Re: + ext4-add-dax-functionality.patch added to -mm tree

[Jan Kara <jack@suse.cz>]

On Thu 19-02-15 08:55:23, Dave Chinner wrote:
> On Wed, Feb 18, 2015 at 11:40:09AM +0100, Jan Kara wrote:
> > On Tue 17-02-15 08:37:45, Matthew Wilcox wrote:
> > > On Tue, Feb 17, 2015 at 09:52:00AM +0100, Jan Kara wrote:
> > > > > > This got added to fix a problem that Dave Chinner pointed out.  We need
> > > > > > the allocated extent to either be zeroed (as ext2 does), or marked as
> > > > > > unwritten (ext4, XFS) so that a racing read/page fault doesn't return
> > > > > > uninitialized data.  If it's marked as unwritten, we need to convert it
> > > > > > to a written extent after we've initialised the contents.  We use the
> > > > > > b_end_io() callback to do this, and it's called from the DAX code, not in
> > > > > > softirq context.
> > > > >   OK, I see. But I didn't find where ->b_end_io gets called from dax code
> > > > > (specifically I don't see it anywhere in dax_do_IO() or dax_io()). Can you
> > > > > point me please?
> > > 
> > > For faults, we call it in dax_insert_mapping(), the very last thing
> > > before returning in the fault path.  The normal I/O path gets to use
> > > the dio_iodone_t for the same purpose.
> >   I see. I didn't think of races with reads (hum, I actually wonder whether
> > we don't have this data exposure problem for ext4 for mmapped write into
> > a hole vs direct read as well). So I guess we do need those unwritten
> > extent dances after all (or we would need to have a page covering hole when
> > writing to it via mmap but I guess unwritten extent dances are somewhat
> > more standard).
> 
> Right, that was the reason for doing it that way - it leveraged all
> the existing methods we have for avoiding data exposure races in
> XFS. but it's also not just for races - it's for ensuring that if we
> crash between the allocation and the write to the persistent store
> we don't expose the underlying contents when the system next comes
> up.
  Well, ext3/4 handles the crash situation differently - we make sure we
flush data to allocated blocks before committing a transaction that
allocates them. That works perfectly for crashes but doesn't avoid the
race with DIO.

> > > > > Also abusing b_end_io of a phony buffer for that looks ugly to me (we are
> > > > > trying to get away from passing phony bh around and this would entangle us
> > > > > even more into that mess). Normally I would think that end_io() callback
> > > > > passed into dax_do_io() should perform necessary conversions and for
> > > > > dax_fault() we could do necessary conversions inside foofs_page_mkwrite()...
> > > 
> > > Dave sees to be the one trying the hardest to get rid of the phony BHs
> > > ... and it was his idea to (ab)use b_end_io for this.  The problem with
> > > doing the conversion in ext4_page_mkwrite() is that we don't know at
> > > that point whether the BH is unwritten or not.
> >   I see, thanks for explanation. So it would be enough to pass a bit of
> > information (unwritten / written) to a caller of do_dax_fault() and that
> > can then call conversion function. IMO doing that (either in a return value
> > or in a separate argument of do_dax_fault()) would be cleaner and more
> > readable than playing with b_private and b_end_io... Thoughts?
> 
> I'm happy for a better mechanism to be thought up. The current one
> was expedient, but not pretty. The need for the end_io function was
> because unwritten conversion needed to happen after the page was
> zeroed. If we change dax_fault() to also take a "end_io" function
> callback (already takes a get_blocks callback), then we can avoid
> the need to use the phony bh for this purpose. i.e filesystems that
> allocate unwritten extents can pass a completion function
  Yeah, that's probably even better.

> And speaking of phony BHs, the pnfs block layout changes introduce
> an struct iomap and a "map_blocks" method to the export_ops in
> exportfs.h. This is the model what we should be using instead of
> phony BHs for block mapping/allocation operations...
  Yup, that'd be nice.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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