linux-kernel - Re: [PATCH] RISC-V: Fix FIXMAP area corruption on RV32 systems

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Message-ID: <20190818181914.GB20217@infradead.org>
Date:   Sun, 18 Aug 2019 11:19:14 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Anup Patel <Anup.Patel@....com>
Cc:     Palmer Dabbelt <palmer@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Anup Patel <anup@...infault.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Atish Patra <Atish.Patra@....com>,
        Alistair Francis <Alistair.Francis@....com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>
Subject: Re: [PATCH] RISC-V: Fix FIXMAP area corruption on RV32 systems

> +#define FIXADDR_TOP      (VMALLOC_START)

Nit: no need for the braces, the definitions below don't use it
either.

> +#ifdef CONFIG_64BIT
> +#define FIXADDR_SIZE     PMD_SIZE
> +#else
> +#define FIXADDR_SIZE     PGDIR_SIZE
> +#endif
> +#define FIXADDR_START    (FIXADDR_TOP - FIXADDR_SIZE)
> +
>  /*
> - * Task size is 0x4000000000 for RV64 or 0xb800000 for RV32.
> + * Task size is 0x4000000000 for RV64 or 0x9fc00000 for RV32.
>   * Note that PGDIR_SIZE must evenly divide TASK_SIZE.
>   */
>  #ifdef CONFIG_64BIT
>  #define TASK_SIZE (PGDIR_SIZE * PTRS_PER_PGD / 2)
>  #else
> -#define TASK_SIZE VMALLOC_START
> +#define TASK_SIZE FIXADDR_START
>  #endif

Mentioning the addresses is a little weird.  IMHO this would be
a much nicer place to explain the high-level memory layout, including
maybe a little ASCII art.  Also we could have one #ifdef CONFIG_64BIT
for both related values.  Last but not least instead of saying that
something should be dividable it would be nice to have a BUILD_BUG_ON
to enforce it.

Either way we are late in the cycle, so I guess this is ok for now:

Reviewed-by: Christoph Hellwig <hch@....de>

But I'd love to see this area improved a little further as it is full
of mine fields.