writeback: Do not sort b_io list only because of block device inode

It is very likely that block device inode will be part of BDI dirty list
as well. However it doesn't make sence to sort inodes on the b_io list
just because of this inode (as it contains buffers all over the device
anyway). So save some CPU cycles which is valuable since we hold relatively
contented wb->list_lock.
Signed-off-by: Jan Kara <jack@suse.cz>

fs/block_dev.c

@@ -606,7 +606,7 @@ static struct block_device *bd_acquire(struct inode *inode)
 	return bdev;
 }
 
-static inline int sb_is_blkdev_sb(struct super_block *sb)
+int sb_is_blkdev_sb(struct super_block *sb)
 {
 	return sb == blockdev_superblock;
 }

fs/fs-writeback.c

@@ -68,7 +68,7 @@ static inline struct backing_dev_info *inode_to_bdi(struct inode *inode)
 {
 	struct super_block *sb = inode->i_sb;
 
-	if (strcmp(sb->s_type->name, "bdev") == 0)
+	if (sb_is_blkdev_sb(sb))
 		return inode->i_mapping->backing_dev_info;
 
 	return sb->s_bdi;
@@ -250,11 +250,13 @@ static int move_expired_inodes(struct list_head *delaying_queue,
 		if (work->older_than_this &&
 		    inode_dirtied_after(inode, *work->older_than_this))
 			break;
+		list_move(&inode->i_wb_list, &tmp);
+		moved++;
+		if (sb_is_blkdev_sb(inode->i_sb))
+			continue;
 		if (sb && sb != inode->i_sb)
 			do_sb_sort = 1;
 		sb = inode->i_sb;
-		list_move(&inode->i_wb_list, &tmp);
-		moved++;
 	}
 
 	/* just one sb in list, splice to dispatch_queue and we're done */

include/linux/fs.h

@@ -2058,6 +2058,7 @@ extern struct super_block *freeze_bdev(struct block_device *);
 extern void emergency_thaw_all(void);
 extern int thaw_bdev(struct block_device *bdev, struct super_block *sb);
 extern int fsync_bdev(struct block_device *);
+extern int sb_is_blkdev_sb(struct super_block *sb);
 #else
 static inline void bd_forget(struct inode *inode) {}
 static inline int sync_blockdev(struct block_device *bdev) { return 0; }
@@ -2077,6 +2078,11 @@ static inline int thaw_bdev(struct block_device *bdev, struct super_block *sb)
 static inline void iterate_bdevs(void (*f)(struct block_device *, void *), void *arg)
 {
 }
+
+static inline int sb_is_blkdev_sb(struct super_block *sb)
+{
+	return 0;
+}
 #endif
 extern int sync_filesystem(struct super_block *);
 extern const struct file_operations def_blk_fops;