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

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Message-ID: <CAHS8izOTWF9PO9N6ZamJ0xSCTOojXV+LfYm+5B5b8Ad1MA0QpA@mail.gmail.com>
Date: Wed, 21 May 2025 10:16:46 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Stanislav Fomichev <stfomichev@...il.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com, 
	kuba@...nel.org, pabeni@...hat.com, viro@...iv.linux.org.uk, horms@...nel.org, 
	andrew+netdev@...n.ch, shuah@...nel.org, sagi@...mberg.me, willemb@...gle.com, 
	asml.silence@...il.com, jdamato@...tly.com, kaiyuanz@...gle.com, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH net-next 1/3] net: devmem: support single IOV with sendmsg

On Tue, May 20, 2025 at 1:30 PM Stanislav Fomichev <stfomichev@...il.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@...iv.linux.org.uk>
> Cc: Pavel Begunkov <asml.silence@...il.com>
> Cc: Mina Almasry <almasrymina@...gle.com>
> Fixes: bd61848900bf ("net: devmem: Implement TX path")
> Signed-off-by: Stanislav Fomichev <stfomichev@...il.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?

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.


-- 
Thanks,
Mina