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    SubjectRe: [PATCH 025 of 35] Treat rq->hard_nr_sectors as setting an overriding limit in the size of the request
    On Thursday August 2, htejun@gmail.com wrote:
    >
    > 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.

    Yes, that reasoning matches mine.

    >
    > 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.

    I suspect that having a byte count in 'struct request' would make
    sense too. However I would rather avoid making that change myself - I
    think it would require reading and understanding a lot more code....

    I cannot see anything that would go wrong with rounding up bio_sectors
    unconditionally, so I think I will take that approach for this patch
    series.

    Thanks.

    NeilBrown
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