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Commit bfcfaa77 authored by Linus Torvalds's avatar Linus Torvalds
Browse files

vfs: use 'unsigned long' accesses for dcache name comparison and hashing 



Ok, this is hacky, and only works on little-endian machines with goo
unaligned handling.  And even then only with CONFIG_DEBUG_PAGEALLOC
disabled, since it can access up to 7 bytes after the pathname.

But it runs like a bat out of hell.

Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 9f8050c4

arch/x86/Kconfig

+1 −0
Original line number Diff line number Diff line
@@ -82,6 +82,7 @@ config X86 
	select CLKEVT_I8253

	select ARCH_HAVE_NMI_SAFE_CMPXCHG

	select GENERIC_IOMAP

	select DCACHE_WORD_ACCESS if !DEBUG_PAGEALLOC



config INSTRUCTION_DECODER

	def_bool (KPROBES || PERF_EVENTS)

fs/Kconfig

+4 −0
Original line number Diff line number Diff line
@@ -4,6 +4,10 @@ 


menu "File systems"



# Use unaligned word dcache accesses

config DCACHE_WORD_ACCESS

       bool



if BLOCK



source "fs/ext2/Kconfig"

fs/dcache.c

+23 −0
Original line number Diff line number Diff line
@@ -144,6 +144,28 @@ int proc_nr_dentry(ctl_table *table, int write, void __user *buffer, 
static inline int dentry_cmp(const unsigned char *cs, size_t scount,

				const unsigned char *ct, size_t tcount)

{

#ifdef CONFIG_DCACHE_WORD_ACCESS

	unsigned long a,b,mask;



	if (unlikely(scount != tcount))

		return 1;



	for (;;) {

		a = *(unsigned long *)cs;

		b = *(unsigned long *)ct;

		if (tcount < sizeof(unsigned long))

			break;

		if (unlikely(a != b))

			return 1;

		cs += sizeof(unsigned long);

		ct += sizeof(unsigned long);

		tcount -= sizeof(unsigned long);

		if (!tcount)

			return 0;

	}

	mask = ~(~0ul << tcount*8);

	return unlikely(!!((a ^ b) & mask));

#else

	if (scount != tcount)

		return 1;
@@ -155,6 +177,7 @@ static inline int dentry_cmp(const unsigned char *cs, size_t scount, 
		tcount--;

	} while (tcount);

	return 0;

#endif

}



static void __d_free(struct rcu_head *head)

fs/namei.c

+122 −0
Original line number Diff line number Diff line
@@ -1374,6 +1374,126 @@ static inline int can_lookup(struct inode *inode) 
	return 1;

}



/*

 * We can do the critical dentry name comparison and hashing

 * operations one word at a time, but we are limited to:

 *

 * - Architectures with fast unaligned word accesses. We could

 *   do a "get_unaligned()" if this helps and is sufficiently

 *   fast.

 *

 * - Little-endian machines (so that we can generate the mask

 *   of low bytes efficiently). Again, we *could* do a byte

 *   swapping load on big-endian architectures if that is not

 *   expensive enough to make the optimization worthless.

 *

 * - non-CONFIG_DEBUG_PAGEALLOC configurations (so that we

 *   do not trap on the (extremely unlikely) case of a page

 *   crossing operation.

 *

 * - Furthermore, we need an efficient 64-bit compile for the

 *   64-bit case in order to generate the "number of bytes in

 *   the final mask". Again, that could be replaced with a

 *   efficient population count instruction or similar.

 */

#ifdef CONFIG_DCACHE_WORD_ACCESS



#ifdef CONFIG_64BIT



/*

 * Jan Achrenius on G+: microoptimized version of

 * the simpler "(mask & ONEBYTES) * ONEBYTES >> 56"

 * that works for the bytemasks without having to

 * mask them first.

 */

static inline long count_masked_bytes(unsigned long mask)

{

	return mask*0x0001020304050608 >> 56;

}



static inline unsigned int fold_hash(unsigned long hash)

{

	hash += hash >> (8*sizeof(int));

	return hash;

}



#else	/* 32-bit case */



/* Carl Chatfield / Jan Achrenius G+ version for 32-bit */

static inline long count_masked_bytes(long mask)

{

	/* (000000 0000ff 00ffff ffffff) -> ( 1 1 2 3 ) */

	long a = (0x0ff0001+mask) >> 23;

	/* Fix the 1 for 00 case */

	return a & mask;

}



#define fold_hash(x) (x)



#endif



unsigned int full_name_hash(const unsigned char *name, unsigned int len)

{

	unsigned long a, mask;

	unsigned long hash = 0;



	for (;;) {

		a = *(unsigned long *)name;

		hash *= 9;

		if (len < sizeof(unsigned long))

			break;

		hash += a;

		name += sizeof(unsigned long);

		len -= sizeof(unsigned long);

		if (!len)

			goto done;

	}

	mask = ~(~0ul << len*8);

	hash += mask & a;

done:

	return fold_hash(hash);

}

EXPORT_SYMBOL(full_name_hash);



#define ONEBYTES	0x0101010101010101ul

#define SLASHBYTES	0x2f2f2f2f2f2f2f2ful

#define HIGHBITS	0x8080808080808080ul



/* Return the high bit set in the first byte that is a zero */

static inline unsigned long has_zero(unsigned long a)

{

	return ((a - ONEBYTES) & ~a) & HIGHBITS;

}



/*

 * Calculate the length and hash of the path component, and

 * return the length of the component;

 */

static inline unsigned long hash_name(const char *name, unsigned int *hashp)

{

	unsigned long a, mask, hash, len;



	hash = a = 0;

	len = -sizeof(unsigned long);

	do {

		hash = (hash + a) * 9;

		len += sizeof(unsigned long);

		a = *(unsigned long *)(name+len);

		/* Do we have any NUL or '/' bytes in this word? */

		mask = has_zero(a) | has_zero(a ^ SLASHBYTES);

	} while (!mask);



	/* The mask *below* the first high bit set */

	mask = (mask - 1) & ~mask;

	mask >>= 7;

	hash += a & mask;

	*hashp = fold_hash(hash);



	return len + count_masked_bytes(mask);

}



#else



unsigned int full_name_hash(const unsigned char *name, unsigned int len)

{

	unsigned long hash = init_name_hash();
@@ -1402,6 +1522,8 @@ static inline unsigned long hash_name(const char *name, unsigned int *hashp) 
	return len;

}



#endif



/*

 * Name resolution.

 * This is the basic name resolution function, turning a pathname into