--- zzzz-none-000/linux-3.10.107/include/linux/hash.h	2017-06-27 09:49:32.000000000 +0000
+++ scorpion-7490-727/linux-3.10.107/include/linux/hash.h	2021-02-04 17:41:59.000000000 +0000
@@ -32,10 +32,29 @@
 #error Wordsize not 32 or 64
 #endif
 
+/*
+ * The above primes are actively bad for hashing, since they are
+ * too sparse. The 32-bit one is mostly ok, the 64-bit one causes
+ * real problems. Besides, the "prime" part is pointless for the
+ * multiplicative hash.
+ *
+ * Although a random odd number will do, it turns out that the golden
+ * ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice
+ * properties.
+ *
+ * These are the negative, (1 - phi) = (phi^2) = (3 - sqrt(5))/2.
+ * (See Knuth vol 3, section 6.4, exercise 9.)
+ */
+#define GOLDEN_RATIO_32 0x61C88647
+#define GOLDEN_RATIO_64 0x61C8864680B583EBull
+
 static __always_inline u64 hash_64(u64 val, unsigned int bits)
 {
 	u64 hash = val;
 
+#if BITS_PER_LONG == 64
+	hash = hash * GOLDEN_RATIO_64;
+#else
 	/*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
 	u64 n = hash;
 	n <<= 18;
@@ -50,6 +69,7 @@
 	hash += n;
 	n <<= 2;
 	hash += n;
+#endif
 
 	/* High bits are more random, so use them. */
 	return hash >> (64 - bits);
@@ -78,4 +98,5 @@
 #endif
 	return (u32)val;
 }
+
 #endif /* _LINUX_HASH_H */