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/

1 files changed, 1 insertions, 13 deletions

diff --git a/fs/fscache/cookie.c b/fs/fscache/cookie.c
index ec9bce33085f..2558814193e9 100644
--- 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;
 }