Re: [PATCH] mm: don't allow fault_around_bytes to be 0

[Dave Hansen]

On 07/28/2014 02:36 AM, Kirill A. Shutemov wrote:
> +++ b/mm/memory.c
> @@ -2786,7 +2786,8 @@ static int fault_around_bytes_set(void *data, u64 val)
>  {
>  	if (val / PAGE_SIZE > PTRS_PER_PTE)
>  		return -EINVAL;
> -	fault_around_bytes = val;
> +	/* rounddown_pow_of_two(0) is not defined */
> +	fault_around_bytes = max(val, PAGE_SIZE);
>  	return 0;
>  }

It's also possible to race and have fault_around_bytes change between
when fault_around_mask() and fault_around_pages() are called so that
they don't match any more.  The min()/max() in do_fault_around() should
keep this from doing anything _too_ nasty, but it's worth thinking about
at least.

The safest thing to do might be to use an ACCESS_ONCE() at the beginning
of do_fault_around() for fault_around_bytes and generate
fault_around_mask() from the ACCESS_ONCE() result.
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