mm: clarify __add_to_swap_cache locking

__add_to_swap_cache unconditionally sets the page locked, which can be a bit
alarming to the unsuspecting reader: in the code paths where the page is
visible to other CPUs, the page should be (and is) already locked.

Instead, just add a check to ensure the page is locked here, and teach the one
path relying on the old behaviour to call SetPageLocked itself.

[hugh@veritas.com: locking fix]
Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swap_state.c

@@ -74,6 +74,7 @@ static int __add_to_swap_cache(struct page *page, swp_entry_t entry,
 {
 	int error;
 
+	BUG_ON(!PageLocked(page));
 	BUG_ON(PageSwapCache(page));
 	BUG_ON(PagePrivate(page));
 	error = radix_tree_preload(gfp_mask);
@@ -83,7 +84,6 @@ static int __add_to_swap_cache(struct page *page, swp_entry_t entry,
 						entry.val, page);
 		if (!error) {
 			page_cache_get(page);
-			SetPageLocked(page);
 			SetPageSwapCache(page);
 			set_page_private(page, entry.val);
 			total_swapcache_pages++;
@@ -99,15 +99,18 @@ static int add_to_swap_cache(struct page *page, swp_entry_t entry)
 {
 	int error;
 
+	BUG_ON(PageLocked(page));
 	if (!swap_duplicate(entry)) {
 		INC_CACHE_INFO(noent_race);
 		return -ENOENT;
 	}
+	SetPageLocked(page);
 	error = __add_to_swap_cache(page, entry, GFP_KERNEL);
 	/*
 	 * Anon pages are already on the LRU, we don't run lru_cache_add here.
 	 */
 	if (error) {
+		ClearPageLocked(page);
 		swap_free(entry);
 		if (error == -EEXIST)
 			INC_CACHE_INFO(exist_race);