Re: [PATCH 7/8] xfs: Support for transparent PUD pages

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Mon, Jan 04, 2016 at 07:33:56AM +1100, Dave Chinner wrote:
> On Sat, Jan 02, 2016 at 11:43:09AM -0500, Matthew Wilcox wrote:
> > On Thu, Dec 31, 2015 at 10:30:27AM +1100, Dave Chinner wrote:
> > > > @@ -1637,6 +1669,7 @@ xfs_filemap_pfn_mkwrite(
> > > >  static const struct vm_operations_struct xfs_file_vm_ops = {
> > > >  	.fault		= xfs_filemap_fault,
> > > >  	.pmd_fault	= xfs_filemap_pmd_fault,
> > > > +	.pud_fault	= xfs_filemap_pud_fault,
> > > 
> > > This is getting silly - we now have 3 different page fault handlers
> > > that all do exactly the same thing. Please abstract this so that the
> > > page/pmd/pud is transparent and gets passed through to the generic
> > > handler code that then handles the differences between page/pmd/pud
> > > internally.
> > > 
> > > This, after all, is the original reason that the ->fault handler was
> > > introduced....
> > 
> > I agree that it's silly, but this is the direction I was asked to go in by
> > the MM people at the last MM summit.  There was agreement that this needs
> > to be abstracted, but that should be left for a separate cleanup round.
> 
> Ok, so it's time to abstract it now, before we end up with another
> round of broken filesystem code (like the first attempts at the
> XFS pmd_fault code).
> 
> > I did prototype something I called a vpte (virtual pte), but that's very
> > much on the back burner for now.
> 
> It's trivial to pack the parameters for pmd_fault and pud_fault
> into the struct vm_fault - all you need to do is add pmd_t/pud_t
> pointers to the structure, and everything else can be put into
> existing members of that structure. There's no need for a "virtual
> pte" type anywhere - you can do this effectively with an anonymous
> union for the pte/pmd/pud pointer and a flag to indicate the fault
> type.
> 
> Then in __dax_fault() you can check vmf->flags and call the
> appropriate __dax_p{te,md,ud}_fault function, all without the
> filesystem having to care about the different fault types. Similar
> can be done with filemap_fault() - if it gets pmd/pud fault flags
> set it can just reject them as they should never occur right now...

I think the first 4 patches of my hugetmpfs RFD patchset[1] are relevant
here. Looks like it shouldn't be a big deal to extend the approach to
cover DAX case.

[1] http://lkml.kernel.org./r/1447889136-6928-1-git-send-email-kirill.shutemov@linux.intel.com
-- 
 Kirill A. Shutemov