diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index a7b522f4851d3869e1ff502cde866e07494532df..bf233b283319cea03fcf76f2d81dce5b9933d536 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1085,6 +1085,12 @@ EXPORT_SYMBOL(vm_unmap_ram);
  * @node: prefer to allocate data structures on this node
  * @prot: memory protection to use. PAGE_KERNEL for regular RAM
  *
+ * If you use this function for less than VMAP_MAX_ALLOC pages, it could be
+ * faster than vmap so it's good.  But if you mix long-life and short-life
+ * objects with vm_map_ram(), it could consume lots of address space through
+ * fragmentation (especially on a 32bit machine).  You could see failures in
+ * the end.  Please use this function for short-lived objects.
+ *
  * Returns: a pointer to the address that has been mapped, or %NULL on failure
  */
 void *vm_map_ram(struct page **pages, unsigned int count, int node, pgprot_t prot)