On Tue, May 02, 2017 at 01:42:26PM +1000, NeilBrown wrote:

blk_bio_segment_split() makes sure bios have no more than
BIO_MAX_PAGES entries in the bi_io_vec.
This was done because bio_clone_bioset() (when given a
mempool bioset) could not handle larger io_vecs.

No driver uses bio_clone_bioset() any more, they all
use bio_clone_fast() if anything, and bio_clone_fast()
doesn't clone the bi_io_vec.

Maybe in future, some drivers still may try to use 
bio_clone_bioset() again, I suggest to add some comments
on bio_clone_bioset() to make this usage explicitly. Also
better to trigger a warning if a big src bio is passed to
bio_clone_bioset().

Thanks,
Ming