tcp: gso: fix truesize tracking

commit 6ff50cd55545 ("tcp: gso: do not generate out of order packets")
had an heuristic that can trigger a warning in skb_try_coalesce(),
because skb->truesize of the gso segments were exactly set to mss.

This breaks the requirement that

skb->truesize >= skb->len + truesizeof(struct sk_buff);

It can trivially be reproduced by :

ifconfig lo mtu 1500
ethtool -K lo tso off
netperf

As the skbs are looped into the TCP networking stack, skb_try_coalesce()
warns us of these skb under-estimating their truesize.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 8 deletions

diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
index 3a7525e6c086..533c58a5cfb7 100644
--- a/net/ipv4/tcp_offload.c
+++ b/net/ipv4/tcp_offload.c
@@ -18,6 +18,7 @@ struct sk_buff *tcp_tso_segment(struct sk_buff *skb,
                                 netdev_features_t features)
 {
         struct sk_buff *segs = ERR_PTR(-EINVAL);
+        unsigned int sum_truesize = 0;
         struct tcphdr *th;
         unsigned int thlen;
         unsigned int seq;
@@ -102,13 +103,7 @@ struct sk_buff *tcp_tso_segment(struct sk_buff *skb,
                 if (copy_destructor) {
                         skb->destructor = gso_skb->destructor;
                         skb->sk = gso_skb->sk;
-                        /* {tcp|sock}_wfree() use exact truesize accounting :
-                         * sum(skb->truesize) MUST be exactly be gso_skb->truesize
-                         * So we account mss bytes of 'true size' for each segment.
-                         * The last segment will contain the remaining.
-                         */
-                        skb->truesize = mss;
-                        gso_skb->truesize -= mss;
+                        sum_truesize += skb->truesize;
                 }
                 skb = skb->next;
                 th = tcp_hdr(skb);
@@ -125,7 +120,9 @@ struct sk_buff *tcp_tso_segment(struct sk_buff *skb,
         if (copy_destructor) {
                 swap(gso_skb->sk, skb->sk);
                 swap(gso_skb->destructor, skb->destructor);
-                swap(gso_skb->truesize, skb->truesize);
+                sum_truesize += skb->truesize;
+                atomic_add(sum_truesize - gso_skb->truesize,
+                           &skb->sk->sk_wmem_alloc);
         }
 
         delta = htonl(oldlen + (skb_tail_pointer(skb) -