linux-kernel - Re: [PATCH v1 4/9] blk-mq: introduce init_flush_rq_fn callback in 'blk_mq

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Date:	Sat, 16 Aug 2014 15:49:27 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Kleikamp <dave.kleikamp@...cle.com>,
	Zach Brown <zab@...bo.net>, Benjamin LaHaise <bcrl@...ck.org>,
	Christoph Hellwig <hch@...radead.org>,
	Kent Overstreet <kmo@...erainc.com>, linux-aio@...ck.org,
	linux-fsdevel@...r.kernel.org, Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH v1 4/9] blk-mq: introduce init_flush_rq_fn callback in 'blk_mq_ops'

On 8/16/14, Jens Axboe <axboe@...nel.dk> wrote:
> On 08/14/2014 09:50 AM, Ming Lei wrote:
>> Currently pdu of the flush rq is simlpy copied from another rq,
>> it isn't enough to initialize pointer field well, so introduce
>> the callback for driver to handle the case easily.
>
> This is the only patch I don't really like. Can't we make do with
> calling ->init_request() for this instead of having to add another
> (weird) hook?

I considered ->init_request() before, but looks there are some problems:

- from API view, both 'hctx_idx' and 'request_idx' parameter don't make
sense for flush rq since it beongs to request queue instead of any one
of hctx

- init_request()/exit_request() are weird too, since they will be called
for queuing every flush req, and they should have been called one
shot

- it is called before queuing each flush req, and might introduce a bit cost
unnecessarily if init_request does lots of stuff

- using init_request may break some current drivers(like scsi)

Now I feel ->init_flush_rq() isn't good too, how about introducing
prepare_flush_rq() and unprepare_flush_rq()? And they can be
lightweight and have document benefit at least.

Thanks,
-- 
Ming Lei
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