From: Eric Dumazet <edumazet@google.com>
Date: Tue, 14 Jun 2022 17:17:34 +0000 (-0700)
Subject: tcp: fix possible freeze in tx path under memory pressure
X-Git-Tag: v5.18.18~756
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=20b6d28f3a51dbcd75aaf14acd193e27d697f52a;p=thirdparty%2Fkernel%2Fstable.git

tcp: fix possible freeze in tx path under memory pressure

[ Upstream commit 849b425cd091e1804af964b771761cfbefbafb43 ]

Blamed commit only dealt with applications issuing small writes.

Issue here is that we allow to force memory schedule for the sk_buff
allocation, but we have no guarantee that sendmsg() is able to
copy some payload in it.

In this patch, I make sure the socket can use up to tcp_wmem[0] bytes.

For example, if we consider tcp_wmem[0] = 4096 (default on x86),
and initial skb->truesize being 1280, tcp_sendmsg() is able to
copy up to 2816 bytes under memory pressure.

Before this patch a sendmsg() sending more than 2816 bytes
would either block forever (if persistent memory pressure),
or return -EAGAIN.

For bigger MTU networks, it is advised to increase tcp_wmem[0]
to avoid sending too small packets.

v2: deal with zero copy paths.

Fixes: 8e4d980ac215 ("tcp: fix behavior for epoll edge trigger")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Reviewed-by: Wei Wang <weiwan@google.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 91735d631a282..51116166e3d21 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -953,6 +953,23 @@ static int tcp_downgrade_zcopy_pure(struct sock *sk, struct sk_buff *skb)
 	return 0;
 }
 
+static int tcp_wmem_schedule(struct sock *sk, int copy)
+{
+	int left;
+
+	if (likely(sk_wmem_schedule(sk, copy)))
+		return copy;
+
+	/* We could be in trouble if we have nothing queued.
+	 * Use whatever is left in sk->sk_forward_alloc and tcp_wmem[0]
+	 * to guarantee some progress.
+	 */
+	left = sock_net(sk)->ipv4.sysctl_tcp_wmem[0] - sk->sk_wmem_queued;
+	if (left > 0)
+		sk_forced_mem_schedule(sk, min(left, copy));
+	return min(copy, sk->sk_forward_alloc);
+}
+
 static struct sk_buff *tcp_build_frag(struct sock *sk, int size_goal, int flags,
 				      struct page *page, int offset, size_t *size)
 {
@@ -988,7 +1005,11 @@ new_segment:
 		tcp_mark_push(tp, skb);
 		goto new_segment;
 	}
-	if (tcp_downgrade_zcopy_pure(sk, skb) || !sk_wmem_schedule(sk, copy))
+	if (tcp_downgrade_zcopy_pure(sk, skb))
+		return NULL;
+
+	copy = tcp_wmem_schedule(sk, copy);
+	if (!copy)
 		return NULL;
 
 	if (can_coalesce) {
@@ -1337,8 +1358,11 @@ new_segment:
 
 			copy = min_t(int, copy, pfrag->size - pfrag->offset);
 
-			if (tcp_downgrade_zcopy_pure(sk, skb) ||
-			    !sk_wmem_schedule(sk, copy))
+			if (tcp_downgrade_zcopy_pure(sk, skb))
+				goto wait_for_space;
+
+			copy = tcp_wmem_schedule(sk, copy);
+			if (!copy)
 				goto wait_for_space;
 
 			err = skb_copy_to_page_nocache(sk, &msg->msg_iter, skb,
@@ -1365,7 +1389,8 @@ new_segment:
 				skb_shinfo(skb)->flags |= SKBFL_PURE_ZEROCOPY;
 
 			if (!skb_zcopy_pure(skb)) {
-				if (!sk_wmem_schedule(sk, copy))
+				copy = tcp_wmem_schedule(sk, copy);
+				if (!copy)
 					goto wait_for_space;
 			}