[PATCH net-next v2 2/7] net: devmem: charge the frag length to TX skb truesize

[Stanislav Fomichev <sdf.kernel@gmail.com>]

Devmem TX charges each frag the net_iov size, PAGE_SIZE today:
zerocopy_fill_skb_from_devmem() adds PAGE_SIZE to skb->truesize per
frag and __zerocopy_sg_from_iter() charges it to sk_wmem_queued. TCP
uncharges in bytes as acked data is trimmed from the skb
(tcp_trim_head()) and uncharges the rest when the skb is freed. A frag
shorter than a page leaves the difference charged until then, which is
small with PAGE_SIZE net_iovs.

TX net_iovs are about to grow up to 2MB, while frags stay bounded by
the skb and many frags share a net_iov. Charging the net_iov size would
charge up to 2MB per frag: a 64KB skb with two frags would charge 4MB
and fill the default 4MB send buffer on its own.

Charge the frag length instead, which also matches what TCP uncharges.

Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
---
 net/core/datagram.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/datagram.c b/net/core/datagram.c
index ecc30a6f8302..489d92fe6dae 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -737,7 +737,7 @@ zerocopy_fill_skb_from_devmem(struct sk_buff *skb, struct iov_iter *from,
 
 		netmem_inc_ref(net_iov_to_netmem(niov));
 		skb_add_rx_frag_netmem(skb, i, net_iov_to_netmem(niov), off,
-				       size, PAGE_SIZE);
+				       size, size);
 		iov_iter_advance(from, size);
 		length -= size;
 		i++;
-- 
2.53.0-Meta