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Re: [dm-devel] [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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