[PATCH] hugetlb: add comment explaining reasons for Bus Errors

I just spent some time researching a Bus Error.  Turns out that the huge
page fault handler can return VM_FAULT_SIGBUS for various conditions where
no huge page is available.

Add a note explaining the reasoning in the source.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Acked-by: William Lee Irwin III <wli@holomorphy.com>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/hugetlb.c

@@ -444,6 +444,15 @@ retry:
 		page = alloc_huge_page(vma, address);
 		if (!page) {
 			hugetlb_put_quota(mapping);
+			/*
+		 	 * No huge pages available. So this is an OOM
+			 * condition but we do not want to trigger the OOM
+			 * killer, so we return VM_FAULT_SIGBUS.
+			 *
+			 * A program using hugepages may fault with Bus Error
+			 * because no huge pages are available in the cpuset, per
+			 * memory policy or because all are in use!
+			 */
 			goto out;
 		}