[PATCH net-next v2] net: devmem: return EMSGSIZE on type mismatch

[Mina Almasry <almasrymina@google.com>]

When a devmem payload mixes with a standard page payload, return
-EMSGSIZE instead of -EFAULT. This enables tcp_sendmsg to seamlessly
fall back to creating a new segment instead of failing the socket send.

Fixes: bd61848900bff ("net: devmem: Implement TX path")
Cc: Pavel Begunkov <asml.silence@gmail.com>
Cc: Stanislav Fomichev <sdf@fomichev.me>
Cc: Bobby Eshleman <bobbyeshleman@gmail.com>
Signed-off-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>

---
v2: Sent to net-next instead of net (Jakub). Otherwise no changes.
v1: https://lore.kernel.org/r/20260801125308.1342897-2-almasrymina@google.com
---
 net/core/datagram.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/core/datagram.c b/net/core/datagram.c
index 173b5d97bd409..6f8ffd61bcab4 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -638,7 +638,7 @@ int zerocopy_fill_skb_from_iter(struct sk_buff *skb,
 	int frag = skb_shinfo(skb)->nr_frags;
 
 	if (!skb_frags_readable(skb))
-		return -EFAULT;
+		return -EMSGSIZE;
 
 	while (length && iov_iter_count(from)) {
 		struct page *head, *last_head = NULL;
@@ -713,7 +713,7 @@ zerocopy_fill_skb_from_devmem(struct sk_buff *skb, struct iov_iter *from,
 	struct net_iov *niov;
 
 	if (i && skb_frags_readable(skb))
-		return -EFAULT;
+		return -EMSGSIZE;
 
 	/* Devmem filling works by taking an IOVEC from the user where the
 	 * iov_addrs are interpreted as an offset in bytes into the dma-buf to

base-commit: 001b5d347d8ba39b2dccaefcc57967b18caec8fe
-- 
2.55.0.679.g6767b8d81c-goog