linux-kernel - Re: data corruption: revalidating a (removable) hdd/flash on re-insert

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Date:	Tue, 4 Nov 2008 20:57:29 +0100
From:	Pavel Machek <pavel@...e.cz>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	Michael Tokarev <mjt@....msk.ru>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: data corruption: revalidating a (removable) hdd/flash on
	re-insert

On Fri 2008-10-31 17:10:26, Kay Sievers wrote:
> On Fri, Oct 31, 2008 at 16:38, Michael Tokarev <mjt@....msk.ru> wrote:
> > To make a long story short: is there a way to force kernel
> > to re-validate a replaced usb-connected hard drive (or a
> > flash) *automatically*?
> >
> > Because right now, the kernel does not see that the drive
> > has been replaced, and uses *some* old cached values, which
> > results in random data corruption here and there, and other
> > similar odd things.
> 
> Maybe your card reader is broken. I can not reproduce this with any of
> the many readers I have. Usually a media change results in media
> revalidation with the next access to the device. You can easily
> reproduce that:
> 
> Insert the media, and force a validation:
>   $ touch /dev/sdb
> 
> Start logging of the kernel uevents to the console:
>   $ udevadm monitor --kernel &
> 
> Access the device:
>   $ touch /dev/sdb
> 
> Nothing should happen, as the reader/kernel knows it is still valid.
> 
> Now remove the media and insert it immediately again.
> 
> Access the device:
>   $ touch /dev/sdb
>   UEVENT[1225468868.803950] change
> /devices/pci0000:00/0000:00:1d.7/usb5/5-2/5-2:1.0/host8/target8:0:0/8:0:0:0
> (scsi)
> 
> and you see the reader told to kernel (scsi unit attention) to
> revalidate the device.
> 
> These events happen only when the device is accessed. That's why
> distros poll removable devices for media changes.
> 
> Every access to removable media is guarded by this revalidation check.
> If you don't see these events, you should not trust this reader, and
> at least never change the media while it is connected.

This is rather nasty data-corrupter. Could we at least blacklist
broken device, and force revalidation on each close or something like
that?


-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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