Re: [PATCH] md: stop using do_sync_mapping_range

[Jamie Lokier <jamie@shareable.org>]

Christoph Hellwig wrote:
> -		do_sync_mapping_range(file->f_mapping, 0, LLONG_MAX,
> -				      SYNC_FILE_RANGE_WAIT_BEFORE |
> -				      SYNC_FILE_RANGE_WRITE |
> -				      SYNC_FILE_RANGE_WAIT_AFTER);
> +		filemap_write_and_wait(file->f_mapping);
>
> It's a very awkward way to write out all data and wait for it, so just
> call filemap_write_and_wait.  I still can't figure what the point of
> all this is, so a comment would surely be helpful.

The SYNC_FILE_RANGE_WAIT_BEFORE is to make sure that writes which were
started before the most recent dirtying are completed before
initiating a second write on the same pages to make sure the most
recently dirty data is written.

Does filemap_write_and_wait() do that?

Fwiw, It's partially redundant: it isn't necessary to have a second
write when the first write request was queued in the elevator but
hadn't yet reached the device.

-- Jamie