Re: [RFC PATCH v8 4/7] mm/mempolicy: Export memory policy symbols

[Gregory Price <gourry@gourry.net>]

On Wed, Jun 18, 2025 at 11:29:32AM +0000, Shivank Garg wrote:
> KVM guest_memfd wants to implement support for NUMA policies just like
> shmem already does using the shared policy infrastructure. As
> guest_memfd currently resides in KVM module code, we have to export the
> relevant symbols.
> 
> In the future, guest_memfd might be moved to core-mm, at which point the
> symbols no longer would have to be exported. When/if that happens is
> still unclear.
> 
> Acked-by: David Hildenbrand <david@redhat.com>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> Signed-off-by: Shivank Garg <shivankg@amd.com>
> ---
>  mm/mempolicy.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 3b1dfd08338b..d98243cdf090 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -354,6 +354,7 @@ struct mempolicy *get_task_policy(struct task_struct *p)
>  
>  	return &default_policy;
>  }
> +EXPORT_SYMBOL_GPL(get_task_policy);
>  
>  static const struct mempolicy_operations {
>  	int (*create)(struct mempolicy *pol, const nodemask_t *nodes);
> @@ -487,6 +488,7 @@ void __mpol_put(struct mempolicy *pol)
>  		return;
>  	kmem_cache_free(policy_cache, pol);
>  }
> +EXPORT_SYMBOL_GPL(__mpol_put);
>  

I'm concerned that get_task_policy doesn't actually increment the policy
refcount - and mpol_cond_put only decrements the refcount for shared
policies (vma policies) - while __mpol_put decrements it unconditionally.

If you look at how get_task_policy is used internally to mempolicy,
you'll find that it either completes the operation in the context of the
task lock (allocation time) or it calls mpol_get afterwards.

Exporting this as-is creates a triping hazard, if only because get/put
naming implies reference counting.

~Gregory