hugetlb: prevent BUG_ON in hugetlb_fault() -> hugetlb_cow()

BugLink: http://bugs.launchpad.net/bugs/1002880

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@tilera.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: Hillf Danton <dhillf@gmail.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
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>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

1 files changed, 0 insertions, 1 deletions

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 0a606a8e77e..4bdaae194c3 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2477,7 +2477,6 @@ 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);
                                 goto retry_avoidcopy;