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

[Jens Axboe <axboe@kernel.dk>]

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? Is bi_next available at this point?

-- 
Jens Axboe