[HUGETLB] Add comment explaining reasons for Bus Errors

[Christoph Lameter <clameter@engr.sgi.com>]

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>

Index: linux-2.6.16-rc2/mm/hugetlb.c
===================================================================
--- linux-2.6.16-rc2.orig/mm/hugetlb.c	2006-02-02 22:03:08.000000000 -0800
+++ linux-2.6.16-rc2/mm/hugetlb.c	2006-02-03 16:03:20.000000000 -0800
@@ -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;
 		}
 
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