Re: [PATCH] mm/mempolicy: add node_empty check in SYSC_migrate_pages

[Vlastimil Babka]

On 10/18/2017 11:34 AM, Yisheng Xie wrote:
>>> For MAX_NUMNODES is 4, so 0x10 nodemask will tread as empty set which makes
>>> 	nodes_subset(*new, node_states[N_MEMORY])
>>
>> According to manpage of migrate_pages:
>>
>>         EINVAL The value specified by maxnode exceeds a kernel-imposed
>> limit.  Or, old_nodes or new_nodes specifies one or more node IDs that
>> are greater than the maximum supported node ID.  Or, none of the node
>> IDs specified by new_nodes are on-line and allowed by the process's
>> current cpuset context, or none of the specified nodes contain memory.
>>
>> if maxnode parameter is 64, but MAX_NUMNODES ("kernel-imposed limit") is
>> 4, we should get EINVAL just because of that. I don't see such check in
>> the migrate_pages implementation though.
> 
> Yes, that is what manpage said, but I have a question about this: if user
> set maxnode exceeds a kernel-imposed and try to access node without enough
> privilege, which errors values we should return ? For I have seen that all
> of the ltp migrate_pages01 will set maxnode to 64 in my system.

Hm I don't think it matters much and don't know if there's some commonly
used priority. Personally I would do the checks resulting in EINVAL
first, before EPERM, but if the code is structured differently, it may
stay as it is.