linux-kernel - 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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Date:	Thu, 02 Aug 2007 14:03:25 +0900
From:	Tejun Heo <htejun@...il.com>
To:	Neil Brown <neilb@...e.de>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 025 of 35] Treat rq->hard_nr_sectors as setting an overriding
 limit in the size of the request

Neil Brown wrote:
> On Thursday August 2, htejun@...il.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.

Yes, converting to nbytes will probably take a lot of work and probably
deserves a separate series if it's ever gonna be done.

Thanks.

-- 
tejun
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