Re: [PATCH v5 2/2] dm: support bio polling

[Ming Lei <ming.lei@redhat.com>]

On Sun, Mar 06, 2022 at 06:48:15PM -0700, Jens Axboe wrote:
> On 3/6/22 2:29 AM, Christoph Hellwig wrote:
> >> +/*
> >> + * Reuse ->bi_end_io as hlist head for storing all dm_io instances
> >> + * associated with this bio, and this bio's bi_end_io has to be
> >> + * stored in one of 'dm_io' instance first.
> >> + */
> >> +static inline struct hlist_head *dm_get_bio_hlist_head(struct bio *bio)
> >> +{
> >> +	WARN_ON_ONCE(!(bio->bi_opf & REQ_DM_POLL_LIST));
> >> +
> >> +	return (struct hlist_head *)&bio->bi_end_io;
> >> +}
> > 
> > So this reuse is what I really hated.  I still think we should be able
> > to find space in the bio by creatively shifting fields around to just
> > add the hlist there directly, which would remove the need for this
> > override and more importantly the quite cumbersome saving and restoring
> > of the end_io handler.
> 
> If it's possible, then that would be preferable. But I don't think
> that's going to be easy to do...

I agree, now basically there isn't gap inside bio, so either adding one
new field or reusing one existed field...


Thanks,
Ming