On Tue, Jul 31, 2007 at 12:17:59PM +1000, NeilBrown wrote: > > For a request to be able to refer to part of a bio, we need to be able > to impose a size limit at the request level. So allow hard_nr_sectors > to be less than the size of the bios (and bio_vecs) and interpret it > such that anything in the last bio beyond that limit is ignored. > > As some bios can be less than one sector - as happens when a SCSI > sense command is being submitted - we need to set hard_nr_sectors to > bi_size rounded up in blk_rq_bio_prep, and we need to abort the > rq_for_each_segment loop if _iter.bio becomes NULL even if _iter.size > is still non-zero
This is pretty confusing. In all other places, bi_size -> #sector conversion is done by rounding down but only in blk_rq_bio_prep() it's being rounded up. Is my following reasoning correct? It was okay till now because unaligned requests don't get merged and also haven't done partial completions (end_that_request_first with partial count)? So till now, hard_nr_sectors and nr_sectors didn't really matter for unaligned requests but now it matters because it's considered while iterating over bvecs in rq. If so, I think the correct thing to do would be changing bio_sectors() to round up first or let block layer measure transfer in bytes not in sectors. I don't think everyone would agree with the latter tho. I (tentatively) think it would be better to represent length in bytes tho. A lot of requests which aren't aligned to 512 bytes pass through the block layer and the mismatch can result in subtle bugs. Thanks. -- tejun - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
