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    SubjectRe: Wrong DIF guard tag on ext2 write
    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:
    > > 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?

    Block layer should retry it with bounce pages. That would be a lot nicer
    than forcing all upper layers to avoid the problem.



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