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(+)

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

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)


> 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
> 
>

On Fri, 26 Jan 2024 08:22:40 -0500 Chunsheng Luo <luochunsheng@ustc.edu> 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.

Thanks.  Under what circumstances does userspace trigger this issue?

For example,  In a system with NUMA nodes 0,1,2,3, i mbind process to 
node 0-2 and set home_node to node 3, it will not be allocated from node 
3, then from node closer to node 3. But i think home_node should be set 
directly from node 0-2, which makes more sense. So i think it needs to 
return failure to prompt user.


On 2024/1/28 16:29, Andrew Morton wrote:
> On Fri, 26 Jan 2024 08:22:40 -0500 Chunsheng Luo <luochunsheng@ustc.edu> 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.
> Thanks.  Under what circumstances does userspace trigger this issue?
>