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SubjectRe: discriminate single bit error hardware failure from slab corruption.
Dave Jones wrote:

>On Fri, Feb 03, 2006 at 02:44:52AM +0200, Avi Kivity wrote:
>
> > total += hweight8(data[offset+i] ^ POISON_FREE);
> >
> > > printk(" %02x", (unsigned char)data[offset + i]);
> > > }
> > > printk("\n");
> > >@@ -1019,6 +1023,18 @@ static void dump_line(char *data, int of
> > > }
> > > }
> > > printk("\n");
> > >+ switch (total) {
> > >+ case 0x36:
> > >+ case 0x6a:
> > >+ case 0x6f:
> > >+ case 0x81:
> > >+ case 0xac:
> > >+ case 0xd3:
> > >+ case 0xd5:
> > >+ case 0xea:
> > >+ printk (KERN_ERR "Single bit error detected.
> > >Possibly bad RAM. Please run memtest86.\n");
> > >+ return;
> > >+ }
> > >
> > >
> > and a
> >
> > if (total == 1)
> > printk(...);
> >
> > here? it seems more readable and more correct as well.
>
>More readable ? Are you kidding ?
>What I wrote is smack-you-in-the-face-obvious what it's doing.
>With your variant, I have to sit down and think it through.
>
>
Looks like we have mirror image brains :) - I had to scratch my scalp to
figure out where all the magic numbers in the switch came from.

Perhaps well named variables will help:

unsigned char modified_bits = data[offset+i] ^ POSION_FREE;
int modified_bits_count = hweight8(modified_bits);
total += modified_bits_count;

>wrt correctness, what do you see wrong with my approach?
>
>
Your code will generate a false positive 8 times in 256 runs, or 1 in
32. A 3% false positive rate seems excessive, It's also sensitive to
changes to POISON_FREE.

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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