On Tue, Aug 05, 2025 at 04:52:11PM -0600, Keith Busch wrote:

On Tue, Aug 05, 2025 at 12:56:07PM -0700, Keith Busch wrote:

quoted

+	bv = next->bio->bi_io_vec[0];
+	bvprv = req->biotail->bi_io_vec[req->biotail->bi_vcnt - 1];

Hm, commit 7bcd79ac50d9d8 suggests I missed something obvious about not
being allowed to use 'bi_vcnt - 1' for the tail bvec of the previous
bio. I guess it's some stacking thing.

It is bio cloning and splitting.  bi_io_vec is the original payload added
to a bio, and directly accessing it is only allowed for the submitter,
i.e. usually the file system.  The I/O stack always need to go through
the bio_iter to deal with split/cloned/truncated/partially completed and
resubmitted bios.