Re: Wrong DIF guard tag on ext2 write

[Chris Mason <chris.mason@oracle.com>]

On Wed, Jun 02, 2010 at 04:46:49AM +1000, Nick Piggin wrote:
> On Tue, Jun 01, 2010 at 02:09:05PM -0400, Chris Mason wrote:
> > On Tue, Jun 01, 2010 at 04:54:53PM +0000, James Bottomley wrote:
> > 
> > > For self
> > > induced errors (as long as we can detect them) I think we can just
> > > forget about it ... if the changed page is important, the I/O request
> > > gets repeated (modulo the problem of too great a frequency of changes
> > > leading to us never successfully writing it) or it gets dropped because
> > > the file was truncated or the data deleted for some other reason.
> > 
> > Sorry, how can we tell the errors that are self induced from the evil
> > bit flipping cable induced errors?
> 
> Block layer should retry it with bounce pages. That would be a lot nicer
> than forcing all upper layers to avoid the problem.
> 

So the idea is that we have sent down a buffer and it changed in flight.
The block layer is going to say: oh look, the crcs don't match, I'll
bounce it, recrc it and send again.  But, there are at least 3 reasons the crc
will change:

1) filesystem changed it
2) corruption on the wire or in the raid controller
3) the page was corrupted while the IO layer was doing the IO.

1 and 2 are easy, we bounce, retry and everyone continues on with
their lives.  With #3, we'll recrc and send the IO down again thinking
the data is correct when really we're writing garbage.

How can we tell these three cases apart?

-chris