[ 21/47] hugetlb: prevent BUG_ON in hugetlb_fault() -> hugetlb_cow()

3.3-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Chris Metcalf <redacted>

commit 4998a6c0edce7fae9c0a5463f6ec3fa585258ee7 upstream.

Commit 66aebce747eaf ("hugetlb: fix race condition in hugetlb_fault()")
added code to avoid a race condition by elevating the page refcount in
hugetlb_fault() while calling hugetlb_cow().

However, one code path in hugetlb_cow() includes an assertion that the
page count is 1, whereas it may now also have the value 2 in this path.

The consensus is that this BUG_ON has served its purpose, so rather than
extending it to cover both cases, we just remove it.

Signed-off-by: Chris Metcalf <redacted>
Acked-by: Mel Gorman <redacted>
Acked-by: Hillf Danton <redacted>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Michal Hocko <redacted>
Cc: KAMEZAWA Hiroyuki <redacted>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 mm/hugetlb.c |    1 -
 1 file changed, 1 deletion(-)

--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c

@@ -2486,7 +2486,6 @@ retry_avoidcopy:
 		if (outside_reserve) {
 			BUG_ON(huge_pte_none(pte));
 			if (unmap_ref_private(mm, vma, old_page, address)) {
-				BUG_ON(page_count(old_page) != 1);
 				BUG_ON(huge_pte_none(pte));
 				spin_lock(&mm->page_table_lock);
 				ptep = huge_pte_offset(mm, address & huge_page_mask(h));