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

[Jens Axboe <axboe@kernel.dk>]

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...

-- 
Jens Axboe