[PATCH 3/3] mm/hugetlb: use get_page_unless_zero() in hugetlb_fault()

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

After fixing the race in follow_page(FOLL_GET) for hugepages, I start to
observe the BUG of "get_page() on refcount 0 page" in hugetlb_fault() in
the same test.

I'm not exactly sure about how this race is triggered, but hugetlb_fault()
calls pte_page() and get_page() outside page table lock, so it's not safe.
This patch checks the refcount of the gotten page, and aborts the page fault
if the refcount is 0, expecting to retry.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: <stable@vger.kernel.org>  # [3.12+]
---
 mm/hugetlb.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git mmotm-2014-07-22-15-58.orig/mm/hugetlb.c mmotm-2014-07-22-15-58/mm/hugetlb.c
index 6793914b6aac..86e7341aad77 100644
--- mmotm-2014-07-22-15-58.orig/mm/hugetlb.c
+++ mmotm-2014-07-22-15-58/mm/hugetlb.c
@@ -3189,7 +3189,8 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	 * so no worry about deadlock.
 	 */
 	page = pte_page(entry);
-	get_page(page);
+	if (!get_page_unless_zero(page))
+		goto out_put_pagecache;
 	if (page != pagecache_page)
 		lock_page(page);
 
@@ -3215,15 +3216,14 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 
 out_ptl:
 	spin_unlock(ptl);
-
+	if (page != pagecache_page)
+		unlock_page(page);
+	put_page(page);
+out_put_pagecache:
 	if (pagecache_page) {
 		unlock_page(pagecache_page);
 		put_page(pagecache_page);
 	}
-	if (page != pagecache_page)
-		unlock_page(page);
-	put_page(page);
-
 out_mutex:
 	mutex_unlock(&htlb_fault_mutex_table[hash]);
 	return ret;
-- 
1.9.3