linux-kernel - Re: [PATCH] riscv: mm: remove the BUG_ON check of mapping the last 4K bytes of memory

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Date:   Tue, 25 Jan 2022 17:10:41 +0100
From:   Alexandre Ghiti <alex@...ti.fr>
To:     Jisheng Zhang <jszhang@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>
Cc:     linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: mm: remove the BUG_ON check of mapping the last 4K
 bytes of memory


On 1/25/22 16:55, Jisheng Zhang wrote:
> remove the BUG_ON check of mapping the last 4K bytes of the addressable
> memory since "this is true for every kernel actually" as pointed out
> by Alexandre.
>
> Signed-off-by: Jisheng Zhang <jszhang@...nel.org>
> Reviewed-by: Alexandre Ghiti <alex@...ti.fr>
> ---
>   arch/riscv/mm/init.c | 8 --------
>   1 file changed, 8 deletions(-)
>
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index cf4d018b7d66..8347d0fda8cd 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -811,14 +811,6 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>   	BUG_ON((PAGE_OFFSET % PGDIR_SIZE) != 0);
>   	BUG_ON((kernel_map.phys_addr % PMD_SIZE) != 0);
>   
> -#ifdef CONFIG_64BIT
> -	/*
> -	 * The last 4K bytes of the addressable memory can not be mapped because
> -	 * of IS_ERR_VALUE macro.
> -	 */
> -	BUG_ON((kernel_map.virt_addr + kernel_map.size) > ADDRESS_SPACE_END - SZ_4K);
> -#endif


This BUG_ON seems pretty legit to me: I re-read the exchanges we had, 
and I see that I didn't notice that in your v2, you actually removed the 
BUG_ON. So that's my bad, what I meant in the first place was that the 
BUG_ON is true for 32-bit and 64-bit kernels actually.

Sorry my RB was not right on this one :(

Alex


> -
>   	pt_ops_set_early();
>   
>   	/* Setup early PGD for fixmap */