netdev - [PATCH net] tcp: fix mem under-charging with zerocopy sendmsg()

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Message-Id: <20220201065254.680532-1-eric.dumazet@gmail.com>
Date:   Mon, 31 Jan 2022 22:52:54 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Talal Ahmad <talalahmad@...gle.com>,
        Arjun Roy <arjunroy@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: [PATCH net] tcp: fix mem under-charging with zerocopy sendmsg()

From: Eric Dumazet <edumazet@...gle.com>

We got reports of following warning in inet_sock_destruct()

	WARN_ON(sk_forward_alloc_get(sk));

Whenever we add a non zero-copy fragment to a pure zerocopy skb,
we have to anticipate that whole skb->truesize will be uncharged
when skb is finally freed.

skb->data_len is the payload length. But the memory truesize
estimated by __zerocopy_sg_from_iter() is page aligned.

Fixes: 9b65b17db723 ("net: avoid double accounting for pure zerocopy skbs")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Talal Ahmad <talalahmad@...gle.com>
Cc: Arjun Roy <arjunroy@...gle.com>
Cc: Soheil Hassas Yeganeh <soheil@...gle.com>
Cc: Willem de Bruijn <willemb@...gle.com>
---
 net/ipv4/tcp.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 78e81465f5f3632f54093495d2f2a064e60c7237..bdf108f544a45a2aa24bc962fb81dfd0ca1e0682 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1322,10 +1322,13 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 
 			/* skb changing from pure zc to mixed, must charge zc */
 			if (unlikely(skb_zcopy_pure(skb))) {
-				if (!sk_wmem_schedule(sk, skb->data_len))
+				u32 extra = skb->truesize -
+					    SKB_TRUESIZE(skb_end_offset(skb));
+
+				if (!sk_wmem_schedule(sk, extra))
 					goto wait_for_space;
 
-				sk_mem_charge(sk, skb->data_len);
+				sk_mem_charge(sk, extra);
 				skb_shinfo(skb)->flags &= ~SKBFL_PURE_ZEROCOPY;
 			}
 
-- 
2.35.0.rc2.247.g8bbb082509-goog