linux-kernel - Re: [PATCH 2/2 v2] UBI: Block: Add blk-mq support

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Date:	Wed, 14 Jan 2015 09:39:06 +0100
From:	Richard Weinberger <richard@....at>
To:	Jens Axboe <axboe@...com>, Christoph Hellwig <hch@...radead.org>
CC:	dedekind1@...il.com, dwmw2@...radead.org,
	computersforpeace@...il.com, linux-mtd@...ts.infradead.org,
	linux-kernel@...r.kernel.org, tom.leiming@...il.com
Subject: Re: [PATCH 2/2 v2] UBI: Block: Add blk-mq support

Am 14.01.2015 um 01:23 schrieb Jens Axboe:
> On 01/13/2015 04:36 PM, Richard Weinberger wrote:
>>
>>
>> Am 14.01.2015 um 00:30 schrieb Jens Axboe:
>>>> If I understand you correctly it can happen that blk_rq_bytes() returns
>>>> more bytes than blk_rq_map_sg() allocated, right?
>>>
>>> No, the number of bytes will be the same, no magic is involved :-)
>>
>> Good to know. :)
>>
>>> But lets say the initial request has 4 bios, with each 2 pages, for a
>>> total of 8 segments. Lets further assume that the pages in each bio are
>>> contiguous, so that blk_rq_map_sg() will map this to 4 sg elements, each
>>> 2xpages long.
>>>
>>> Now, this may already be handled just fine, and you don't need to
>>> update/store the actual sg count. I just looked at the source, and I'm
>>> assuming it'll do the right thing (ubi_read_sg() will bump the active sg
>>> element, when that size has been consumed), but I don't have
>>> ubi_read_sg() in my tree to verify.
>>
>> Currently the sg count is hard coded to UBI_MAX_SG_COUNT.
> 
> The max count doesn't matter, that just provides you a guarantee that
> you'll never receive a request that maps to more than that. The point
> I'm trying to make is that if you receive 8 segments and it maps to 4,
> then you better not look at segments 5..8 after it being mapped.
> Whatever the max is, doesn't matter in this conversation.
> 
>> I'm sorry, I forgot to CC you and hch to this patch:
> 
> Which is as I suspected, you'll do each segment to the length specified,
> hence you don't need to track the returned count from blk_rq_map_sg().

Thanks a lot for the kind explanation, Jens!
I'll add a comment the usage of blk_rq_map_sg() to avoid further confusion.
Am I allowed to add your Reviewed-by too?

Thanks,
//richard
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