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

[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 <edumazet@google.com>
Cc: Soheil Hassas Yeganeh <soheil@google.com>
Cc: Willem de Bruijn <willemb@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

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

[Soheil Hassas Yeganeh]

On Fri, Apr 19, 2019 at 7:02 PM Eric Dumazet <edumazet@google.com> wrote:
>
> 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 <edumazet@google.com>
> Cc: Soheil Hassas Yeganeh <soheil@google.com>
> Cc: Willem de Bruijn <willemb@google.com>

Acked-by: Soheil Hassas Yeganeh <soheil@google.com>

Nice catch! Thank you for the fix.

> ---
>  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
>

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

[David Miller]

From: Eric Dumazet <edumazet@google.com>
Date: Fri, 19 Apr 2019 16:02:03 -0700

> 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 <edumazet@google.com>

Applied, thanks Eric.