[PATCH] shmem fixes

A shmem cleanup/bugfix patch from Hugh Dickins.

- Minor: in try_to_unuse(), only wait on writeout if we actually
  started new writeout.  Otherwise, there is no need because a
  wait_on_page_writeback() has already been executed against this page.
  And it's locked, so no new writeback can start.

- Minor: in shmem_unuse_inode(): remove all the
  wait_on_page_writeback() logic.  We already did that in
  try_to_unuse(), adn the page is locked so no new writeback can start.

- Less minor: add a missing a page_cache_release() to
  shmem_get_page_locked() in the uncommon case where the page was found
  to be under writeout.

mm/shmem.c

@@ -426,21 +426,14 @@ static int shmem_unuse_inode(struct shmem_inode_info *info, swp_entry_t entry, s
 	swap_free(entry);
 	ptr[offset] = (swp_entry_t) {0};
 
-	while (inode && (PageWriteback(page) ||
-			move_from_swap_cache(page, idx, inode->i_mapping))) {
+	while (inode && move_from_swap_cache(page, idx, inode->i_mapping)) {
 		/*
 		 * Yield for kswapd, and try again - but we're still
 		 * holding the page lock - ugh! fix this up later on.
 		 * Beware of inode being unlinked or truncated: just
 		 * leave try_to_unuse to delete_from_swap_cache if so.
-		 *
-		 * AKPM: We now wait on writeback too.  Note that it's
-		 * the page lock which prevents new writeback from starting.
 		 */
 		spin_unlock(&info->lock);
-		if (PageWriteback(page))
-			wait_on_page_writeback(page);
-		else
 		yield();
 		spin_lock(&info->lock);
 		ptr = shmem_swp_entry(info, idx, 0);
@@ -607,6 +600,7 @@ static struct page * shmem_getpage_locked(struct shmem_inode_info *info, struct
 			spin_unlock(&info->lock);
 			wait_on_page_writeback(page);
 			unlock_page(page);
+			page_cache_release(page);
 			goto repeat;
 		}
 		error = move_from_swap_cache(page, idx, mapping);

mm/swapfile.c

@@ -687,11 +687,10 @@ static int try_to_unuse(unsigned int type)
 		if ((*swap_map > 1) && PageDirty(page) && PageSwapCache(page)) {
 			swap_writepage(page);
 			lock_page(page);
-		}
-		if (PageSwapCache(page)) {
 			wait_on_page_writeback(page);
-			delete_from_swap_cache(page);
 		}
+		if (PageSwapCache(page))
+			delete_from_swap_cache(page);
 
 		/*
 		 * So we could skip searching mms once swap count went