Messages in this thread | | | From | Neil Brown <> | Date | Thu, 2 Aug 2007 13:31:16 +1000 | Subject | Re: [PATCH 025 of 35] Treat rq->hard_nr_sectors as setting an overriding limit in the size of the request |
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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.
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