mm/mm_slot.h: add comments for mm_slot_lookup/insert

mm_slot_lookup() and mm_slot_insert() are the only helpers in this header
that are implemented as macros rather than static inline functions.  This
may look inconsistent without explanation.

Explain they must be macros because hash_for_each_possible() needs the
table as an array (for sizeof), not a pointer.

Link: https://lore.kernel.org/[email protected]
Signed-off-by: xu xin <[email protected]>
Reviewed-by: Barry Song <[email protected]>
Acked-by: David Hildenbrand (Arm) <[email protected]>
Reviewed-by: Qi Zheng <[email protected]>
Reviewed-by: Lorenzo Stoakes (ARM) <[email protected]>
Reviewed-by: SJ Park <[email protected]>
Reviewed-by: Zi Yan <[email protected]>
Cc: Baolin Wang <[email protected]>
Cc: Chengming Zhou <[email protected]>
Cc: Dev Jain <[email protected]>
Cc: Lance Yang <[email protected]>
Cc: Nico Pache <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Wang Yaxin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/mm_slot.h

@@ -33,6 +33,12 @@ static inline void mm_slot_free(struct kmem_cache *cache, void *objp)
 	kmem_cache_free(cache, objp);
 }
 
+/*
+ * Note: mm_slot_lookup and mm_slot_insert cannot be converted to static inline
+ * functions because the hash helpers (hash_for_each_possible and hash_add) rely
+ * on the actual array argument 'hashtable' for sizeof() instead of pointers.
+ */
+
 #define mm_slot_lookup(_hashtable, _mm) 				       \
 ({									       \
 	struct mm_slot *tmp_slot, *mm_slot = NULL;			       \