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

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

On Sun, Mar 06, 2022 at 07:25:11PM -0700, Jens Axboe wrote:
> On 3/6/22 7:20 PM, Ming Lei wrote:
> > 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...
> 
> There'd no amount of re-arranging that'll free up 8 bytes, that's just
> not happening. I'm not a huge fan of growing struct bio for that, and
> the oddity here is mostly (to me) that ->bi_end_io is the one overlayed.
> That would usually belong to the owner of the bio.
> 
> Maybe some commenting would help?

OK, ->bi_end_io is safe because it is only called until the bio is
ended, so we can retrieve the list head and recover ->bi_end_io before
polling.

> Is bi_next available at this point?

The same bio can be re-submitted to block layer because of splitting, and
will be linked to current->bio_list[].

BTW, bio splitting can be very often for some dm target, that is why we
don't ignore bio splitting for dm polling.


Thanks,
Ming