Re: [RFC] block integrity: Fix write after checksum calculationproblem

From: Jan Kara
Date: Thu Feb 24 2011 - 11:48:10 EST

On Wed 23-02-11 15:35:11, Chris Mason wrote:
> Excerpts from Joel Becker's message of 2011-02-23 15:24:47 -0500:
> > On Tue, Feb 22, 2011 at 11:45:44AM -0500, Martin K. Petersen wrote:
> > > Also, DIX is only the tip of the iceberg. Many other impending
> > > technologies feature checksums and require pages to be stable during I/O
> > > due to checksumming, encryption and so on.
> > >
> > > The VM is already trying to do the right thing. We just need the
> > > relevant filesystems to catch up.
> >
> > ocfs2 handles stable metadata for its checksums when feeding
> > things to the journal. If we're doing pagecache-based I/O, is the
> > pagecache going to help here for data?
> Data is much easier than metadata. All you really need is to wait on
> writeback in file_write, wait on writeback in page_mkwrite, and make
> sure you don't free blocks back to the allocator that are actively under
> IO.
> I expect the hard part to be jbd and metadata in ext34.
But JBD already has to do data copy if a buffer is going to be modified
before/while it is written to the journal. So we should alredy do all that
is needed for metadata. I don't say there aren't any bugs as they could be
triggered only by crashing at the wrong moment and observing fs corruption.
But most of the work should be there...

Jan Kara <jack@xxxxxxx>
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