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Date:   Sat, 12 Sep 2020 13:45:02 +0300
From:   Mike Rapoport <rppt@kernel.org>
To:     Atish Patra <atish.patra@wdc.com>
Cc:     linux-kernel@vger.kernel.org, Albert Ou <aou@eecs.berkeley.edu>,
        Anup Patel <anup.patel@wdc.com>,
        linux-riscv@lists.infradead.org,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Zong Li <zong.li@sifive.com>
Subject: Re: [PATCH] RISC-V: Consider sparse memory while removing unusable
 memory
Message-ID: <20200912104502.GF2142832@kernel.org>
References: <20200912002341.4869-1-atish.patra@wdc.com>
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Hello Atish,

On Fri, Sep 11, 2020 at 05:23:41PM -0700, Atish Patra wrote:
> Currently, any usable memory area beyond page_offset is removed by adding the
> memory sizes from each memblock. That may not work for sparse memory
> as memory regions can be very far apart resulting incorrect removal of some
> usable memory.

If I understand correctly, the memory with physical addresses larger
than (-PAGE_OFFSET) cannot be used. Since it was aready
memblock_add()'ed during device tree parsing, you need to remove it from
memblock.

For that you can use memblock_enforce_memory_limit(-PAGE_OFFSET).

> Just use the start of the first memory block and the end of the last memory
> block to compute the size of the total memory that can be used.
> 
> Signed-off-by: Atish Patra <atish.patra@wdc.com>
> ---
>  arch/riscv/mm/init.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 787c75f751a5..188281fc2816 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -147,7 +147,6 @@ void __init setup_bootmem(void)
>  {
>  	struct memblock_region *reg;
>  	phys_addr_t mem_size = 0;
> -	phys_addr_t total_mem = 0;
>  	phys_addr_t mem_start, end = 0;
>  	phys_addr_t vmlinux_end = __pa_symbol(&_end);
>  	phys_addr_t vmlinux_start = __pa_symbol(&_start);
> @@ -155,18 +154,17 @@ void __init setup_bootmem(void)
>  	/* Find the memory region containing the kernel */
>  	for_each_memblock(memory, reg) {
>  		end = reg->base + reg->size;
> -		if (!total_mem)
> +		if (!mem_start)
>  			mem_start = reg->base;
>  		if (reg->base <= vmlinux_start && vmlinux_end <= end)
>  			BUG_ON(reg->size == 0);
> -		total_mem = total_mem + reg->size;
>  	}
>  
>  	/*
>  	 * Remove memblock from the end of usable area to the
>  	 * end of region
>  	 */
> -	mem_size = min(total_mem, (phys_addr_t)-PAGE_OFFSET);
> +	mem_size = min(end - mem_start, (phys_addr_t)-PAGE_OFFSET);
>  	if (mem_start + mem_size < end)
>  		memblock_remove(mem_start + mem_size,
>  				end - mem_start - mem_size);
> -- 
> 2.24.0
> 

-- 
Sincerely yours,
Mike.