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

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Message-ID: <20160103203356.GD6682@dastard>
Date:	Mon, 4 Jan 2016 07:33:56 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Matthew Wilcox <willy@...ux.intel.com>
Cc:	Matthew Wilcox <matthew.r.wilcox@...el.com>, linux-mm@...ck.org,
	linux-nvdimm@...ts.01.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH 7/8] xfs: Support for transparent PUD pages

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...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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