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Date:   Wed, 8 Jul 2020 15:58:26 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Kanchan Joshi <joshi.k@samsung.com>, viro@zeniv.linux.org.uk,
        bcrl@kvack.org, hch@infradead.org, Damien.LeMoal@wdc.com,
        asml.silence@gmail.com, linux-fsdevel@vger.kernel.org,
        mb@lightnvm.io, linux-kernel@vger.kernel.org, linux-aio@kvack.org,
        io-uring@vger.kernel.org, linux-block@vger.kernel.org,
        Selvakumar S <selvakuma.s1@samsung.com>,
        Nitesh Shetty <nj.shetty@samsung.com>,
        Javier Gonzalez <javier.gonz@samsung.com>
Subject: Re: [PATCH v3 4/4] io_uring: add support for zone-append
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On Wed, Jul 08, 2020 at 08:54:07AM -0600, Jens Axboe wrote:
> On 7/8/20 6:58 AM, Kanchan Joshi wrote:
> >>> +#define IOCB_NO_CMPL		(15 << 28)
> >>>
> >>>  struct kiocb {
> >>> [...]
> >>> -	void (*ki_complete)(struct kiocb *iocb, long ret, long ret2);
> >>> +	loff_t __user *ki_uposp;
> >>> -	int			ki_flags;
> >>> +	unsigned int		ki_flags;
> >>>
> >>> +typedef void ki_cmpl(struct kiocb *, long ret, long ret2);
> >>> +static ki_cmpl * const ki_cmpls[15];
> >>>
> >>> +void ki_complete(struct kiocb *iocb, long ret, long ret2)
> >>> +{
> >>> +	unsigned int id = iocb->ki_flags >> 28;
> >>> +
> >>> +	if (id < 15)
> >>> +		ki_cmpls[id](iocb, ret, ret2);
> >>> +}
> >>>
> >>> +int kiocb_cmpl_register(void (*cb)(struct kiocb *, long, long))
> >>> +{
> >>> +	for (i = 0; i < 15; i++) {
> >>> +		if (ki_cmpls[id])
> >>> +			continue;
> >>> +		ki_cmpls[id] = cb;
> >>> +		return id;
> >>> +	}
> >>> +	WARN();
> >>> +	return -1;
> >>> +}
> >>
> >> That could work, we don't really have a lot of different completion
> >> types in the kernel.
> > 
> > Thanks, this looks sorted.
> 
> Not really, someone still needs to do that work. I took a quick look, and
> most of it looks straight forward. The only potential complication is
> ocfs2, which does a swap of the completion for the kiocb. That would just
> turn into an upper flag swap. And potential sync kiocb with NULL
> ki_complete. The latter should be fine, I think we just need to reserve
> completion nr 0 for being that.

I was reserving completion 15 for that ;-)

+#define IOCB_NO_CMPL		(15 << 28)
...
+	if (id < 15)
+		ki_cmpls[id](iocb, ret, ret2);

Saves us one pointer in the array ...