Re: discriminate single bit error hardware failure from slab corruption.

[Avi Kivity]

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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