Re: [RFC] block integrity: Fix write after checksum calculation problem

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

Excerpts from Jan Kara's message of 2011-02-24 11:47:58 -0500:
> 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...

Most of it is there, but there are always little bits and pieces.  The
ext4 journal csumming code was one semi-recent example where we found
metadata changing in flight.

A big part of testing this is getting some way to detect the bugs
without dif/dix.  With btrfs I have patches to do set_memory_ro on
pages once I've don the crc, hopefully we can generalize that idea or
some up with something smarter.

-chris