f2fs: check s_dirty under cp_mutex

It needs to check s_dirty under cp_mutex, since s_dirty is reset under that
mutex.
And previous condition was not correct, since we can omit doing checkpoint
when checkpoint was done followed by all the node pages were written back.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/checkpoint.c

@@ -952,6 +952,10 @@ void write_checkpoint(struct f2fs_sb_info *sbi, bool is_umount)
 	trace_f2fs_write_checkpoint(sbi->sb, is_umount, "start block_ops");
 
 	mutex_lock(&sbi->cp_mutex);
+
+	if (!sbi->s_dirty)
+		goto out;
+
 	block_operations(sbi);
 
 	trace_f2fs_write_checkpoint(sbi->sb, is_umount, "finish block_ops");
@@ -976,9 +980,9 @@ void write_checkpoint(struct f2fs_sb_info *sbi, bool is_umount)
 	do_checkpoint(sbi, is_umount);
 
 	unblock_operations(sbi);
-	mutex_unlock(&sbi->cp_mutex);
-
 	stat_inc_cp_count(sbi->stat_info);
+out:
+	mutex_unlock(&sbi->cp_mutex);
 	trace_f2fs_write_checkpoint(sbi->sb, is_umount, "finish checkpoint");
 }
 

fs/f2fs/super.c

@@ -460,9 +460,6 @@ int f2fs_sync_fs(struct super_block *sb, int sync)
 
 	trace_f2fs_sync_fs(sb, sync);
 
-	if (!sbi->s_dirty && !get_pages(sbi, F2FS_DIRTY_NODES))
-		return 0;
-
 	if (sync) {
 		mutex_lock(&sbi->gc_mutex);
 		write_checkpoint(sbi, false);