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

[Jing Xiangfeng]

On 2019/2/20 7:45, Mike Kravetz wrote:
> On 2/18/19 1:27 AM, Michal Hocko wrote:
>> On Sat 16-02-19 21:31:12, Jingxiangfeng wrote:
>>> From: Jing Xiangfeng <jingxiangfeng@xxxxxxxxxx>
>>>
>>> We can use the following command to dynamically allocate huge pages:
>>> 	echo NR_HUGEPAGES > /proc/sys/vm/nr_hugepages
>>> The count in  __nr_hugepages_store_common() is parsed from /proc/sys/vm/nr_hugepages,
>>> The maximum number of count is ULONG_MAX,
>>> the operation 'count += h->nr_huge_pages - h->nr_huge_pages_node[nid]' overflow and count will be a wrong number.
>>
>> Could you be more specific of what is the runtime effect on the
>> overflow? I haven't checked closer but I would assume that we will
>> simply shrink the pool size because count will become a small number.
>>
> 
> Well, the first thing to note is that this code only applies to case where
> someone is changing a node specific hugetlb count.  i.e.
> /sys/devices/system/node/node1/hugepages/hugepages-2048kB
> In this case, the calculated value of count is a maximum or minimum total
> number of huge pages.  However, only the number of huge pages on the specified
> node is adjusted to try and achieve this maximum or minimum.
> 
> So, in the case of overflow the number of huge pages on the specified node
> could be reduced.  I say 'could' because it really is dependent on previous
> values.  In some situations the node specific value will be increased.
> 
> Minor point is that the description in the commit message does not match
> the code changed.
> 
Thanks for your reply.as you said, the case is where someone is changing a node
specific hugetlb count when CONFIG_NUMA is enable. I will modify the commit message.

>> Is there any reason to report an error in that case? We do not report
>> errors when we cannot allocate the requested number of huge pages so why
>> is this case any different?
> 
> Another issue to consider is that h->nr_huge_pages is an unsigned long,
> and h->nr_huge_pages_node[] is an unsigned int.  The sysfs store routines
> treat them both as unsigned longs.  Ideally, the store routines should
> distinguish between the two.
> 
> In reality, an actual overflow is unlikely.  If my math is correct (not
> likely) it would take something like 8 Petabytes to overflow the node specific
> counts.
> 
> In the case of a user entering a crazy high value and causing an overflow,
> an error return might not be out of line.  Another option would be to simply
> set count to ULONG_MAX if we detect overflow (or UINT_MAX if we are paranoid)
> and continue on.  This may be more in line with user's intention of allocating
> as many huge pages as possible.
> 
> Thoughts?
> 
It is better to set count to ULONG_MAX if we detect overflow, and continue to
allocate as many huge pages as possible.
I will send v2 soon.