On Sat, Oct 29, 2016 at 04:08:25PM +0800, Ming Lei wrote:

There are lots of direct access to .bi_vcnt & .bi_io_vec
of bio, and it isn't ready to support multipage bvecs
for BTRFS, so set NO_MP for these request queues.

For one bio is an I/O submitter, it has absolutely no business changing
queue flags - if we need to stick to this limitation it simply needs
a version of bio_add_page that doesn't create multi-page bvecs.

Second I don't think making it multipage bvec aware is all that hard,
and we should aim for doing the proper thing.