Re: Wrong DIF guard tag on ext2 write

[Nick Piggin]

On Tue, Jun 01, 2010 at 03:35:28PM -0400, Chris Mason wrote:
> 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?

Do we really need to handle #3? It could have happened before the
checksum was calculated.

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