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

From: Jens Axboe <axboe@kernel.dk>
To: Christoph Hellwig <hch@lst.de>, Mike Snitzer <snitzer@redhat.com>
Cc: ming.lei@redhat.com, dm-devel@redhat.com, linux-block@vger.kernel.org
Subject: Re: [PATCH v5 2/2] dm: support bio polling
Date: Sun, 6 Mar 2022 18:48:15 -0700	[thread overview]
Message-ID: <2ced53d5-d87b-95db-a612-6896f73ce895@kernel.dk> (raw)
In-Reply-To: <20220306092937.GC22883@lst.de>

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