f2fs: reduce competition among node page writes

We do not need to block on ->node_write among different node page writers e.g.
fsync/flush, unless we have a node page writer from write_checkpoint.
So it's better use rw_semaphore instead of mutex type for ->node_write to
promote performance.
Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/checkpoint.c

@@ -762,10 +762,10 @@ static void block_operations(struct f2fs_sb_info *sbi)
 	 * until finishing nat/sit flush.
 	 */
 retry_flush_nodes:
-	mutex_lock(&sbi->node_write);
+	down_write(&sbi->node_write);
 
 	if (get_pages(sbi, F2FS_DIRTY_NODES)) {
-		mutex_unlock(&sbi->node_write);
+		up_write(&sbi->node_write);
 		sync_node_pages(sbi, 0, &wbc);
 		goto retry_flush_nodes;
 	}
@@ -774,7 +774,7 @@ static void block_operations(struct f2fs_sb_info *sbi)
 
 static void unblock_operations(struct f2fs_sb_info *sbi)
 {
-	mutex_unlock(&sbi->node_write);
+	up_write(&sbi->node_write);
 	f2fs_unlock_all(sbi);
 }
 

fs/f2fs/f2fs.h

@@ -452,7 +452,7 @@ struct f2fs_sb_info {
 	struct inode *meta_inode;		/* cache meta blocks */
 	struct mutex cp_mutex;			/* checkpoint procedure lock */
 	struct rw_semaphore cp_rwsem;		/* blocking FS operations */
-	struct mutex node_write;		/* locking node writes */
+	struct rw_semaphore node_write;		/* locking node writes */
 	struct mutex writepages;		/* mutex for writepages() */
 	bool por_doing;				/* recovery is doing or not */
 	wait_queue_head_t cp_wait;

fs/f2fs/node.c

@@ -1231,12 +1231,12 @@ static int f2fs_write_node_page(struct page *page,
 	if (wbc->for_reclaim)
 		goto redirty_out;
 
-	mutex_lock(&sbi->node_write);
+	down_read(&sbi->node_write);
 	set_page_writeback(page);
 	write_node_page(sbi, page, &fio, nid, ni.blk_addr, &new_addr);
 	set_node_addr(sbi, &ni, new_addr, is_fsync_dnode(page));
 	dec_page_count(sbi, F2FS_DIRTY_NODES);
-	mutex_unlock(&sbi->node_write);
+	up_read(&sbi->node_write);
 	unlock_page(page);
 	return 0;
 

fs/f2fs/super.c

@@ -953,7 +953,7 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
 	mutex_init(&sbi->gc_mutex);
 	mutex_init(&sbi->writepages);
 	mutex_init(&sbi->cp_mutex);
-	mutex_init(&sbi->node_write);
+	init_rwsem(&sbi->node_write);
 	sbi->por_doing = false;
 	spin_lock_init(&sbi->stat_lock);