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Date:   Tue, 7 Jul 2020 23:18:12 +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,
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        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 Tue, Jul 07, 2020 at 02:40:06PM -0600, Jens Axboe wrote:
> >> so we have another 24 bytes before io_kiocb takes up another cacheline.
> >> If that's a serious problem, I have an idea about how to shrink struct
> >> kiocb by 8 bytes so struct io_rw would have space to store another
> >> pointer.
> > Yes, io_kiocb has room. Cache-locality wise whether that is fine or
> > it must be placed within io_rw - I'll come to know once I get to
> > implement this. Please share the idea you have, it can come handy.
> 
> Except it doesn't, I'm not interested in adding per-request type fields
> to the generic part of it. Before we know it, we'll blow past the next
> cacheline.
> 
> If we can find space in the kiocb, that'd be much better. Note that once
> the async buffered bits go in for 5.9, then there's no longer a 4-byte
> hole in struct kiocb.

Well, poot, I was planning on using that.  OK, how about this:

+#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;
+}

... etc, also need an unregister.