# RE: \[PATCH net-next v2 01/17\] net: Copy slab data for sendmsg(MSG\_SPLICE\_PAGES)

- From: Willem de Bruijn \<willemdebruijn.kernel@gmail.com\>
- Date: 2023-06-18 16:43:27 
- List: netdev
- Page: https://ratatoskr.run/netdev/2023/06/2237828
- Thread: https://ratatoskr.run/netdev/2023/06/2237442/t.md

```text
David Howells wrote:
> If sendmsg() is passed MSG_SPLICE_PAGES and is given a buffer that contains
> some data that's resident in the slab, copy it rather than returning EIO.
> This can be made use of by a number of drivers in the kernel, including:
> iwarp, ceph/rds, dlm, nvme, ocfs2, drdb.  It could also be used by iscsi,
> rxrpc, sunrpc, cifs and probably others.
> 
> skb_splice_from_iter() is given it's own fragment allocator as
> page_frag_alloc_align() can't be used because it does no locking to prevent
> parallel callers from racing.  alloc_skb_frag() uses a separate folio for
> each cpu and locks to the cpu whilst allocating, reenabling cpu migration
> around folio allocation.
> 
> This could allocate a whole page instead for each fragment to be copied, as
> alloc_skb_with_frags() would do instead, but that would waste a lot of
> space (most of the fragments look like they're going to be small).
> 
> This allows an entire message that consists of, say, a protocol header or
> two, a number of pages of data and a protocol footer to be sent using a
> single call to sock_sendmsg().
> 
> The callers could be made to copy the data into fragments before calling
> sendmsg(), but that then penalises them if MSG_SPLICE_PAGES gets ignored.
> 
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Alexander Duyck <redacted>
> cc: Eric Dumazet <redacted>
> cc: "David S. Miller" <davem@davemloft.net>
> cc: David Ahern <dsahern@kernel.org>
> cc: Jakub Kicinski <kuba@kernel.org>
> cc: Paolo Abeni <pabeni@redhat.com>
> cc: Jens Axboe <axboe@kernel.dk>
> cc: Matthew Wilcox <willy@infradead.org>
> cc: Menglong Dong <redacted>
> cc: <redacted>
> ---
> 
> Notes:
>     ver #2)
>     - Fix parameter to put_cpu_ptr() to have an '&'.
> 
>  include/linux/skbuff.h |   5 ++
>  net/core/skbuff.c      | 171 ++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 173 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 91ed66952580..0ba776cd9be8 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -5037,6 +5037,11 @@ static inline void skb_mark_for_recycle(struct sk_buff *skb)
>  #endif
>  }
>  
> +void *alloc_skb_frag(size_t fragsz, gfp_t gfp);
> +void *copy_skb_frag(const void *s, size_t len, gfp_t gfp);
> +ssize_t skb_splice_from_iter(struct sk_buff *skb, struct iov_iter *iter,
> +			     ssize_t maxsize, gfp_t gfp);
> +
>  ssize_t skb_splice_from_iter(struct sk_buff *skb, struct iov_iter *iter,
>  			     ssize_t maxsize, gfp_t gfp);
>

duplicate declaration

(no need to respin just for this, imho)
```