f2fs: fix race between write_checkpoint and write_begin

The following race could lead to inconsistent SIT bitmap:

Task A                          Task B
======                          ======
f2fs_write_checkpoint
  block_operations
    f2fs_lock_all
      down_write(node_change)
      down_write(node_write)
      ... sync ...
      up_write(node_change)
                                f2fs_file_write_iter
                                  set_inode_flag(FI_NO_PREALLOC)
                                  ......
                                  f2fs_write_begin(index=0, has inline data)
                                    prepare_write_begin
                                      __do_map_lock(AIO) => down_read(node_change)
                                      f2fs_convert_inline_page => update SIT
                                      __do_map_lock(AIO) => up_read(node_change)
  f2fs_flush_sit_entries <= inconsistent SIT
  finish write checkpoint
  sudden-power-off

If SPO occurs after checkpoint is finished, SIT bitmap will be set
incorrectly.
Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -2322,6 +2322,7 @@ static int prepare_write_begin(struct f2fs_sb_info *sbi,
 	bool locked = false;
 	struct extent_info ei = {0,0,0};
 	int err = 0;
+	int flag;
 
 	/*
 	 * we already allocated all the blocks, so we don't need to get
@@ -2331,9 +2332,15 @@ static int prepare_write_begin(struct f2fs_sb_info *sbi,
 			!is_inode_flag_set(inode, FI_NO_PREALLOC))
 		return 0;
 
+	/* f2fs_lock_op avoids race between write CP and convert_inline_page */
+	if (f2fs_has_inline_data(inode) && pos + len > MAX_INLINE_DATA(inode))
+		flag = F2FS_GET_BLOCK_DEFAULT;
+	else
+		flag = F2FS_GET_BLOCK_PRE_AIO;
+
 	if (f2fs_has_inline_data(inode) ||
 			(pos & PAGE_MASK) >= i_size_read(inode)) {
-		__do_map_lock(sbi, F2FS_GET_BLOCK_PRE_AIO, true);
+		__do_map_lock(sbi, flag, true);
 		locked = true;
 	}
 restart:
@@ -2371,6 +2378,7 @@ static int prepare_write_begin(struct f2fs_sb_info *sbi,
 				f2fs_put_dnode(&dn);
 				__do_map_lock(sbi, F2FS_GET_BLOCK_PRE_AIO,
 								true);
+				WARN_ON(flag != F2FS_GET_BLOCK_PRE_AIO);
 				locked = true;
 				goto restart;
 			}
@@ -2384,7 +2392,7 @@ static int prepare_write_begin(struct f2fs_sb_info *sbi,
 	f2fs_put_dnode(&dn);
 unlock_out:
 	if (locked)
-		__do_map_lock(sbi, F2FS_GET_BLOCK_PRE_AIO, false);
+		__do_map_lock(sbi, flag, false);
 	return err;
 }