net: set SK_MEM_QUANTUM to 4096

Systems with large pages (64KB pages for example) do not always have
huge quantity of memory.

A big SK_MEM_QUANTUM value leads to fewer interactions with the
global counters (like tcp_memory_allocated) but might trigger
memory pressure much faster, giving suboptimal TCP performance
since windows are lowered to ridiculous values.

Note that sysctl_mem units being in pages and in ABI, we also need
to change sk_prot_mem_limits() accordingly.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -1162,11 +1162,6 @@ static inline void sk_enter_memory_pressure(struct sock *sk)
 	sk->sk_prot->enter_memory_pressure(sk);
 }
 
-static inline long sk_prot_mem_limits(const struct sock *sk, int index)
-{
-	return sk->sk_prot->sysctl_mem[index];
-}
-
 static inline long
 sk_memory_allocated(const struct sock *sk)
 {
@@ -1281,11 +1276,27 @@ int __sk_mem_schedule(struct sock *sk, int size, int kind);
 void __sk_mem_reduce_allocated(struct sock *sk, int amount);
 void __sk_mem_reclaim(struct sock *sk, int amount);
 
-#define SK_MEM_QUANTUM ((int)PAGE_SIZE)
+/* We used to have PAGE_SIZE here, but systems with 64KB pages
+ * do not necessarily have 16x time more memory than 4KB ones.
+ */
+#define SK_MEM_QUANTUM 4096
 #define SK_MEM_QUANTUM_SHIFT ilog2(SK_MEM_QUANTUM)
 #define SK_MEM_SEND	0
 #define SK_MEM_RECV	1
 
+/* sysctl_mem values are in pages, we convert them in SK_MEM_QUANTUM units */
+static inline long sk_prot_mem_limits(const struct sock *sk, int index)
+{
+	long val = sk->sk_prot->sysctl_mem[index];
+
+#if PAGE_SIZE > SK_MEM_QUANTUM
+	val <<= PAGE_SHIFT - SK_MEM_QUANTUM_SHIFT;
+#elif PAGE_SIZE < SK_MEM_QUANTUM
+	val >>= SK_MEM_QUANTUM_SHIFT - PAGE_SHIFT;
+#endif
+	return val;
+}
+
 static inline int sk_mem_pages(int amt)
 {
 	return (amt + SK_MEM_QUANTUM - 1) >> SK_MEM_QUANTUM_SHIFT;