On Mon, Apr 08, 2019 at 11:52:59AM +0200, Johannes Thumshirn wrote: > On 06/04/2019 02:16, Ming Lei wrote: > > Hi Nikolay, > > > > On Fri, Apr 05, 2019 at 07:04:18PM +0300, Nikolay Borisov wrote: > >> Hello Ming, > >> > >> Following the mp biovec rework what is the maximum > >> data that a bio could contain? Should it be PAGE_SIZE * bio_vec > > > > There isn't any maximum data limit on the bio submitted from fs, > > and block layer will make the final bio sent to driver correct > > by applying all kinds of queue limit, such as max segment size, > > max segment number, max sectors, ... > > Naive question, why are we creating possibly huge bios just to split > them according the the LLDD's limits afterwards? bio split is one important IO model in block layer, which simplifies stacked driver(dm, md, bcache, ...) a lot. It is very reasonable to apply the queue limits in its. make_request_fn(). Otherwise, it will cause huge mess in stacking driver if queue limits are applied in bio_add_page(), see previous .merge_bvec_fn's implementation in these stacking drivers. Not only bio_add_page(), there is also bio clone involved. > > Can't we look at the limits in e.g. bio_add_page() and decide if we need > to split there? bio_add_page() is absolutely the fast path, and it is much more efficient to apply the limit just once in the queue's .make_request_fn. Thanks, Ming
