On Tue, Mar 29, 2016 at 09:42:33AM -0700, Shaohua Li wrote:

bio_alloc_bioset() allocates bvecs from bvec_slabs which can only
allocate maximum 256 bvec (eg, 1M for 4k pages). We can't bump
BLK_DEF_MAX_SECTORS to exceed this value otherwise bio_alloc_bioset will
fail.

This might be true, but it's not a good enough reason.  Request based
driver couldn't care less about the limits of bio_alloc_bioset.

It seems the bug is that somone (would be great to know whoe exactly)
passes a too large value to bio_alloc_bioset. And given that we still
have bio_add_page around for actually adding pages to a bio it seems
like the proper fix would be to simply clamp down the actual allocation
and segment limit inside bio_alloc_bioset.  Which would also help to
eventually remove code doing just that in tons of callers.