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Date: Mon, 28 Jul 2014 08:26:41 -0700
From: Dave Hansen <dave.hansen@linux.intel.com>
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To: "Kirill A. Shutemov" <kirill@shutemov.name>,
	Andrey Ryabinin <a.ryabinin@samsung.com>,
	Sasha Levin <sasha.levin@oracle.com>
CC: Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andi Kleen <ak@linux.intel.com>,
	Matthew Wilcox <matthew.r.wilcox@intel.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Dave Chinner <david@fromorbit.com>, Ning Qu <quning@gmail.com>,
	linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, Dave Jones <davej@redhat.com>,
	stable@vger.kernel.org,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>,
	Konstantin Khlebnikov <koct9i@gmail.com>,
	Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH] mm: don't allow fault_around_bytes to be 0
References: <53D07E96.5000006@oracle.com> <1406533400-6361-1-git-send-email-a.ryabinin@samsung.com> <20140728093611.GA3975@node.dhcp.inet.fi>
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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.