vfs: use 'unsigned long' accesses for dcache name comparison and hashing

Ok, this is hacky, and only works on little-endian machines with goo
unaligned handling.  And even then only with CONFIG_DEBUG_PAGEALLOC
disabled, since it can access up to 7 bytes after the pathname.

But it runs like a bat out of hell.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 122 insertions, 0 deletions

diff --git a/fs/namei.c b/fs/namei.c
index e2ba62820a0f..378497a744b4 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1374,6 +1374,126 @@ static inline int can_lookup(struct inode *inode)
         return 1;
 }
 
+/*
+ * We can do the critical dentry name comparison and hashing
+ * operations one word at a time, but we are limited to:
+ *
+ * - Architectures with fast unaligned word accesses. We could
+ *   do a "get_unaligned()" if this helps and is sufficiently
+ *   fast.
+ *
+ * - Little-endian machines (so that we can generate the mask
+ *   of low bytes efficiently). Again, we *could* do a byte
+ *   swapping load on big-endian architectures if that is not
+ *   expensive enough to make the optimization worthless.
+ *
+ * - non-CONFIG_DEBUG_PAGEALLOC configurations (so that we
+ *   do not trap on the (extremely unlikely) case of a page
+ *   crossing operation.
+ *
+ * - Furthermore, we need an efficient 64-bit compile for the
+ *   64-bit case in order to generate the "number of bytes in
+ *   the final mask". Again, that could be replaced with a
+ *   efficient population count instruction or similar.
+ */
+#ifdef CONFIG_DCACHE_WORD_ACCESS
+
+#ifdef CONFIG_64BIT
+
+/*
+ * Jan Achrenius on G+: microoptimized version of
+ * the simpler "(mask & ONEBYTES) * ONEBYTES >> 56"
+ * that works for the bytemasks without having to
+ * mask them first.
+ */
+static inline long count_masked_bytes(unsigned long mask)
+{
+        return mask*0x0001020304050608 >> 56;
+}
+
+static inline unsigned int fold_hash(unsigned long hash)
+{
+        hash += hash >> (8*sizeof(int));
+        return hash;
+}
+
+#else   /* 32-bit case */
+
+/* Carl Chatfield / Jan Achrenius G+ version for 32-bit */
+static inline long count_masked_bytes(long mask)
+{
+        /* (000000 0000ff 00ffff ffffff) -> ( 1 1 2 3 ) */
+        long a = (0x0ff0001+mask) >> 23;
+        /* Fix the 1 for 00 case */
+        return a & mask;
+}
+
+#define fold_hash(x) (x)
+
+#endif
+
+unsigned int full_name_hash(const unsigned char *name, unsigned int len)
+{
+        unsigned long a, mask;
+        unsigned long hash = 0;
+
+        for (;;) {
+                a = *(unsigned long *)name;
+                hash *= 9;
+                if (len < sizeof(unsigned long))
+                        break;
+                hash += a;
+                name += sizeof(unsigned long);
+                len -= sizeof(unsigned long);
+                if (!len)
+                        goto done;
+        }
+        mask = ~(~0ul << len*8);
+        hash += mask & a;
+done:
+        return fold_hash(hash);
+}
+EXPORT_SYMBOL(full_name_hash);
+
+#define ONEBYTES        0x0101010101010101ul
+#define SLASHBYTES      0x2f2f2f2f2f2f2f2ful
+#define HIGHBITS        0x8080808080808080ul
+
+/* Return the high bit set in the first byte that is a zero */
+static inline unsigned long has_zero(unsigned long a)
+{
+        return ((a - ONEBYTES) & ~a) & HIGHBITS;
+}
+
+/*
+ * Calculate the length and hash of the path component, and
+ * return the length of the component;
+ */
+static inline unsigned long hash_name(const char *name, unsigned int *hashp)
+{
+        unsigned long a, mask, hash, len;
+
+        hash = a = 0;
+        len = -sizeof(unsigned long);
+        do {
+                hash = (hash + a) * 9;
+                len += sizeof(unsigned long);
+                a = *(unsigned long *)(name+len);
+                /* Do we have any NUL or '/' bytes in this word? */
+                mask = has_zero(a) | has_zero(a ^ SLASHBYTES);
+        } while (!mask);
+
+        /* The mask *below* the first high bit set */
+        mask = (mask - 1) & ~mask;
+        mask >>= 7;
+        hash += a & mask;
+        *hashp = fold_hash(hash);
+
+        return len + count_masked_bytes(mask);
+}
+
+#else
+
 unsigned int full_name_hash(const unsigned char *name, unsigned int len)
 {
         unsigned long hash = init_name_hash();
@@ -1402,6 +1522,8 @@ static inline unsigned long hash_name(const char *name, unsigned int *hashp)
         return len;
 }
 
+#endif
+
 /*
  * Name resolution.
  * This is the basic name resolution function, turning a pathname into