Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:8 as permitted sender) client-ip=2620:137:e000::3:8;
From:   Abel Wu <wuyun.abel@bytedance.com>
To:     "David S . Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Shakeel Butt <shakeelb@google.com>
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        Abel Wu <wuyun.abel@bytedance.com>
Subject: [PATCH net v3 2/3] sock: Doc behaviors for pressure heurisitics
Date:   Thu, 19 Oct 2023 20:00:25 +0800
Message-Id: <20231019120026.42215-2-wuyun.abel@bytedance.com>
In-Reply-To: <20231019120026.42215-1-wuyun.abel@bytedance.com>
References: <20231019120026.42215-1-wuyun.abel@bytedance.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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@google.com>
Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
Acked-by: Shakeel Butt <shakeelb@google.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