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

[Matthew Wilcox]

On Sun, Jul 05, 2020 at 03:00:47PM -0600, Jens Axboe wrote:
> On 7/5/20 12:47 PM, Kanchan Joshi wrote:
> > From: Selvakumar S <selvakuma.s1@xxxxxxxxxxx>
> > 
> > For zone-append, block-layer will return zone-relative offset via ret2
> > of ki_complete interface. Make changes to collect it, and send to
> > user-space using cqe->flags.
> > 
> > Signed-off-by: Selvakumar S <selvakuma.s1@xxxxxxxxxxx>
> > Signed-off-by: Kanchan Joshi <joshi.k@xxxxxxxxxxx>
> > Signed-off-by: Nitesh Shetty <nj.shetty@xxxxxxxxxxx>
> > Signed-off-by: Javier Gonzalez <javier.gonz@xxxxxxxxxxx>
> > ---
> >  fs/io_uring.c | 21 +++++++++++++++++++--
> >  1 file changed, 19 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/io_uring.c b/fs/io_uring.c
> > index 155f3d8..cbde4df 100644
> > --- a/fs/io_uring.c
> > +++ b/fs/io_uring.c
> > @@ -402,6 +402,8 @@ struct io_rw {
> >  	struct kiocb			kiocb;
> >  	u64				addr;
> >  	u64				len;
> > +	/* zone-relative offset for append, in sectors */
> > +	u32			append_offset;
> >  };
> 
> I don't like this very much at all. As it stands, the first cacheline
> of io_kiocb is set aside for request-private data. io_rw is already
> exactly 64 bytes, which means that you're now growing io_rw beyond
> a cacheline and increasing the size of io_kiocb as a whole.
> 
> Maybe you can reuse io_rw->len for this, as that is only used on the
> submission side of things.

I'm surprised you aren't more upset by the abuse of cqe->flags for the
address.

What do you think to my idea of interpreting the user_data as being a
pointer to somewhere to store the address?  Obviously other things
can be stored after the address in the user_data.

Or we could have a separate flag to indicate that is how to interpret
the user_data.