linux-kernel - Re: [PATCH] mm: don't allow fault_around

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Date:	Mon, 28 Jul 2014 08:26:41 -0700
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Andrey Ryabinin <a.ryabinin@...sung.com>,
	Sasha Levin <sasha.levin@...cle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Dave Chinner <david@...morbit.com>, Ning Qu <quning@...il.com>,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Dave Jones <davej@...hat.com>,
	stable@...r.kernel.org,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Konstantin Khlebnikov <koct9i@...il.com>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH] mm: don't allow fault_around_bytes to be 0

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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