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Subject[PATCH v3 07/68] fscache: Implement a hash function
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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. le32_to_cpu() is added to
make it endianness-independent.

Changes
=======
ver #3:
- Read the data being hashed in an endianness-independent way[1].
- Change the size parameter to be in bytes rather than words.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/CAHk-=whtkzB446+hX0zdLsdcUJsJ=8_-0S1mE_R+YurThfUbLA@mail.gmail.com [1]
Link: https://lore.kernel.org/r/163819586113.215744.1699465806130102367.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906888735.143852.10944614318596881429.stgit@warthog.procyon.org.uk/ # v2
---

fs/fscache/internal.h | 2 ++
fs/fscache/main.c | 40 ++++++++++++++++++++++++++++++++++++++++
2 files changed, 42 insertions(+)

diff --git a/fs/fscache/internal.h b/fs/fscache/internal.h
index ea52f8594a77..f345bdb018ba 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, const void *data, size_t len);
+
/*
* proc.c
*/
diff --git a/fs/fscache/main.c b/fs/fscache/main.c
index 819de2ee1276..687b34903d5b 100644
--- a/fs/fscache/main.c
+++ b/fs/fscache/main.c
@@ -24,6 +24,46 @@ 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 must guarantee that
+ * the source data is a multiple of four bytes in size.
+ */
+unsigned int fscache_hash(unsigned int salt, const void *data, size_t len)
+{
+ const __le32 *p = data;
+ unsigned int a, x = 0, y = salt, n = len / sizeof(__le32);
+
+ for (; n; n--) {
+ a = le32_to_cpu(*p++);
+ HASH_MIX(x, y, a);
+ }
+ return fold_hash(x, y);
+}
+
/*
* initialise the fs caching module
*/

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