netdev - [PATCH net v3 2/3] sock: Doc behaviors for pressure heurisitics

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Message-Id: <20231019120026.42215-2-wuyun.abel@bytedance.com>
Date: Thu, 19 Oct 2023 20:00:25 +0800
From: Abel Wu <wuyun.abel@...edance.com>
To: "David S . Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Shakeel Butt <shakeelb@...gle.com>
Cc: netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Abel Wu <wuyun.abel@...edance.com>
Subject: [PATCH net v3 2/3] sock: Doc behaviors for pressure heurisitics

There are now two accounting infrastructures for skmem, while the
heuristics in __sk_mem_raise_allocated() were actually introduced
before memcg was born.

Add some comments to clarify whether they can be applied to both
infrastructures or not.

Suggested-by: Shakeel Butt <shakeelb@...gle.com>
Signed-off-by: Abel Wu <wuyun.abel@...edance.com>
Acked-by: Shakeel Butt <shakeelb@...gle.com>
---
 net/core/sock.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/net/core/sock.c b/net/core/sock.c
index 4412c47466a7..45841a5689b6 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -3069,7 +3069,14 @@ int __sk_mem_raise_allocated(struct sock *sk, int size, int amt, int kind)
 	if (allocated > sk_prot_mem_limits(sk, 2))
 		goto suppress_allocation;
 
-	/* guarantee minimum buffer size under pressure */
+	/* Guarantee minimum buffer size under pressure (either global
+	 * or memcg) to make sure features described in RFC 7323 (TCP
+	 * Extensions for High Performance) work properly.
+	 *
+	 * This rule does NOT stand when exceeds global or memcg's hard
+	 * limit, or else a DoS attack can be taken place by spawning
+	 * lots of sockets whose usage are under minimum buffer size.
+	 */
 	if (kind == SK_MEM_RECV) {
 		if (atomic_read(&sk->sk_rmem_alloc) < sk_get_rmem0(sk, prot))
 			return 1;
@@ -3090,6 +3097,11 @@ int __sk_mem_raise_allocated(struct sock *sk, int size, int amt, int kind)
 
 		if (!sk_under_memory_pressure(sk))
 			return 1;
+
+		/* Try to be fair among all the sockets under global
+		 * pressure by allowing the ones that below average
+		 * usage to raise.
+		 */
 		alloc = sk_sockets_allocated_read_positive(sk);
 		if (sk_prot_mem_limits(sk, 2) > alloc *
 		    sk_mem_pages(sk->sk_wmem_queued +
-- 
2.37.3