[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>

1 files changed, 9 insertions, 0 deletions

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index b21d78c941b5..ceb3ebb3c399 100644
--- a/mm/hugetlb.c
+++ b/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;
                 }