On Mon, Apr 07, 2025 at 09:59:16AM -0400, Sean Anderson wrote: > > drivers (case 1a), or generated by bio_split_to_limits (case 1b), which > > is called for every blk-mq driver before calling into ->queue_rq(s) or > > explicitly called by a few bio based driver. > > > > The other is the bio-vec synthesized by bio_for_each_segment (case 2). > > I'm referring to the bio_vecs you get from queue_mq. Which I think is the > latter. The bios hanging off the request have bio_vecs that are case 1b. If you use bio_for_each_segment or an open coded variant of that on the bio you get on-stack bio_vecs for case 2. > > > - Is it possible to have a bio where the total length is a multiple of > > > logical_sector_size, but the data is split across several segments > > > where each segment is a multiple of SECTOR_SIZE? > > > > Yes. > > ...if this is the case, then for some of those segments wouldn't bv_len > not be a multiple of logical_sector_size? Maybe I'm misunderstanding the question. But if you have e.g. a transfer that is 2MiB, it could be perfectly fine that each bio_vec is say 1kiB. > > > - Can I somehow request to only get segments with bv_len aligned to > > > logical_sector_size? > > > > For drivers that use bio_split_to_limits implicitly or explicitly you can > > do that by setting the right seg_boundary_mask. > > Is that the right knob? It operates on the physical address, so it looked > more like something for broken DMA engines. For example (if I recall correctly) > MMC SDMA can't cross a page boundary, so you could use seg_boundary_mask to > enforce that. seg_boundary_mask is indeed primarily about physical addresses. That being said if your seg_boundary_mask is logical_sector_size all bio_vecs in cases 1a/b will be aligned to it for their base address, and thus automatically their length too. > > in the short run the best fix would be to synthesized a > > bio_for_each_segment like bio_vec that stays inside a single page > > using bio_iter_iovec) at the top of do_blktrans_request and use > > that for all references to the data. > > > > OK, but if you have to stay inside a single page couldn't you end up > with a sector spanning a page boundary due to only being aligned to > dma_alignment? Or maybe we set seg_boundary_mask to PAGE_MASK to enforce that? bio_iter_iovec ensures that the synthetic request never crosses a page boundary. But as Keith pointed out that means if your dma_alignment or seg_boundary_mask are smaller than the logical block size you might actually get a non-aligned length.
