linux-kernel - Re: single bit flip detector.

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Message-ID: <20060801232724.GB5738@redhat.com>
Date:	Tue, 1 Aug 2006 19:27:24 -0400
From:	Dave Jones <davej@...hat.com>
To:	Alexey Dobriyan <adobriyan@...il.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: single bit flip detector.

On Tue, Aug 01, 2006 at 07:16:03PM -0400, Dave Jones wrote:
 > On Wed, Aug 02, 2006 at 03:00:03AM +0400, Alexey Dobriyan wrote:
 > 
 >  > Turn on CONFIG_DEBUG_SLAB before compiling. ;-)
 > 
 > Well, that was silly.   Here's a properly compile tested patch :-)

The grammar police found me, here's hopefully the final rendition..

		Dave


In case where we detect a single bit has been flipped, we spew
the usual slab corruption message, which users instantly think
is a kernel bug.  In a lot of cases, single bit errors are
down to bad memory, or other hardware failure.

This patch adds an extra line to the slab debug messages
in those cases, in the hope that users will try memtest before
they report a bug.

000: 6b 6b 6b 6b 6a 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b
Single bit error detected. Probably bad RAM.

Signed-off-by: Dave Jones <davej@...hat.com>

diff --git a/mm/slab.c b/mm/slab.c
index 21ba060..39f1183 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -1638,10 +1638,29 @@ static void poison_obj(struct kmem_cache
 static void dump_line(char *data, int offset, int limit)
 {
 	int i;
+	unsigned char total = 0, bad_count = 0, errors = 0;
 	printk(KERN_ERR "%03x:", offset);
-	for (i = 0; i < limit; i++)
+	for (i = 0; i < limit; i++) {
+		if (data[offset + i] != POISON_FREE) {
+			total += data[offset + i];
+			bad_count++;
+		}
 		printk(" %02x", (unsigned char)data[offset + i]);
+	}
 	printk("\n");
+
+	if (bad_count == 1) {
+		errors = total ^ POISON_FREE;
+		if (errors && !(errors & (errors-1))) {
+			printk (KERN_ERR "Single bit error detected. Probably bad RAM.\n");
+#ifdef CONFIG_X86
+			printk (KERN_ERR "Run memtest86+ or a similar memory test tool.\n");
+#else
+			printk (KERN_ERR "Run a memory test tool.\n");
+#endif
+			return;
+		}
+	}
 }
 #endif
 

-- 
http://www.codemonkey.org.uk
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