[PATCH v2 07/67] fscache: Implement a hash function

[David Howells <dhowells@redhat.com>]

Implement a function to generate hashes.  It needs to be stable over time
and endianness-independent as the hashes will appear on disk in future
patches.  It can assume that its input is a multiple of four bytes in size
and alignment.

This is borrowed from the VFS and simplified.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819586113.215744.1699465806130102367.stgit@warthog.procyon.org.uk/ # v1
---

 fs/fscache/internal.h |    2 ++
 fs/fscache/main.c     |   39 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/fs/fscache/internal.h b/fs/fscache/internal.h
index ea52f8594a77..64767992bd15 100644
--- a/fs/fscache/internal.h
+++ b/fs/fscache/internal.h
@@ -22,6 +22,8 @@
  */
 extern unsigned fscache_debug;
 
+extern unsigned int fscache_hash(unsigned int salt, unsigned int *data, unsigned int n);
+
 /*
  * proc.c
  */
diff --git a/fs/fscache/main.c b/fs/fscache/main.c
index 819de2ee1276..a4afba1b9d3b 100644
--- a/fs/fscache/main.c
+++ b/fs/fscache/main.c
@@ -24,6 +24,45 @@ MODULE_PARM_DESC(fscache_debug,
 struct workqueue_struct *fscache_wq;
 EXPORT_SYMBOL(fscache_wq);
 
+/*
+ * Mixing scores (in bits) for (7,20):
+ * Input delta: 1-bit      2-bit
+ * 1 round:     330.3     9201.6
+ * 2 rounds:   1246.4    25475.4
+ * 3 rounds:   1907.1    31295.1
+ * 4 rounds:   2042.3    31718.6
+ * Perfect:    2048      31744
+ *            (32*64)   (32*31/2 * 64)
+ */
+#define HASH_MIX(x, y, a)	\
+	(	x ^= (a),	\
+	y ^= x,	x = rol32(x, 7),\
+	x += y,	y = rol32(y,20),\
+	y *= 9			)
+
+static inline unsigned int fold_hash(unsigned long x, unsigned long y)
+{
+	/* Use arch-optimized multiply if one exists */
+	return __hash_32(y ^ __hash_32(x));
+}
+
+/*
+ * Generate a hash.  This is derived from full_name_hash(), but we want to be
+ * sure it is arch independent and that it doesn't change as bits of the
+ * computed hash value might appear on disk.  The caller also guarantees that
+ * the hashed data will be a series of aligned 32-bit words.
+ */
+unsigned int fscache_hash(unsigned int salt, unsigned int *data, unsigned int n)
+{
+	unsigned int a, x = 0, y = salt;
+
+	for (; n; n--) {
+		a = *data++;
+		HASH_MIX(x, y, a);
+	}
+	return fold_hash(x, y);
+}
+
 /*
  * initialise the fs caching module
  */