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

[Shivank Garg <shivankg@amd.com>]

On 6/18/2025 8:42 PM, Gregory Price wrote:
> 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.

I agree. But the semantics of my usage isn't new. shmem use this in same way.

I think the alloc_frozen_pages_noprof(), alloc_pages_bulk_mempolicy_noprof()
calls get_task_policy without task_lock or calling mpol_get.

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

Since KVM is the only user, we could consider newly added EXPORT_SYMBOL_GPL_FOR_MODULES(..., "kvm")
to avoid wider exposure.
Does this solve your concern?
Or should we rename these functions.
What should be the preferred approach?

Thanks,
Shivank