shmem: cleanup shmem_add_to_page_cache

shmem_add_to_page_cache() has three callsites, but only one of them wants
the radix_tree_preload() (an exceptional entry guarantees that the radix
tree node is present in the other cases), and only that site can achieve
mem_cgroup_uncharge_cache_page() (PageSwapCache makes it a no-op in the
other cases).  We did it this way originally to reflect
add_to_page_cache_locked(); but it's confusing now, so move the radix_tree
preloading and mem_cgroup uncharging to that one caller.
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/shmem.c

@@ -288,25 +288,21 @@ static int shmem_add_to_page_cache(struct page *page,
 				   struct address_space *mapping,
 				   pgoff_t index, gfp_t gfp, void *expected)
 {
-	int error = 0;
+	int error;
 
 	VM_BUG_ON(!PageLocked(page));
 	VM_BUG_ON(!PageSwapBacked(page));
 
-	if (!expected)
-		error = radix_tree_preload(gfp & GFP_RECLAIM_MASK);
-	if (!error) {
 	page_cache_get(page);
 	page->mapping = mapping;
 	page->index = index;
 
 	spin_lock_irq(&mapping->tree_lock);
 	if (!expected)
-			error = radix_tree_insert(&mapping->page_tree,
-							index, page);
+		error = radix_tree_insert(&mapping->page_tree, index, page);
 	else
-			error = shmem_radix_tree_replace(mapping, index,
-							expected, page);
+		error = shmem_radix_tree_replace(mapping, index, expected,
+								 page);
 	if (!error) {
 		mapping->nrpages++;
 		__inc_zone_page_state(page, NR_FILE_PAGES);
@@ -317,11 +313,6 @@ static int shmem_add_to_page_cache(struct page *page,
 		spin_unlock_irq(&mapping->tree_lock);
 		page_cache_release(page);
 	}
-		if (!expected)
-			radix_tree_preload_end();
-	}
-	if (error)
-		mem_cgroup_uncharge_cache_page(page);
 	return error;
 }
 
@@ -1202,11 +1193,18 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
 		__set_page_locked(page);
 		error = mem_cgroup_cache_charge(page, current->mm,
 						gfp & GFP_RECLAIM_MASK);
-		if (!error)
+		if (error)
+			goto decused;
+		error = radix_tree_preload(gfp & GFP_RECLAIM_MASK);
+		if (!error) {
 			error = shmem_add_to_page_cache(page, mapping, index,
 							gfp, NULL);
-		if (error)
+			radix_tree_preload_end();
+		}
+		if (error) {
+			mem_cgroup_uncharge_cache_page(page);
 			goto decused;
+		}
 		lru_cache_add_anon(page);
 
 		spin_lock(&info->lock);