jbd2: add a missing data flush during file and fs synchronization

When the filesystem performs file or filesystem synchronization (e.g.,
ext4_sync_file()), it queries the journal to determine whether to flush
the file device through jbd2_trans_will_send_data_barrier(). If the
target transaction has not started committing, it assumes that the
journal will submit the flush command, allowing the filesystem to bypass
a redundant flush command. However, this assumption is not always valid.
If the journal is not located on the filesystem device, the journal
commit thread will not submit the flush command unless the variable
->t_need_data_flush is set to 1. Consequently, the flush may be missed,
and data may be lost following a power failure or system crash, even if
the synchronization appears to succeed.

Unfortunately, we cannot determine with certainty whether the target
transaction will flush to the filesystem device before it commits.
However, if it has not started committing, it must be the running
transaction. Therefore, fix it by always set its t_need_data_flush to 1,
ensuring that the committing thread will flush the filesystem device.

Fixes: bbd2be36 ("jbd2: Add function jbd2_trans_will_send_data_barrier()")
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20241206111327.4171337-1-yi.zhang@huaweicloud.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>