Implement a function to generate hashes. It needs to be stable over time and endianness-independent as the hashes will appear on disk in future patches. It can assume that its input is a multiple of four bytes in size and alignment. This is borrowed from the VFS and simplified. le32_to_cpu() is added to make it endianness-independent. Changes ======= ver #3: - Read the data being hashed in an endianness-independent way[1]. - Change the size parameter to be in bytes rather than words. Signed-off-by: David Howells <dhowells@xxxxxxxxxx> cc: linux-cachefs@xxxxxxxxxx Link: https://lore.kernel.org/r/CAHk-=whtkzB446+hX0zdLsdcUJsJ=8_-0S1mE_R+YurThfUbLA@xxxxxxxxxxxxxx [1] Link: https://lore.kernel.org/r/163819586113.215744.1699465806130102367.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v1 Link: https://lore.kernel.org/r/163906888735.143852.10944614318596881429.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v2 --- fs/fscache/internal.h | 2 ++ fs/fscache/main.c | 40 ++++++++++++++++++++++++++++++++++++++++ 2 files changed, 42 insertions(+) diff --git a/fs/fscache/internal.h b/fs/fscache/internal.h index ea52f8594a77..f345bdb018ba 100644 --- a/fs/fscache/internal.h +++ b/fs/fscache/internal.h @@ -22,6 +22,8 @@ */ extern unsigned fscache_debug; +extern unsigned int fscache_hash(unsigned int salt, const void *data, size_t len); + /* * proc.c */ diff --git a/fs/fscache/main.c b/fs/fscache/main.c index 819de2ee1276..687b34903d5b 100644 --- a/fs/fscache/main.c +++ b/fs/fscache/main.c @@ -24,6 +24,46 @@ MODULE_PARM_DESC(fscache_debug, struct workqueue_struct *fscache_wq; EXPORT_SYMBOL(fscache_wq); +/* + * 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 must guarantee that + * the source data is a multiple of four bytes in size. + */ +unsigned int fscache_hash(unsigned int salt, const void *data, size_t len) +{ + const __le32 *p = data; + unsigned int a, x = 0, y = salt, n = len / sizeof(__le32); + + for (; n; n--) { + a = le32_to_cpu(*p++); + HASH_MIX(x, y, a); + } + return fold_hash(x, y); +} + /* * initialise the fs caching module */
