On Wed, Dec 20, 2023 at 05:27:14PM -0800, Song Liu wrote:

Upper layer (fs, etc.) may issue flush with something like:

  bio_reset(bio, bdev, REQ_OP_FLUSH);
  bio->bi_end_io = xxx;
  submit_bio(bio);

No, they can't.  REQ_OP_FLUSH is currently only used by request
based drivers and only generated by the flush state machine.

(Not that I particularly like this wart, I'd much prefer file systems
to submit a REQ_OP_FLUSH than an empty write with a preflush flag,
but that's not a trivial change).