Re: [PATCH v4] mm/hugetlb: Fix unsigned overflow in __nr_hugepages_store_common()

[Andrew Morton <akpm@linux-foundation.org>]

> 
> The number of node specific huge pages can be set via a file such as:
> /sys/devices/system/node/node1/hugepages/hugepages-2048kB/nr_hugepages
> When a node specific value is specified, the global number of huge
> pages must also be adjusted.  This adjustment is calculated as the
> specified node specific value + (global value - current node value).
> If the node specific value provided by the user is large enough, this
> calculation could overflow an unsigned long leading to a smaller
> than expected number of huge pages.
> 
> To fix, check the calculation for overflow.  If overflow is detected,
> use ULONG_MAX as the requested value.  This is inline with the user
> request to allocate as many huge pages as possible.
> 
> It was also noticed that the above calculation was done outside the
> hugetlb_lock.  Therefore, the values could be inconsistent and result
> in underflow.  To fix, the calculation is moved to within the routine
> set_max_huge_pages() where the lock is held.
> 
> ...
>
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2274,7 +2274,7 @@ static int adjust_pool_surplus(struct hstate *h,
> nodemask_t *nodes_allowed,

Please tweak that email client to prevent the wordwraps.

> +	/*
> +	 * Check for a node specific request.  Adjust global count, but
> +	 * restrict alloc/free to the specified node.
> +	 */
> +	if (nid != NUMA_NO_NODE) {
> +		unsigned long old_count = count;
> +		count += h->nr_huge_pages - h->nr_huge_pages_node[nid];
> +		/*
> +		 * If user specified count causes overflow, set to
> +		 * largest possible value.
> +		 */
> +		if (count < old_count)
> +			count = ULONG_MAX;
> +	}

The above two comments explain the code, but do not reveal the
reasoning behind the policy decisions which that code implements.

> ...
>
> +	} else {
>  		/*
> -		 * per node hstate attribute: adjust count to global,
> -		 * but restrict alloc/free to the specified node.
> +		 * Node specific request, but we could not allocate
> +		 * node mask.  Pass in ALL nodes, and clear nid.
>  		 */

Ditto here, somewhat.

The old mantra: comments should explain "why", not "what".  Reading the
code tells us the "what".

Thanks.