Index: sys/kern/kern_malloc.c
===================================================================
--- sys/kern/kern_malloc.c
+++ sys/kern/kern_malloc.c
@@ -1030,6 +1030,23 @@
 	return (mem);
 }
 
+/*
+ * 	malloc_size2zone: returns the usable size if an allocation is performed.
+ */
+size_t
+malloc_size2zone(size_t size)
+{
+	int indx;
+
+	if (size > kmem_zmax) {
+		return (-1);
+	}
+	if (size & KMEM_ZMASK)
+		size = (size & ~KMEM_ZMASK) + KMEM_ZBASE;
+	indx = kmemsize[size >> KMEM_ZSHIFT];
+	return (kmemzones[indx].kz_size);
+}
+
 /*
  *	malloc_usable_size: returns the usable size of the allocation.
  */
Index: sys/sys/malloc.h
===================================================================
--- sys/sys/malloc.h
+++ sys/sys/malloc.h
@@ -251,6 +251,7 @@
 void	malloc_type_list(malloc_type_list_func_t *, void *);
 void	malloc_uninit(void *);
 size_t	malloc_usable_size(const void *);
+size_t	malloc_size2zone(size_t);
 void	*realloc(void *addr, size_t size, struct malloc_type *type, int flags)
 	    __result_use_check __alloc_size(2);
 void	*reallocf(void *addr, size_t size, struct malloc_type *type, int flags)