Thread (32 messages) 32 messages, 6 authors, 2025-06-23

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

From: Gregory Price <gourry@gourry.net>
Date: 2025-06-18 15:12:12
Also in: kvm, linux-coco, linux-fsdevel, linux-kselftest, linux-mm, lkml

On Wed, Jun 18, 2025 at 11:29:32AM +0000, Shivank Garg wrote:
quoted hunk ↗ jump to hunk
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 <redacted>
Acked-by: Vlastimil Babka <redacted>
Signed-off-by: Shivank Garg <redacted>
---
 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
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