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

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

On Mon, Mar 07, 2022 at 10:41:31AM +0800, Ming Lei wrote:
> 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.

->bi_private can be reused too, is that better?

Yeah, both belong to owner(higher level storage), then block layer can't touch
them inside submit_bio_noacct(), that is also why this trick is safe.

Thanks,
Ming