On 10/14/19 11:12 AM, Michal Hocko wrote:
>> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
>> index 4ae967b..a23509f 100644
>> --- a/mm/mempolicy.c
>> +++ b/mm/mempolicy.c
>> @@ -1328,9 +1328,11 @@ static int get_nodes(nodemask_t *nodes, const unsigned long __user *nmask,
>> unsigned long nlongs;
>> unsigned long endmask;
>>
>> - --maxnode;
>> nodes_clear(*nodes);
>> - if (maxnode == 0 || !nmask)
>> + /*
>> + * If the user specified only one node, no need to set nodemask
>> + */
>> + if (maxnode - 1 == 0 || !nmask)
>> return 0;
>> if (maxnode > PAGE_SIZE*BITS_PER_BYTE)
>> return -EINVAL;
>
> I am afraid this is a wrong fix. It is really hard to grasp the code but my
> understanding is that the caller is supposed to provide maxnode larger
> than than the nodemask. So if you want 2 nodes then maxnode should be 3.
> Have a look at the libnuma (which is a reference implementation)
>
> static void setpol(int policy, struct bitmask *bmp)
> {
> if (set_mempolicy(policy, bmp->maskp, bmp->size + 1) < 0)
> numa_error("set_mempolicy");
> }
>
> The semantic is quite awkward but it is that way for years.
Yes, unfortunately. Too late to change. We could just update the
manpages at this point.
get_mempolicy(2) says:
maxnode specifies the number of node IDs that can be stored into
nodemask—that is, the maximum node ID plus one.
- Since node ID starts with 0, it should be actually "plus two".
set_mempolicy(2) says:
nodemask points to a bit mask of node IDs that contains up to maxnode
bits.
- should be also clarified.
