hugetlb, rmap: fix confusing page locking in hugetlb_cow()

The "if (!trylock_page)" block in the avoidcopy path of hugetlb_cow()
looks confusing and is buggy.  Originally this trylock_page() was
intended to make sure that old_page is locked even when old_page !=
pagecache_page, because then only pagecache_page is locked.

This patch fixes it by moving page locking into hugetlb_fault().

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 10 deletions

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 9519f3f6c1da..c03273807182 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2324,11 +2324,8 @@ retry_avoidcopy:
          * and just make the page writable */
         avoidcopy = (page_mapcount(old_page) == 1);
         if (avoidcopy) {
-                if (!trylock_page(old_page)) {
-                        if (PageAnon(old_page))
-                                page_move_anon_rmap(old_page, vma, address);
-                } else
-                        unlock_page(old_page);
+                if (PageAnon(old_page))
+                        page_move_anon_rmap(old_page, vma, address);
                 set_huge_ptep_writable(vma, address, ptep);
                 return 0;
         }
@@ -2631,10 +2628,16 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
                                                                 vma, address);
         }
 
-        if (!pagecache_page) {
-                page = pte_page(entry);
+        /*
+         * hugetlb_cow() requires page locks of pte_page(entry) and
+         * pagecache_page, so here we need take the former one
+         * when page != pagecache_page or !pagecache_page.
+         * Note that locking order is always pagecache_page -> page,
+         * so no worry about deadlock.
+         */
+        page = pte_page(entry);
+        if (page != pagecache_page)
                 lock_page(page);
-        }
 
         spin_lock(&mm->page_table_lock);
         /* Check for a racing update before calling hugetlb_cow */
@@ -2661,9 +2664,8 @@ out_page_table_lock:
         if (pagecache_page) {
                 unlock_page(pagecache_page);
                 put_page(pagecache_page);
-        } else {
-                unlock_page(page);
         }
+        unlock_page(page);
 
 out_mutex:
         mutex_unlock(&hugetlb_instantiation_mutex);