Re: Error testing ext3 on brd ramdisk

[Jorge Boncompte [DTI2]]

Nick Piggin escribió:
> On Mon, Mar 02, 2009 at 06:42:18PM +0100, Jorge Boncompte [DTI2] wrote:
> > Nick Piggin escribió:
> > > On Fri, Feb 27, 2009 at 07:08:46PM +0100, Jorge Boncompte [DTI2] wrote:
> > > > 	Hi,
> > > >
> > > > 	I have added Nick Piggin to the CC: as maintainer of the brd driver.
> > > >
> > > > 	After switching an embedded distribution that /etc on a ramdisk 
> > > > 	based minix filesystem from 2.6.23.17 to 2.6.29-rcX i am too getting 
> > > > 	errors ant the filesystem is corrupted. Does not happen always. The 
> > > > visible effect with text files after reboot is getting the old version of 
> > > > the file and "\0"'s at the end.
> > > >
> > > > 	Did you found a solution?
> > > What architectures are you using? It's possible that brd is missing
> > > a cacheflush. I test it pretty heavily on x86 and no problems, so
> > > this might point to an arch specific problem.
> > >
> > > ---
> > > drivers/block/brd.c |    4 +++-
> > > 1 file changed, 3 insertions(+), 1 deletion(-)
> > >
> > > Index: linux-2.6/drivers/block/brd.c
> > > ===================================================================
> > > --- linux-2.6.orig/drivers/block/brd.c
> > > +++ linux-2.6/drivers/block/brd.c
> > > @@ -275,8 +275,10 @@ static int brd_do_bvec(struct brd_device
> > > 	if (rw == READ) {
> > > 		copy_from_brd(mem + off, brd, sector, len);
> > > 		flush_dcache_page(page);
> > > -	} else
> > > +	} else {
> > > +		flush_dcache_page(page);
> > > 		copy_to_brd(brd, mem + off, sector, len);
> > > +	}
> > > 	kunmap_atomic(mem, KM_USER0);
> > >
> > > out:
> > 	Hi, I am on 32bits x86, 2 x Xeon with HT CPUs, but I have seen the 
> > 	same corruption on a KVM/QEMU guest with single emulated CPU.
> >
> > 	With your patch on top of vanilla 2.6.29-rc3+plus some networking 
> > patches I still get corruption sometimes.
> >
> > 	The script that saves the configuration does...
> >
> > ------------
> > mount -no remount,ro /dev/ram0
> > dd if=/dev/ram0 of=config.bin bs=1k count=1000
> > mount -no remount,rw /dev/ram0
> > md5sum config.bin
> > dd if=config.bin of=/dev/hda1
> > echo $md5sum | dd of=/dev/hda1 bs=1k seek=1100 count=32
> > ------------
> >
> > on system boot
> >
> > ------------
> > CHECK MD5SUM
> > dd if=/dev/hda1 of=/dev/ram0 bs=1k count=1000
> > fsck.minix -a /dev/ram0
> > mount -nt minix /dev/ram0 /etc -o rw
> > ------------
> >
> > 	I have never seen a MD5 failure on boot, just sometimes the 
> > 	filesystem is corrupted. Kernel config attached.
>
> Hi Jorge,
>
> Well I found and fixed something :) (see other mail) but I don't know
> whether that applies to you here if you're running with a single CPU
> and no preemption. But still, it might be worth trying that patch? I'm

	I first saw the corruption on the 2 x Xeon system. I'll try it and let 
you know.

> sorry I'm still unable to reproduce a problem with your script
> (although you don't describe how you create the filesystem before
> you remount it).

	If the MD5Sum check fails the startup script copy a templated image of 
the filesystem contained on a file on another partition to the ramdisk.

> > From your description, it suggests that the corrupted image is being
> read from /dev/ram0 (becuase the md5sum passes).

	No, it is read from /dev/hda1.

> In your script, can you run fsck.minix on config.bin when you first
> create it? What if you unmount /dev/ram0 before copying the image?

	Yesterday I did some tests and found that doing...

-----------
umount /etc (/etc is what is mounted from /dev/ram0)
dd if=/dev/zero of=/dev/ram0 bs=1k count=1000
mount /dev/ram0 /etc -t minix -o rw
-----------
...succeds and mounts a corrupted filesystem with the old content. Doing 
the same with the all ramdisk driver fails on mount with "no filesystem 
found".

If I do...
-----------
umount /etc (/etc is what is mounted from /dev/ram0)
echo 3 > /proc/sys/vm/drop_caches
dd if=/dev/zero of=/dev/ram0 bs=1k count=1000
mount /dev/ram0 /etc -t minix -o rw
----------
... then the mount fails with no filesystem found as it should.

	Does this ring any bell? :-)

	Regards,

	Jorge

--
==============================================================
Jorge Boncompte - Ingenieria y Gestion de RED
DTI2 - Desarrollo de la Tecnologia de las Comunicaciones
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Tlf: +34 957 761395 / FAX: +34 957 450380
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