Fixing random corruption in raw IO on 2.2.x kernel with bigmem enabled

Fixing random corruption in raw IO on 2.2.x kernel with bigmem enabled

[Peng Dai]

[-- Attachment #1: Type: text/plain, Size: 1460 bytes --]

Hi,

This patch fixes a subtle corruption when doing raw IO on the 2.2.x
kernel
with bigmem enabled. The problem was first reported by Markus Döhr while

running SAP DB on a variation of the 2.2.16 kernel with among others the

following patches installed,

> linux-2.2.16-rawio.patch
> linux-2.2.16-raw-fixup.patch
> linux-2.2.16-raw-fixup2.patch
> linux-2.2.16-bigmem.patch
> linux-2.2.16-bigmem-raw.patch
> linux-2.2.16-bigmem-dcache.patch
> linux-2.2.16-bigmem-initrd.patch

See http://marc.theaimsgroup.com/?l=linux-kernel&m=97038067229365&w=2
for the original description.

The corruption is caused by a bug in kiobuf_copy_bounce in fs/iobuf.c -
the
bigmem page containing the user data is not always copied to its
associated
bounce buffer. This would occur when an array of pages (>1) are passed
to
kiobuf_copy_bounce and a normal page happens to appear before a bigmem
page in the array, which in turn leads to the writing of uninitialized
(garbage)
pages to the raw device.

The corruption happens rather infrequently. It has been reproduced both
with
the SAP DB and without. It appears that intense paging activity
increases its
chance of occurrence.

Regards,

Peng Dai
Kernel Engineering                                          Tel:
978-446-9166 ext. 276
Mission Critical Linux Inc.                              Fax:
978-446-9470
100 Foot of John                                               Email:
dai@missioncriticallinux.com
Lowell, MA 01852


[-- Attachment #2: iobuf.c.patch --]
[-- Type: text/plain, Size: 485 bytes --]

--- linux/fs/iobuf.c.orig	Tue Nov 28 13:54:26 2000
+++ linux/fs/iobuf.c	Tue Nov 28 13:55:17 2000
@@ -213,10 +213,10 @@
 		unsigned long kin, kout;
 		int pagelen = length;
 		
-		if (bounce_page) {
-			if ((pagelen+offset) > PAGE_SIZE)
-				pagelen = PAGE_SIZE - offset;
-		
+		if ((pagelen+offset) > PAGE_SIZE)
+			pagelen = PAGE_SIZE - offset;
+
+		if (bounce_page) {		
 			if (direction == COPY_TO_BOUNCE) {
 				kin  = kmap(page, KM_READ);
 				kout = kmap(bounce_page, KM_WRITE);

Re: Fixing random corruption in raw IO on 2.2.x kernel with bigmem enabled

[Stephen C. Tweedie]

Hi,

On Wed, Dec 06, 2000 at 12:28:54PM -0500, Peng Dai wrote:
> 
> This patch fixes a subtle corruption when doing raw IO on the 2.2.x
> kernel
> with bigmem enabled. The problem was first reported by Markus Döhr while

That patch is already part of the full bugfixed raw IO patchset I
posted out a few days ago.  Look for kiobuf-2.2.18pre24-B.tar.gz in

	ftp.uk.linux.org:/pub/linux/sct/fs/raw-io
or	ftp.*.kernel.org:/pub/linux/kernel/people/sct/raw-io

Cheers,
 Stephen
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