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

[Kanchan Joshi]

On Tue, Jul 07, 2020 at 04:37:55PM -0600, Jens Axboe wrote:
> On 7/7/20 4:18 PM, Matthew Wilcox wrote:
> > 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:
>
> Figured you might have had your sights set on that one, which is why I
> wanted to bring it up upfront :-)
>
> > +#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.
The last thing is about the flag used to trigger this processing. 
Will it be fine to intoduce new flag (RWF_APPEND2 or RWF_APPEND_OFFSET)
instead of using RWF_APPEND? 

New flag will do what RWF_APPEND does and will also return the 
written-location (and therefore expects pointer setup in application).