Re: [PATCH v3 4/4] io_uring: add support for zone-append

Matthew Wilcox
2020-07-07T22:18:32Z

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.