Date:	Sat, 28 May 2016 15:57:14 -0400
From:	George Spelvin <linux@...encehorizons.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	lkml <linux-kernel@...r.kernel.org>
Cc:	"J . Bruce Fields" <bfields@...hat.com>,
	George Spelvin <linux@...encehorizons.net>
Subject: [PATCH v3 01/10] Pull out string hash to <linux/stringhash.h>

... so they can be used without the rest of <linux/dcache.h>

The hashlen_* macros will make sense next patch.

Signed-off-by: George Spelvin <linux@...encehorizons.net>
---
 include/linux/dcache.h     | 27 +----------------
 include/linux/stringhash.h | 72 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 73 insertions(+), 26 deletions(-)
 create mode 100644 include/linux/stringhash.h

diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 7e9422cb..0f9a977c 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -10,6 +10,7 @@
 #include <linux/cache.h>
 #include <linux/rcupdate.h>
 #include <linux/lockref.h>
+#include <linux/stringhash.h>
 
 struct path;
 struct vfsmount;
@@ -52,9 +53,6 @@ struct qstr {
 };
 
 #define QSTR_INIT(n,l) { { { .len = l } }, .name = n }
-#define hashlen_hash(hashlen) ((u32) (hashlen))
-#define hashlen_len(hashlen)  ((u32)((hashlen) >> 32))
-#define hashlen_create(hash,len) (((u64)(len)<<32)|(u32)(hash))
 
 struct dentry_stat_t {
 	long nr_dentry;
@@ -65,29 +63,6 @@ struct dentry_stat_t {
 };
 extern struct dentry_stat_t dentry_stat;
 
-/* Name hashing routines. Initial hash value */
-/* Hash courtesy of the R5 hash in reiserfs modulo sign bits */
-#define init_name_hash()		0
-
-/* partial hash update function. Assume roughly 4 bits per character */
-static inline unsigned long
-partial_name_hash(unsigned long c, unsigned long prevhash)
-{
-	return (prevhash + (c << 4) + (c >> 4)) * 11;
-}
-
-/*
- * Finally: cut down the number of bits to a int value (and try to avoid
- * losing bits)
- */
-static inline unsigned long end_name_hash(unsigned long hash)
-{
-	return (unsigned int) hash;
-}
-
-/* Compute the hash for a name string. */
-extern unsigned int full_name_hash(const unsigned char *, unsigned int);
-
 /*
  * Try to keep struct dentry aligned on 64 byte cachelines (this will
  * give reasonable cacheline footprint with larger lines without the
diff --git a/include/linux/stringhash.h b/include/linux/stringhash.h
new file mode 100644
index 00000000..2eaaaf6d
--- /dev/null
+++ b/include/linux/stringhash.h
@@ -0,0 +1,72 @@
+#ifndef __LINUX_STRINGHASH_H
+#define __LINUX_STRINGHASH_H
+
+#include <linux/types.h>
+
+/*
+ * Routines for hashing strings of bytes to a 32-bit hash value.
+ *
+ * These hash functions are NOT GUARANTEED STABLE between kernel
+ * versions, architectures, or even repeated boots of the same kernel.
+ * (E.g. they may depend on boot-time hardware detection or be
+ * deliberately randomized.)
+ *
+ * They are also not intended to be secure against collisions caused by
+ * malicious inputs; much slower hash functions are required for that.
+ *
+ * They are optimized for pathname components, meaning short strings.
+ * Even if a majority of files have longer names, the dynamic profile of
+ * pathname components skews short due to short directory names.
+ * (E.g. /usr/lib/libsesquipedalianism.so.3.141.)
+ */
+
+/*
+ * Version 1: one byte at a time.  Example of use:
+ *
+ * unsigned long hash = init_name_hash;
+ * while (*p)
+ *	hash = partial_name_hash(tolower(*p++), hash);
+ * hash = end_name_hash(hash);
+ *
+ * Although this is designed for bytes, fs/hfsplus/unicode.c
+ * abuses it to hash 16-bit values.
+ */
+
+/* Hash courtesy of the R5 hash in reiserfs modulo sign bits */
+#define init_name_hash()		0
+
+/* partial hash update function. Assume roughly 4 bits per character */
+static inline unsigned long
+partial_name_hash(unsigned long c, unsigned long prevhash)
+{
+	return (prevhash + (c << 4) + (c >> 4)) * 11;
+}
+
+/*
+ * Finally: cut down the number of bits to a int value (and try to avoid
+ * losing bits)
+ */
+static inline unsigned long end_name_hash(unsigned long hash)
+{
+	return (unsigned int)hash;
+}
+
+/*
+ * Version 2: One word (32 or 64 bits) at a time.
+ * If CONFIG_DCACHE_WORD_ACCESS is defined (meaning <asm/word-at-a-time.h>
+ * exists, which describes major Linux platforms like x86 and ARM), then
+ * this computes a different hash function much faster.
+ *
+ * If not set, this falls back to a wrapper around the preceding.
+ */
+extern unsigned int full_name_hash(const unsigned char *, unsigned int);
+
+/*
+ * A hash_len is a u64 with the hash of a string in the low
+ * half and the length in the high half.
+ */
+#define hashlen_hash(hashlen) ((u32)(hashlen))
+#define hashlen_len(hashlen)  ((u32)((hashlen) >> 32))
+#define hashlen_create(hash, len) ((u64)(len)<<32 | (u32)(hash))
+
+#endif	/* __LINUX_STRINGHASH_H */
-- 
2.8.1

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