[PATCH 5.19 0364/1157] tcp: fix possible freeze in tx path under memory pressure

Greg Kroah-Hartman
2022-08-15T23:34:12Z

From: Eric Dumazet <edumazet@google.com>

[ 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>
---
net/ipv4/tcp.c | 33 +++++++++++++++++++++++++++++----
1 file changed, 29 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 766881775abb..3ae2ea048883 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -952,6 +952,23 @@ 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)
{
@@ -987,7 +1004,11 @@ static struct sk_buff *tcp_build_frag(struct sock *sk, int size_goal, int flags,
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) {
@@ -1336,8 +1357,11 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)

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,
@@ -1364,7 +1388,8 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
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;
}

--
2.35.1