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

[Chunsheng Luo <luochunsheng@ustc.edu>]

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On 2024/1/30 10:29, Chunsheng Luo wrote:
>
>
> 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
Sorry, I make a mistake. The mpol_rebind_nodemask() only modifies the 
nodemask of the vma policy,
but doesn't modify the home_node, so the behavior is different from 
do_mbind(), so you are right.
>>> 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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