[PATCH 5.14 144/432] fscache: Fix cookie key hashing

Greg Kroah-Hartman
2021-09-16T17:24:57Z

From: David Howells <dhowells@redhat.com>

[ Upstream commit 35b72573e977ed6b18b094136a4fa3e0ffb13603 ]

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/
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/fscache/cookie.c | 14 +-------------
fs/fscache/internal.h | 2 ++
fs/fscache/main.c | 39 +++++++++++++++++++++++++++++++++++++++
3 files changed, 42 insertions(+), 13 deletions(-)

diff --git a/fs/fscache/cookie.c b/fs/fscache/cookie.c
index 751bc5b1cddf..6104f627cc71 100644
--- a/fs/fscache/cookie.c
+++ b/fs/fscache/cookie.c
@@ -74,10 +74,8 @@ 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));

@@ -91,17 +89,7 @@ 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 c483863b740a..aee639d980ba 100644
--- a/fs/fscache/internal.h
+++ b/fs/fscache/internal.h
@@ -97,6 +97,8 @@ 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 c1e6cc9091aa..4207f98e405f 100644
--- 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
*/
--
2.30.2