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 12:03 PM, Alexander V. Buev wrote:
> >> 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
> 
> Option 2 really sucks, because then obviously everyone wants their
> feature enabled, and then we are back to square one. So never rely on a
> config option, if it can be avoided.
> 
> I'd like to see what option 1 looks like, that sounds like a far better
> solution.
> 
Accepted. I am starting to prepare v3 in this way. 

Thanks to all for feedback!

-- 
Alexander Buev