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

[Dave Chinner <david@fromorbit.com>]

On Fri, Feb 20, 2015 at 08:12:10AM +1100, Dave Chinner wrote:
> On Thu, Feb 19, 2015 at 04:42:41PM +0100, Jan Kara wrote:
> > 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.
> 
> I was talking about direct IO, not buffered IO. DAX is modeled on
> the direct IO stack, not buffered IO. I did go and look at the ext4
> IO completion path, and I can see where ext4_end_io_dio() triggers a
                            ^^^^^^^
                            can't see.

Stupid fingers can type what I'm thinking. :/

> commit outside of doing unwritten extent conversion. Can you clue me
> in - IO completion in ext4 is a maze of twisty passages...
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com
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-- 
Dave Chinner
david@fromorbit.com

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