Re: [PATCH net-next 1/3] net: devmem: support single IOV with sendmsg

[Stanislav Fomichev <stfomichev@gmail.com>]

On 05/21, Mina Almasry wrote:
> On Tue, May 20, 2025 at 1:30 PM Stanislav Fomichev <stfomichev@gmail.com> wrote:
> >
> > sendmsg() with a single iov becomes ITER_UBUF, sendmsg() with multiple
> > iovs becomes ITER_IOVEC. iter_iov_len does not return correct
> > value for UBUF, so teach to treat UBUF differently.
> >
> > Cc: Al Viro <viro@zeniv.linux.org.uk>
> > Cc: Pavel Begunkov <asml.silence@gmail.com>
> > Cc: Mina Almasry <almasrymina@google.com>
> > Fixes: bd61848900bf ("net: devmem: Implement TX path")
> > Signed-off-by: Stanislav Fomichev <stfomichev@gmail.com>
> > ---
> >  include/linux/uio.h | 8 +++++++-
> >  net/core/datagram.c | 3 ++-
> >  2 files changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/uio.h b/include/linux/uio.h
> > index 49ece9e1888f..393d0622cc28 100644
> > --- a/include/linux/uio.h
> > +++ b/include/linux/uio.h
> > @@ -99,7 +99,13 @@ static inline const struct iovec *iter_iov(const struct iov_iter *iter)
> >  }
> >
> >  #define iter_iov_addr(iter)    (iter_iov(iter)->iov_base + (iter)->iov_offset)
> > -#define iter_iov_len(iter)     (iter_iov(iter)->iov_len - (iter)->iov_offset)
> > +
> > +static inline size_t iter_iov_len(const struct iov_iter *i)
> > +{
> > +       if (i->iter_type == ITER_UBUF)
> > +               return i->count;
> > +       return iter_iov(i)->iov_len - i->iov_offset;
> > +}
> >
> 
> This change looks good to me from devmem perspective, but aren't you
> potentially breaking all these existing callers to iter_iov_len?
> 
> ackc -i iter_iov_len
> fs/read_write.c
> 846:                                            iter_iov_len(iter), ppos);
> 849:                                            iter_iov_len(iter), ppos);
> 858:            if (nr != iter_iov_len(iter))
> 
> mm/madvise.c
> 1808:           size_t len_in = iter_iov_len(iter);
> 1838:           iov_iter_advance(iter, iter_iov_len(iter));
> 
> io_uring/rw.c
> 710:                    len = iter_iov_len(iter);
> 
> Or are you confident this change is compatible with these callers for
> some reason?
 
Pavel did go over all callers, see:
https://lore.kernel.org/netdev/7f06216e-1e66-433e-a247-2445dac22498@gmail.com/

> Maybe better to handle this locally in zerocopy_fill_skb_from_devmem,
> and then follow up with a more ambitious change that streamlines how
> all the iters behave.

Yes, I can definitely do that, but it seems a bit strange that the
callers need to distinguish between IOVEC and UBUF (which is a 1-entry
IOVEC), so having working iter_iov_len seems a bit cleaner.