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

[Jan Kara <jack@suse.cz>]

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     | 2 +-
 fs/fs-writeback.c  | 8 +++++---
 include/linux/fs.h | 6 ++++++
 3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 2091db8..2562330 100644
--- a/fs/block_dev.c
+++ b/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;
 }
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 3be5718..cabdece 100644
--- a/fs/fs-writeback.c
+++ b/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 */
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 43db02e..8f897d0 100644
--- a/include/linux/fs.h
+++ b/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;
-- 
1.8.1.4