Re: [PATCH 21/21] block: Bio cancellation

[Jens Axboe <axboe@kernel.dk>]

On Mon, May 13 2013, Kent Overstreet wrote:
> If a bio is associated with a kiocb, allow it to be cancelled.
> 
> This is accomplished by adding a pointer to a kiocb in struct bio, and
> when we go to dequeue a request we check if its bio has been cancelled -
> if so, we end the request with -ECANCELED.
> 
> We don't currently try to cancel bios if IO has already been started -
> that'd require a per bio callback function, and a way to find all the
> outstanding bios for a given kiocb. Such a mechanism may or may not be
> added in the future but this patch tries to start simple.
> 
> Currently this can only be triggered with aio and io_cancel(), but the
> mechanism can be used for sync io too.
> 
> It can also be used for bios created by stacking drivers, and bio clones
> in general - when cloning a bio, if the bi_iocb pointer is copied as
> well the clone will then be cancellable. bio_clone() could be modified
> to do this, but hasn't in this patch because all the bio_clone() users
> would need to be auditied to make sure that it's safe. We can't blindly
> make e.g. raid5 writes cancellable without the knowledge of the md code.

This is a pretty ugly hack, to be honest. It only works for aio. And it
grows struct bio just for that.

I do like the staged approach, where we just check whether a bio is
canceled when we come across it in the various parts of bio allocate to
completion.

> @@ -2124,6 +2130,12 @@ struct request *blk_peek_request(struct request_queue *q)
>  			trace_block_rq_issue(q, rq);
>  		}
>  
> +		if (rq->bio && !rq->bio->bi_next && bio_cancelled(rq->bio)) {
> +			blk_start_request(rq);
> +			__blk_end_request_all(rq, -ECANCELED);
> +			continue;
> +		}

Pretty hacky too, given that it only works for the generic case of a
non-merged bio.

So nack on this one.

-- 
Jens Axboe