[PATCH] hugetlb: detect race if fail to COW

[Hillf Danton <dhillf@gmail.com>]

In the error path that we fail to allocate new huge page, before try again, we
have to check race since page_table_lock is re-acquired.

If racing, our job is done.

Signed-off-by: Hillf Danton <dhillf@gmail.com>
---

--- a/mm/hugetlb.c	Fri Nov 18 21:38:30 2011
+++ b/mm/hugetlb.c	Fri Nov 18 21:48:15 2011
@@ -2407,7 +2407,14 @@ retry_avoidcopy:
 				BUG_ON(page_count(old_page) != 1);
 				BUG_ON(huge_pte_none(pte));
 				spin_lock(&mm->page_table_lock);
-				goto retry_avoidcopy;
+				ptep = huge_pte_offset(mm, address & huge_page_mask(h));
+				if (likely(pte_same(huge_ptep_get(ptep), pte)))
+					goto retry_avoidcopy;
+				/*
+				 * race occurs while re-acquiring page_table_lock, and
+				 * our job is done.
+				 */
+				return 0;
 			}
 			WARN_ON_ONCE(1);
 		}

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