Re: [PATCH 2/4] RISC-V: Set current memblock limit

[Anup Patel]

On Thu, Jan 7, 2021 at 2:57 PM Atish Patra <atish.patra@xxxxxxx> wrote:
>
> Currently, linux kernel can not use last 4k bytes of addressable space because
> IS_ERR_VALUE macro treats those as an error. This will be an issue for RV32
> as any memblock allocator potentially allocate chunk of memory from the end
> of DRAM (2GB) leading bad address error even though the address was technically
> valid.
>
> Fix this issue by limiting the memblock if available memory spans the entire
> address space.
>
> Signed-off-by: Atish Patra <atish.patra@xxxxxxx>
> ---
>  arch/riscv/mm/init.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index bf5379135e39..da53902ef0fc 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -157,9 +157,10 @@ static void __init setup_initrd(void)
>  void __init setup_bootmem(void)
>  {
>         phys_addr_t mem_start = 0;
> -       phys_addr_t start, end = 0;
> +       phys_addr_t start, dram_end, end = 0;
>         phys_addr_t vmlinux_end = __pa_symbol(&_end);
>         phys_addr_t vmlinux_start = __pa_symbol(&_start);
> +       phys_addr_t max_mapped_addr = __pa(PHYS_ADDR_MAX);

Using PHYS_ADDR_MAX as the max virtual address does not look right.

Better use __pa(~(ulong)0) here. Otherwise looks good to me.

Reviewed-by: Anup Patel <anup@xxxxxxxxxxxxxx>

>         u64 i;
>
>         /* Find the memory region containing the kernel */
> @@ -181,7 +182,18 @@ void __init setup_bootmem(void)
>         /* Reserve from the start of the kernel to the end of the kernel */
>         memblock_reserve(vmlinux_start, vmlinux_end - vmlinux_start);
>
> -       max_pfn = PFN_DOWN(memblock_end_of_DRAM());
> +       dram_end = memblock_end_of_DRAM();
> +
> +       /*
> +        * memblock allocator is not aware of the fact that last 4K bytes of
> +        * the addressable memory can not be mapped because of IS_ERR_VALUE
> +        * macro. Make sure that last 4k bytes are not usable by memblock
> +        * if end of dram is equal to maximum addressable memory.
> +        */
> +       if (max_mapped_addr == (dram_end - 1))
> +               memblock_set_current_limit(max_mapped_addr - 4096);
> +
> +       max_pfn = PFN_DOWN(dram_end);
>         max_low_pfn = max_pfn;
>         dma32_phys_limit = min(4UL * SZ_1G, (unsigned long)PFN_PHYS(max_low_pfn));
>         set_max_mapnr(max_low_pfn);
> --
> 2.25.1
>
>
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Regards,
Anup