[ 07/53] hugetlb: prevent BUG_ON in hugetlb_fault() -> hugetlb_cow()

[Ben Hutchings]

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

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

From: Chris Metcalf <cmetcalf@xxxxxxxxxx>

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 <cmetcalf@xxxxxxxxxx>
Acked-by: Mel Gorman <mel@xxxxxxxxx>
Acked-by: Hillf Danton <dhillf@xxxxxxxxx>
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxx>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 mm/hugetlb.c |    1 -
 1 file changed, 1 deletion(-)

--- linux.orig/mm/hugetlb.c
+++ linux/mm/hugetlb.c
@@ -2405,7 +2405,6 @@
 		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);
 				goto retry_avoidcopy;



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