Re: [PATCH 025 of 35] Treat rq->hard_nr_sectors as setting an overriding limit in the size of the request

[Tejun Heo]

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
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