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

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Date:   Mon, 19 Aug 2019 10:19:01 +0530
From:   Anup Patel <anup@...infault.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Anup Patel <Anup.Patel@....com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.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

On Sun, Aug 18, 2019 at 11:49 PM Christoph Hellwig <hch@...radead.org> wrote:
>
> > +#define FIXADDR_TOP      (VMALLOC_START)
>
> Nit: no need for the braces, the definitions below don't use it
> either.

Sure, I will update and send v2 soon.

>
> > +#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.

I agree with you. We also have Sparsemem and KASAN patches which
touch virtual memory layout so it's important to have virtual memory layout
documented clearly. I can add the required documentation as separate patch.

I think the best place to add ASCII art would be asm/pgtable.h where all
virtual memory related defines are placed. Suggestions??

Regards,
Anup