linux-kernel - Re: [PATCH 1/2] block: export __make

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Message-ID: <4E70E1C1.8040907@panasas.com>
Date:	Wed, 14 Sep 2011 20:17:53 +0300
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	Christoph Hellwig <hch@...radead.org>
CC:	Jens Axboe <axboe@...nel.dk>, <linux-raid@...r.kernel.org>,
	<dm-devel@...hat.com>, <linux-kernel@...r.kernel.org>,
	<nab@...ux-iscsi.org>, <ryanh@...ibm.com>
Subject: Re: [PATCH 1/2] block: export __make_request

On 9/14/2011 12:19 AM, Christoph Hellwig wrote:
> On Mon, Sep 12, 2011 at 08:04:46PM +0200, Jens Axboe wrote:
>>> I really hate naming things different from the method they are
>>> implementing.  I've tried to figure out what the point of the
>>> old blk_make_request is - why would we not go through
>>> generic_make_request for this?
>>>
>>> Boaz, any idea?
>>
>> I tend to agree, we could rename the existing blk_make_request(). It
>> could be blk_make_request_from_bio() or something like that, since
>> that's what it does.
>
> It should at very least be renamed.  But I still can't figure out what
> it is for exactly.
>
> There are three users:
>
>   (1) virtio_blk::virtblk_get_id():
> 	This looks like it really should just use blk_rq_map_kern.
>   (2) osd_initiator::_make_request():
> 	This one looks like it should just use the same scheme as
> 	sg_io(), as it's doing the same thing.

Good god what sg_io? That broken pointr+length from user-mode that sg.c
and bsg.c are using? no can do it's not user-mode pointers, and it's not
pointer+length it's pages pointers of a bio. The only other structure
that could carry the same information is struct sg, but we work very 
hard to get rid of this contraption. (scsi_execute_async or something 
that it was)

blk_make_request() was made to be the parallel of __make_request, to be
used from filesystem level users. But with two differences.
1. Mainly support for none-FS BLOCK_PC requests
2. Also support chained bios. (was added later)

>   (3) target_core_pscsi::__pscsi_map_SG():
> 	Same as (2).
>

There is no better suitable structure in current Kernel to carry a list
of pages, with optional offset and length, then bio struct. Given a bio
at hand. how do you make a block request out of it? (If it's not an 
FS_PC type IO?)

As I remember target_core had their own pages-linked-list structure, and
how do you make a request out of that? again best at hand is bio.

bio is for a long time a page-pointers-carrier-structure and is out of 
private block-level use. The filesystem level is full of it.

Thanks
Boaz

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