Re: [PATCH REBASED] hugetlbfs: fix potential over/underflow setting node specific nr_hugepages

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Thu, Mar 28, 2019 at 03:05:33PM -0700, Mike Kravetz wrote:
> 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 within the routine
> set_max_huge_pages() where the lock is held.
> 
> In addition, the code in __nr_hugepages_store_common() which tries to
> handle the case of not being able to allocate a node mask would likely
> result in incorrect behavior.  Luckily, it is very unlikely we will
> ever take this path.  If we do, simply return ENOMEM.
> 
> Reported-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>

Looks good to me.

Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

> ---
> This was sent upstream during 5.1 merge window, but dropped as it was
> based on an earlier version of Alex Ghiti's patch which was dropped.
> Now rebased on top of Alex Ghiti's "[PATCH v8 0/4] Fix free/allocation
> of runtime gigantic pages" series which was just added to mmotm.
> 
>  mm/hugetlb.c | 41 ++++++++++++++++++++++++++++++++++-------
>  1 file changed, 34 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index f3e84c1bef11..f79ae4e42159 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2287,13 +2287,33 @@ static int adjust_pool_surplus(struct hstate *h, nodemask_t *nodes_allowed,
>  }
>  
>  #define persistent_huge_pages(h) (h->nr_huge_pages - h->surplus_huge_pages)
> -static int set_max_huge_pages(struct hstate *h, unsigned long count,
> +static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
>  			      nodemask_t *nodes_allowed)
>  {
>  	unsigned long min_count, ret;
>  
>  	spin_lock(&hugetlb_lock);
>  
> +	/*
> +	 * Check for a node specific request.
> +	 * Changing node specific huge page count may require a corresponding
> +	 * change to the global count.  In any case, the passed node mask
> +	 * (nodes_allowed) will 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];
> +		/*
> +		 * User may have specified a large count value which caused the
> +		 * above calculation to overflow.  In this case, they wanted
> +		 * to allocate as many huge pages as possible.  Set count to
> +		 * largest possible value to align with their intention.
> +		 */
> +		if (count < old_count)
> +			count = ULONG_MAX;
> +	}
> +
>  	/*
>  	 * Gigantic pages runtime allocation depend on the capability for large
>  	 * page range allocation.
> @@ -2445,15 +2465,22 @@ static ssize_t __nr_hugepages_store_common(bool obey_mempolicy,
>  		}
>  	} else if (nodes_allowed) {
>  		/*
> -		 * per node hstate attribute: adjust count to global,
> -		 * but restrict alloc/free to the specified node.
> +		 * Node specific request.  count adjustment happens in
> +		 * set_max_huge_pages() after acquiring hugetlb_lock.
>  		 */
> -		count += h->nr_huge_pages - h->nr_huge_pages_node[nid];
>  		init_nodemask_of_node(nodes_allowed, nid);
> -	} else
> -		nodes_allowed = &node_states[N_MEMORY];
> +	} else {
> +		/*
> +		 * Node specific request, but we could not allocate the few
> +		 * words required for a node mask.  We are unlikely to hit
> +		 * this condition.  Since we can not pass down the appropriate
> +		 * node mask, just return ENOMEM.
> +		 */
> +		err = -ENOMEM;
> +		goto out;
> +	}
>  
> -	err = set_max_huge_pages(h, count, nodes_allowed);
> +	err = set_max_huge_pages(h, count, nid, nodes_allowed);
>  
>  out:
>  	if (nodes_allowed != &node_states[N_MEMORY])
> -- 
> 2.20.1
> 
>