Re: [RFC PATCH 2/8] rqbased-dm: add block layer hook

Kiyoshi Ueda
2006-12-21T22:23:35Z

Hi Jens,

On Thu, 21 Dec 2006 08:49:47 +0100, Jens Axboe <jens.axboe@oracle.com> wrote:
> > The new hook is needed for error handling in dm.
> > For example, when an error occurred on a request, dm-multipath
> > wants to try another path before returning EIO to application.
> > Without the new hook, at the point of end_that_request_last(),
> > the bios are already finished with error and can't be retried.
>
> Ok, I see what you are getting at. The current ->end_io() is called when
> the request has fully completed, you want notification for each chunk
> potentially completed.
>
> I think a better design here would be to use ->end_io() as the full
> completion handler, similar to how bio->bi_end_io() works. A request
> originating from __make_request() would set something ala:
>
> int fs_end_io(struct request *rq, int error, unsigned int nr_bytes)
> {
> if (!__end_that_request_first(rq, err, nr_bytes)) {
> end_that_request_last(rq, error);
> return 0;
> }
>
> return 1;
> }
>
> and normal io completion from a driver would use a helper:
>
> int blk_complete_io(struct request *rq, int error, unsigned int nr_bytes)
> {
> return rq->end_io(rq, error, nr_bytes);
> }
>
> instead of calling the functions manually. That would allow you to get
> notification right at the beginning and do what you need, without adding
> a special hook for this.

I'm not confident about what you mean.
Something like this?
- __make_request() sets fs_end_io() to req->end_io()
- The driver calls blk_complete_io()
* if it succeeds, the request is done
* if it fails, the request is not completed
and the driver needs retry or something
- Current users of req->end_io() have to update/rewrite thier end_io.
- Features like mine will set its own end_io.
It checks error and decides whether calling fs_end_io() or not.

Depending on drivers, there are some functions called between
__end_that_request_first() and end_that_request_last().
For example:
- add_disk_randomness()
- blk_queue_end_tag()
- floppy_off()
So they might prevent such generalization.

In addition to the suggested approach, what do you think about
adding a new flag to req->cmd_flags which lets the end_io() handler
not to return bio to upper layer?
It will be useful for multipathing and can be done even within
the current __end_that_request_first().
For example,

static int __end_that_request_first()
{
.....
error = 0;
if (end_io_error(uptodate))
error = !uptodate ? -EIO : uptodate;
.....
if (error && (req->cmd_flags & "NEW_FLAG"))
return 0; /* Tell the driver to call end_that_request_last() */

total_types = bio_nbytes = 0;
while ((bio = req->bio) != NULL) {
..... /* process of finishing bios */
}
.....
}

Thanks,
Kiyoshi Ueda