[PATCH net-next] tcp: properly reset skb->truesize for tx recycling

[Eric Dumazet]

tcp sendmsg() and sendpage() normally advance skb->data_len
and skb->truesize by the payload added to an skb.

But sendmsg(fd, ..., MSG_ZEROCOPY) has to account for whole pages,
even if a single byte of payload is used in the page.

This means that we can not assume skb->truesize can be adjusted
by skb->data_len. We must instead overwrite its value.

Otherwise skb->truesize is too big and can hit socket sndbuf limit,
especially if the skb is recycled multiple times :/

Fixes: 472c2e07eef0 ("tcp: add one skb cache for tx")
Signed-off-by: Eric Dumazet <eduma...@google.com>
Cc: Soheil Hassas Yeganeh <soh...@google.com>
Cc: Willem de Bruijn <will...@google.com>
---
 net/ipv4/tcp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 
603e770d59b3db96adca9602319d2f6970a56285..f7567a3698eb4d6c24dbe6a2f110413ce14d24a7
 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -868,7 +868,7 @@ struct sk_buff *sk_stream_alloc_skb(struct sock *sk, int 
size, gfp_t gfp,
        if (likely(!size)) {
                skb = sk->sk_tx_skb_cache;
                if (skb && !skb_cloned(skb)) {
-                       skb->truesize -= skb->data_len;
+                       skb->truesize = SKB_TRUESIZE(skb_end_offset(skb));
                        sk->sk_tx_skb_cache = NULL;
                        pskb_trim(skb, 0);
                        INIT_LIST_HEAD(&skb->tcp_tsorted_anchor);
-- 
2.21.0.593.g511ec345e18-goog