Subject: Re: Wrong DIF guard tag on ext2 write
From: James Bottomley <James.Bottomley@suse.de>
To: Chris Mason <chris.mason@oracle.com>
Cc: Matthew Wilcox <matthew@wil.cx>,
       Christof Schmitt <christof.schmitt@de.ibm.com>,
       Boaz Harrosh <bharrosh@panasas.com>,
       "Martin K. Petersen" <martin.petersen@oracle.com>,
       linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org,
       linux-fsdevel@vger.kernel.org
In-Reply-To: <20100601180905.GR8980@think>
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Date: Tue, 01 Jun 2010 16:07:43 -0500
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On Tue, 2010-06-01 at 14:09 -0400, Chris Mason wrote:
> On Tue, Jun 01, 2010 at 04:54:53PM +0000, James Bottomley wrote:
> > On Tue, 2010-06-01 at 12:47 -0400, Chris Mason wrote:
> > > On Tue, Jun 01, 2010 at 10:29:30AM -0600, Matthew Wilcox wrote:
> > > > On Tue, Jun 01, 2010 at 09:49:51AM -0400, Chris Mason wrote:
> > > > > > I agree that a block based retry would close all the holes ... it just
> > > > > > doesn't look elegant to me that the fs will already be repeating the I/O
> > > > > > if it changed the page and so will block.
> > > > > 
> > > > > We might not ever repeat the IO.  We might change the page, write it,
> > > > > change it again, truncate the file and toss the page completely.
> > > > 
> > > > Why does it matter that it was never written in that case?
> > > 
> > > It matters is the storage layer is going to wait around for the block to
> > > be written again with a correct crc.
> > 
> > Actually, I wasn't advocating that.  I think block should return a guard
> > mismatch error.  I think somewhere in filesystem writeout is the place
> > to decide whether the error was self induced or systematic.
> 
> In that case the io error goes to the async page writeback bio-endio
> handlers.  We don't have a reference on the inode and no ability to
> reliably restart the IO, but we can set a bit on the address space
> indicating that somewhere, sometime in the past we had an IO error.
> 
> > 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?

We have all the information ... the fs will eventually mark the page
dirty when it finishes the alterations, we just have to find a way to
express that.

If you're thinking of the double fault scenario where the page
spontaneously corrupts *and* the filesystem alters it, then the only way
of detecting that is to freeze the page as it undergoes I/O ... which
involves quite a bit of filesystem surgery, doesn't it?

James


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