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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, 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


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