Merge branch 'salted-string-hash'

This changes the vfs dentry hashing to mix in the parent pointer at the
_beginning_ of the hash, rather than at the end.

That actually improves both the hash and the code generation, because we
can move more of the computation to the "static" part of the dcache
setup, and do less at lookup runtime.

It turns out that a lot of other hash users also really wanted to mix in
a base pointer as a 'salt' for the hash, and so the slightly extended
interface ends up working well for other cases too.

Users that want a string hash that is purely about the string pass in a
'salt' pointer of NULL.

* merge branch 'salted-string-hash':
  fs/dcache.c: Save one 32-bit multiply in dcache lookup
  vfs: make the string hashes salt the hash

2 files changed, 9 insertions, 7 deletions

diff --git a/include/linux/stringhash.h b/include/linux/stringhash.h
index 451771d9b9c0..7c2d95170d01 100644
--- a/include/linux/stringhash.h
+++ b/include/linux/stringhash.h
@@ -3,6 +3,7 @@
 
 #include <linux/compiler.h>	/* For __pure */
 #include <linux/types.h>	/* For u32, u64 */
+#include <linux/hash.h>
 
 /*
  * Routines for hashing strings of bytes to a 32-bit hash value.
@@ -34,7 +35,7 @@
  */
 
 /* Hash courtesy of the R5 hash in reiserfs modulo sign bits */
-#define init_name_hash()		0
+#define init_name_hash(salt)		(unsigned long)(salt)
 
 /* partial hash update function. Assume roughly 4 bits per character */
 static inline unsigned long
@@ -45,11 +46,12 @@ partial_name_hash(unsigned long c, unsigned long prevhash)
 
 /*
  * Finally: cut down the number of bits to a int value (and try to avoid
- * losing bits)
+ * losing bits).  This also has the property (wanted by the dcache)
+ * that the msbits make a good hash table index.
  */
 static inline unsigned long end_name_hash(unsigned long hash)
 {
-	return (unsigned int)hash;
+	return __hash_32((unsigned int)hash);
 }
 
 /*
@@ -60,7 +62,7 @@ static inline unsigned long end_name_hash(unsigned long hash)
  *
  * If not set, this falls back to a wrapper around the preceding.
  */
-extern unsigned int __pure full_name_hash(const char *, unsigned int);
+extern unsigned int __pure full_name_hash(const void *salt, const char *, unsigned int);
 
 /*
  * A hash_len is a u64 with the hash of a string in the low
@@ -71,6 +73,6 @@ extern unsigned int __pure full_name_hash(const char *, unsigned int);
 #define hashlen_create(hash, len) ((u64)(len)<<32 | (u32)(hash))
 
 /* Return the "hash_len" (hash and length) of a null-terminated string */
-extern u64 __pure hashlen_string(const char *name);
+extern u64 __pure hashlen_string(const void *salt, const char *name);
 
 #endif	/* __LINUX_STRINGHASH_H */
diff --git a/include/linux/sunrpc/svcauth.h b/include/linux/sunrpc/svcauth.h
index 91d5a5d6f52b..d03932055328 100644
--- a/include/linux/sunrpc/svcauth.h
+++ b/include/linux/sunrpc/svcauth.h
@@ -172,12 +172,12 @@ extern void unix_gid_cache_destroy(struct net *net);
  */
 static inline unsigned long hash_str(char const *name, int bits)
 {
-	return hashlen_hash(hashlen_string(name)) >> (32 - bits);
+	return hashlen_hash(hashlen_string(NULL, name)) >> (32 - bits);
 }
 
 static inline unsigned long hash_mem(char const *buf, int length, int bits)
 {
-	return full_name_hash(buf, length) >> (32 - bits);
+	return full_name_hash(NULL, buf, length) >> (32 - bits);
 }
 
 #endif /* __KERNEL__ */