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 <edumazet@google.com>
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: netdev@vger.kernel.org
---

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);