Commit e8a07c9d authored by David Howells's avatar David Howells
Browse files

fscache: Implement a hash function 



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: default avatarDavid Howells <dhowells@redhat.com>
Reviewed-by: default avatarJeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/CAHk-=whtkzB446+hX0zdLsdcUJsJ=8_-0S1mE_R+YurThfUbLA@mail.gmail.com [1]
Link: https://lore.kernel.org/r/163819586113.215744.1699465806130102367.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906888735.143852.10944614318596881429.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967082342.1823006.8915671045444488742.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021493624.640689.9990442668811178628.stgit@warthog.procyon.org.uk/ # v4
parent 1e1236b8

fs/fscache/internal.h

+2 −0
Original line number Diff line number Diff line
@@ -22,6 +22,8 @@ 
 */

extern unsigned fscache_debug;



extern unsigned int fscache_hash(unsigned int salt, const void *data, size_t len);



/*

 * proc.c

 */

fs/fscache/main.c

+40 −0
Original line number Diff line number Diff line
@@ -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

 */