bio segment constraints

[Sean Anderson]

Hi all,

I'm not really sure what guarantees the block layer makes regarding the
segments in a bio as part of a request submitted to a block driver. As
far as I can tell this is not documented anywhere. In particular,

- Is bv_len aligned to SECTOR_SIZE?
- To logical_sector_size?
- What if logical_sector_size > PAGE_SIZE?
- What about bv_offset?
- 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?
- Is is possible to have segments not even aligned to SECTOR_SIZE?
- Can I somehow request to only get segments with bv_len aligned to
   logical_sector_size? Or do I need to do my own coalescing and bounce
   buffering for that?

I've been reading some drivers (as well as stuff in block/) to try and
figure things out, but it's hard to figure out all the places where
constraints are enforced. In particular, I've read several drivers that
make some big assumptions (which might be bugs?) For example, in
drivers/mtd/mtd_blkdevs.c, do_blktrans_request looks like:

	block = blk_rq_pos(req) << 9 >> tr->blkshift;
	nsect = blk_rq_cur_bytes(req) >> tr->blkshift;

	switch (req_op(req)) {
	/* ... snip ... */
	case REQ_OP_READ:
		buf = kmap(bio_page(req->bio)) + bio_offset(req->bio);
		for (; nsect > 0; nsect--, block++, buf += tr->blksize) {
			if (tr->readsect(dev, block, buf)) {
				kunmap(bio_page(req->bio));
				return BLK_STS_IOERR;
			}
		}
		kunmap(bio_page(req->bio));

		rq_for_each_segment(bvec, req, iter)
			flush_dcache_page(bvec.bv_page);
		return BLK_STS_OK;

For context, tr->blkshift is either 512 or 4096, depending on the
backend. From what I can tell, this code assumes the following:

- There is only one bio in a request. This one is a bit of a soft
   assumption since we should only flush the pages in the bio and not the
   whole request otherwise.
- There is only one segment in a bio. This one could be reasonable if
   max_segments was set to 1, but it's not as far as I can tell. So I
   guess we just go off the end of the bio if there's a second segment?
- The data is in lowmem OR bv_offset + bv_len <= PAGE_SIZE. kmap() only
   maps a single page, so if we go past one page we end up in adjacent
   kmapped pages.

Am I missing something here? Handling highmem seems like a persistent
issue. E.g. drivers/mtd/ubi/block.c doesn't even bother doing a kmap.
Should both of these have BLK_FEAT_BOUNCE_HIGH?

--Sean