linux-kernel - Re: [PATCH] mempolicy: check home

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Date: Mon, 29 Jan 2024 10:12:16 -0500
From: Gregory Price <gregory.price@...verge.com>
To: Chunsheng Luo <luochunsheng@...c.edu>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mempolicy: check home_node is in the nodes of policy

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@...c.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
> 
>