Block device request merging - repeated requests.

[Jens Axboe]

On Mon, Jan 31 2000, David Woodhouse wrote:
> axboe en suse.de said:
> >  No, CURRENT->sector is always the "current" sector. When you call
> > end_request, end_that_request_first will adjust rq->sector for you. 
> 
> I only see a modification of req->sector in the !uptodate case.
> 
> I get 'correct' behaviour if I add the following line immediately before 
> every occurrence of end_request(1) in my driver:
> 	CURRENT->sector += CURRENT->current_nr_sectors;
> 
> # dd if=nftl.c bs=512 of=/dev/nftla count=6
> NFTL write request of 0x2 sectors @ 0 (CURRENT->nr_sectors == 6
> NFTL write request completed OK
> NFTL write request of 0x2 sectors @ 2 (CURRENT->nr_sectors == 6
> NFTL write request completed OK
> NFTL write request of 0x2 sectors @ 4 (CURRENT->nr_sectors == 6
> NFTL write request completed OK

You are right, I spoke too quickly, you do have to keep track of
what sector you are working on. But I don't see why we shouldn't
update rq->sector regardless of outcome.

> > The logic goes something like this:
> > - receive request
> > - transfer CURRENT->current_nr_sectors 
> > - end_request
> 
> That's what I'm doing, modulo any stupid mistakes on my part - which is 
> unlikely since this code hasn't changed for a while, and it worked before.

The rest of your request function looks fine.

> > FWIW, I have verified request handling with a dummy driver I wrote. It
> > even handles merged requests with "holes", i.e. non-consequitive
> > buffer heads. 
> 
> I think it would be a very good idea to include that in the kernel tree, as 
> with drivers/net/skeleton.c.
> 
> Please could I have a copy?

If people think that would be a good idea, I'd be happy to provide it.
It needs some cleaning up, as it also does read gathering of
adjacent blocks before writing (it's actually a block layer between
ll_rw_blk and another block driver).

-- 
*  Jens Axboe <axboe en suse.de>
*  Linux CD-ROM Maintainer
*  http://www.kernel.dk

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