[PATCH v2] riscv: Only extend kernel reservation if mapped read-only

[Palmer Dabbelt]

On Thu, 29 Apr 2021 08:05:00 PDT (-0700), geert+renesas at glider.be wrote:
> 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. depending on a simplified version of 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 at glider.be>
> ---
> v2:
>   - Simplify the conditional, as STRICT_KERNEL_RWX depends on
>     MMU && !XIP_KERNEL.
>
> 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 | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 788eb222deacf994..3ebc0f5d2b73b42b 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -136,11 +136,16 @@ 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_64BIT) && defined(CONFIG_STRICT_KERNEL_RWX)
> +	/*
> +	 * Make sure we align the reservation on PMD_SIZE since we will
>  	 * map the kernel in the linear mapping as read-only: we do not want
>  	 * any allocation to happen between _end and the next pmd aligned page.
>  	 */
> -	memblock_reserve(vmlinux_start, (vmlinux_end - vmlinux_start + PMD_SIZE - 1) & PMD_MASK);
> +	vmlinux_end = (vmlinux_end + PMD_SIZE - 1) & PMD_MASK;
> +#endif
> +	memblock_reserve(vmlinux_start, vmlinux_end - vmlinux_start);
>
>  	/*
>  	 * memblock allocator is not aware of the fact that last 4K bytes of

Thanks, this is on for-next