Re: 2.4.X inode cache bug

["Quim K Holland" <qkholland@my-deja.com>]

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In <200101310202.EAA11357@smtp2.kolumbus.fi>, jukka.santala@kolumbus.fi
writes:

> Excuse me for the lack of patch in this mail,...
> In 2.4.x, linux/fs/inode.c has a hash() function with a small
> slip-up.  The inode hash-value is initialized with "unsigned
> long tmp = i_ino | ((unsigned long) sb / L1_CACHE_BYTES);".
> ... I believe this is a
> slip-up, because you should NEVER use bitwise-or in a hash
> formula. This creates a slanted distribution, and depending on
> the address of the superblock block, can cause severe
> inefficiency in the code.
> Just replacing the | with ^ imroves hash-table efficiency
> noticeably,...

Then maybe the attached patch is what you want?  This also replaces
`+' on the next line with `^' to avoid slanted distribution.


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--- 2.4.1/fs/inode.c	Mon Jan 15 18:20:14 2001
+++ 2.4.1/fs/inode.c	Tue Jan 30 22:54:42 2001
@@ -728,8 +728,8 @@
 
 static inline unsigned long hash(struct super_block *sb, unsigned long i_ino)
 {
-	unsigned long tmp = i_ino | ((unsigned long) sb / L1_CACHE_BYTES);
-	tmp = tmp + (tmp >> I_HASHBITS) + (tmp >> I_HASHBITS*2);
+	unsigned long tmp = i_ino ^ ((unsigned long) sb / L1_CACHE_BYTES);
+	tmp = tmp ^ (tmp >> I_HASHBITS) ^ (tmp >> I_HASHBITS*2);
 	return tmp & I_HASHMASK;
 }