Re: [PATCH] mempolicy: check home_node is in the nodes of policy

[Chunsheng Luo <luochunsheng@ustc.edu>]

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On 2024/1/29 23:12, Gregory Price wrote:
> On Fri, Jan 26, 2024 at 08:22:40AM -0500, Chunsheng Luo wrote:
>> set_mempolicy_home_node should be used after setting the memory
>> policy. If the home_node isn't in the nodes of policy, we should
>> return failure to avoid misunderstanding.
>>
>> Signed-off-by: Chunsheng Luo<luochunsheng@ustc.edu>
>> ---
>>   mm/mempolicy.c | 6 ++++++
>>   1 file changed, 6 insertions(+)
>>
> Since it's not possible to add/remove a node to a mask without also
> erasing the home node, this seems reasonable.
>
> e.g. this is what happens presently
> mbind(0-2)    :   mask(0,1,2),   home_node(NUMA_NO_NODE)
> home_node(3)  :   mask(0,1,2),   home_node(3)
> mbind(0-3)    :   mask(0,1,2,3), home_node(NUMA_NO_NODE)
>
> However, it is possible for a cgroup migration or a change to
> cpusets.mems_allowed to change a nodemask without somping the home_node.
>
> e.g.:
> mbind(2-3)    :   mask(2-3),  home_node(NUMA_NO_NODE)
> home_node(3)  :   mask(2-3),  home_node(3)
> cpusets(0-1)  :   mask(0-1),  home_node(3)
>
> Should the rebind code also shift the home-node or un-set it accordingly
> to keep the mask/home_node behavior consistent with the syscalls?
>
> (see mpol_rebind_nodemask)

Thank you for your reply.

First, home_node can only be set for VMA policy.

second, the result of cgroup migration is consistent with the result of the first e.g.

when process mem_allowed updates, vma policy will alse be updated.

Function call:

update_tasks_nodemask

      ->mpol_rebind_mm

          -> for_each_vma: mpol_rebind_policy

              -> mpol_rebind_nodemask

>
>> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
>> index 10a590ee1c89..9282be2ae18e 100644
>> --- a/mm/mempolicy.c
>> +++ b/mm/mempolicy.c
>> @@ -1536,6 +1536,12 @@ SYSCALL_DEFINE4(set_mempolicy_home_node, unsigned long, start, unsigned long, le
>>   			err = -EOPNOTSUPP;
>>   			break;
>>   		}
>> +
>> +		if (!node_isset(home_node, old->nodes)) {
>> +			err = -EINVAL;
>> +			break;
>> +		}
>> +
>>   		new = mpol_dup(old);
>>   		if (IS_ERR(new)) {
>>   			err = PTR_ERR(new);
>> -- 
>> 2.43.0
>>
>>

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