Re: [PATCH v2 2/3] block: io_uring: add READV_PI/WRITEV_PI operations

["Alexander V. Buev" <a.buev@yadro.com>]

> On 2/10/22 6:08 AM, Alexander V. Buev wrote:
> > Added new READV_PI/WRITEV_PI operations to io_uring.
> > Added new pi_addr & pi_len fields to SQE struct.
> > Added new pi_iter field and IOCB_USE_PI flag to kiocb struct.
> > Make corresponding corrections to io uring trace event.
> > 
> > +struct io_rw_pi_state {
> > +	struct iov_iter			iter;
> > +	struct iov_iter_state		iter_state;
> > +	struct iovec			fast_iov[UIO_FASTIOV_PI];
> > +};
> > +
> > +struct io_rw_pi {
> > +	struct io_rw			rw;
> > +	struct iovec			*pi_iov;
> > +	u32				nr_pi_segs;
> > +	struct io_rw_pi_state		*s;
> > +};
> 
> One immediate issue I see here is that io_rw_pi is big, and we try very
> hard to keep the per-command payload to 64-bytes. This would be 88 bytes
> by my count :-/
> 
> Do you need everything from io_rw? If not, I'd just make io_rw_pi
> contain the bits you need and see if you can squeeze it into the
> existing cacheline.

In short - Yes. Current patch code call existing io_read/io_write functions.
This functions use io_rw struct information and process this data.
I wanted to use existing functions but may be this is wrong way in this 
case.
                                                                                
The second problem with request size is that the patch adds pi_iter   
pointer to kiocb struct. This also increase whole request union
length.

So I can see some (may be possible) solution for this: 

 1) do not store whole kiocb struct in request
    and write fully separated io_read/write_pi functions

 2) make special CONFIG_XXX variable and simplify hide this code
    as default

Any other variants?



-- 
Alexander Buev