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

[Christoph Hellwig <hch@infradead.org>]

On Sun, Jul 05, 2020 at 03:00:47PM -0600, Jens Axboe wrote:
> > 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.

We don't actually need any new field at all.  By the time the write
returned ki_pos contains the offset after the write, and the res
argument to ->ki_complete contains the amount of bytes written, which
allow us to trivially derive the starting position.