fscache: Fix cookie key hashing

The current hash algorithm used for hashing cookie keys is really bad,
producing almost no dispersion (after a test kernel build, ~30000 files
were split over just 18 out of the 32768 hash buckets).

Borrow the full_name_hash() hash function into fscache to do the hashing
for cookie keys and, in the future, volume keys.

I don't want to use full_name_hash() as-is because I want the hash value to
be consistent across arches and over time as the hash value produced may
get used on disk.

I can also optimise parts of it away as the key will always be a padded
array of aligned 32-bit words.

Fixes: ec0328e46d6e ("fscache: Maintain a catalogue of allocated cookies")
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/162431201844.2908479.8293647220901514696.stgit@warthog.procyon.org.uk/

diff --git a/fs/fscache/cookie.c b/fs/fscache/cookie.c
index ec9bce3..2558814 100644 (file)
--- a/fs/fscache/cookie.c
+++ b/fs/fscache/cookie.c
@@ -87,10 +87,8 @@ void fscache_free_cookie(struct fscache_cookie *cookie)
 static int fscache_set_key(struct fscache_cookie *cookie,
                           const void *index_key, size_t index_key_len)
 {
-       unsigned long long h;
        u32 *buf;
        int bufs;
-       int i;
 
        bufs = DIV_ROUND_UP(index_key_len, sizeof(*buf));
 
@@ -104,17 +102,7 @@ static int fscache_set_key(struct fscache_cookie *cookie,
        }
 
        memcpy(buf, index_key, index_key_len);
-
-       /* Calculate a hash and combine this with the length in the first word
-        * or first half word
-        */
-       h = (unsigned long)cookie->parent;
-       h += index_key_len + cookie->type;
-
-       for (i = 0; i < bufs; i++)
-               h += buf[i];
-
-       cookie->key_hash = h ^ (h >> 32);
+       cookie->key_hash = fscache_hash(0, buf, bufs);
        return 0;
 }
 

diff --git a/fs/fscache/internal.h b/fs/fscache/internal.h
index 200082c..a49136c 100644 (file)
--- a/fs/fscache/internal.h
+++ b/fs/fscache/internal.h
@@ -74,6 +74,8 @@ extern struct workqueue_struct *fscache_object_wq;
 extern struct workqueue_struct *fscache_op_wq;
 DECLARE_PER_CPU(wait_queue_head_t, fscache_object_cong_wait);
 
+extern unsigned int fscache_hash(unsigned int salt, unsigned int *data, unsigned int n);
+
 static inline bool fscache_object_congested(void)
 {
        return workqueue_congested(WORK_CPU_UNBOUND, fscache_object_wq);

diff --git a/fs/fscache/main.c b/fs/fscache/main.c
index c1e6cc9..4207f98 100644 (file)
--- a/fs/fscache/main.c
+++ b/fs/fscache/main.c
@@ -93,6 +93,45 @@ static struct ctl_table fscache_sysctls_root[] = {
 };
 #endif
 
+/*
+ * 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
  */