Re: Wrong DIF guard tag on ext2 write

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

On Tue, Jun 01, 2010 at 08:40:37AM -0500, James Bottomley wrote:
> On Tue, 2010-06-01 at 09:33 -0400, Chris Mason wrote:
> > On Tue, Jun 01, 2010 at 01:27:56PM +0000, James Bottomley wrote:
> > > On Tue, 2010-06-01 at 12:30 +0200, Christof Schmitt wrote:
> > > > What is the best strategy to continue with the invalid guard tags on
> > > > write requests? Should this be fixed in the filesystems?
> > > 
> > > For write requests, as long as the page dirty bit is still set, it's
> > > safe to drop the request, since it's already going to be repeated.  What
> > > we probably want is an error code we can return that the layer that sees
> > > both the request and the page flags can make the call.
> > 
> > I'm afraid this isn't entirely true.  The FS tends to do this:
> > 
> > change the page
> > <---------> truck sized race right here where the page is clean
> > mark the page dirty
> 
> Would it be too much work in the fs to mark the page dirty before you
> begin altering it (and again after you finish, just in case some cleaner
> noticed and initiated a write)?  Or some other flag that indicates page
> under modification?  All the process controlling the writeout (which is
> pretty high up in the stack) needs to know is if we triggered the check
> error by altering the page while it was in flight.

I expect that once we went down that path we would end up waiting for
the IO to finish before changing the page.  Maybe there is a less
complex way, but I sure didn't see it.

> 
> 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.

-chris