL_RWX) && defined(CONFIG_64BIT) && \
> > +    defined(CONFIG_MMU) && !defined(CONFIG_XIP_KERNEL)
>
> ARCH_HAS_STRICT_KERNEL_RWX depends on MMU and !XIP_KERNEL so I think you
> can get rid of those checks.

Thanks, you're right.  Will simplify in v2.

I'll also clean up the other location where this was copied from.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds