On Wed, 3 Mar 2010, Jens Axboe wrote: > On Wed, Mar 03 2010, Dmitry Monakhov wrote: > > Mike Snitzer <snitzer@xxxxxxxxxx> writes: > > > > > Linux has all sorts of internal interfaces that are "odd"... the current > > > 'q->merge_bvec_fn' interface included. But odd is not a problem (nor is > > > it "broken") unless you make changes that don't consider how the current > > > interface is defined. > > Ok. then cant you please explain more historical questions > > 1) Why bio_add_page() can not add less data than requested? > > Seems that it doesn't make caller's code much complicate > > Off course barrier bio is special case. I don't consider it here. > > Because the caller may not expect that, a partial add may not make any > sense to the caller. The bio code obviously doesn't care. And it > certainly could complicate the caller a lot, if they need to now issue > and wait for several bio's instead of just a single one. Now a single > completion queue and wait_for_completion() is not enough. The whole thing is lame by design. As a consequence, the code for splitting these page-sized bios is being duplicated in md, dm and request-based device (and maybe in other drivers). And there is a long standing unfixable bug --- adding a device to raid1 may fail if the device has smaller request size than the other raid1 leg. Think about this: * thread 1: bio_add_page() -> returns ok ... bio_add_page() -> returns ok * scheduling to thread 2 * * thread 2: add a raid1 leg to the block device * scheduling to thread 1 * * thread 1: submit_bio() --- the bio already oversized for the new raid1 leg and there is no way to shrink it. I think it should be redesigned in this way: * the bio should be allowed to have arbitrary size, up to BIO_MAX_PAGES * the code for splitting bios should be just at one place --- in the generic block layer and it should work with all drivers. I.e. q->make_request_fn will return that the request is too big for the given driver and the generic code will allocate few another bios from per-request_queue mempool and split the bio. * then, we would fix the raid1 bug and we would also simplify md and dm --- remove the splitting code. Mikulas -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
