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Date:   Tue, 15 Mar 2022 09:57:45 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Kanchan Joshi <joshiiitr@gmail.com>
Cc:     Christoph Hellwig <hch@lst.de>,
        Kanchan Joshi <joshi.k@samsung.com>,
        Jens Axboe <axboe@kernel.dk>, Keith Busch <kbusch@kernel.org>,
        Pavel Begunkov <asml.silence@gmail.com>,
        io-uring@vger.kernel.org, linux-nvme@lists.infradead.org,
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        Javier =?iso-8859-1?Q?Gonz=E1lez?= <javier@javigon.com>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Adam Manzanares <a.manzanares@samsung.com>,
        Anuj Gupta <anuj20.g@samsung.com>
Subject: Re: [PATCH 14/17] io_uring: add polling support for uring-cmd
Message-ID: <20220315085745.GE4132@lst.de>
References: <20220308152105.309618-1-joshi.k@samsung.com> <CGME20220308152723epcas5p34460b4af720e515317f88dbb78295f06@epcas5p3.samsung.com> <20220308152105.309618-15-joshi.k@samsung.com> <20220311065007.GC17728@lst.de> <CA+1E3rKKCE53TJ9mJesK3UrPPa=Vqx6fxA+TAhj9v5hT452AuQ@mail.gmail.com>
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On Mon, Mar 14, 2022 at 03:46:08PM +0530, Kanchan Joshi wrote:
> But, after you did bio based polling, we need just the bio to poll.
> iocb is a big structure (48 bytes), and if we try to place it in
> struct io_uring_cmd, we will just blow up the cacheline in io_uring
> (first one in io_kiocb).
> So we just store that bio pointer in io_uring_cmd on submission
> (please see patch 15).
> And here on completion we pick that bio, and put that into this local
> iocb, simply because  ->iopoll needs it.
> Do you see I am still missing anything here?

Yes.  The VFS layer interface for polling is the kiocb.  Don't break
it.  The bio is just an implementation detail.