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>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 9519f3f..c032738 100644 (file)
--- 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);