linux-kernel - Re: [PATCH 2/2] blk-flush: don't need to end rq twice for non post-flush requests

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Date:   Mon, 10 Jul 2023 15:33:08 +0200
From:   Christoph Hellwig <hch@....de>
To:     chengming.zhou@...ux.dev
Cc:     axboe@...nel.dk, hch@....de, ming.lei@...hat.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        zhouchengming@...edance.com
Subject: Re: [PATCH 2/2] blk-flush: don't need to end rq twice for non
 post-flush requests

On Mon, Jul 10, 2023 at 02:47:05PM +0800, chengming.zhou@...ux.dev wrote:
> From: Chengming Zhou <zhouchengming@...edance.com>
> 
> Now we unconditionally blk_rq_init_flush() to replace rq->end_io to
> make rq return twice back to the flush state machine for post-flush.
> 
> Obviously, non post-flush requests don't need it, they don't need to
> end request twice, so they don't need to replace rq->end_io callback.
> And the same for requests with the FUA bit on hardware with FUA support.
> 
> So we move blk_rq_init_flush() to REQ_FSEQ_DATA stage and only replace
> rq->end_io if it needs post-flush. Otherwise, it can end like normal
> request and doesn't need to return back to the flush state machine.

I really like the idea behind this optimization, but I kinda hate
adding more magic to the already way too magic flush sequence.

I wonder if a better idea would be to kill the flush sequence entirely,
and just split the flush_queue into a preflush and a postflush queue.
This would remove a field from struct request and lead to more readable
code as well.