linux-ext4 - Re: [Lsf-pc] [LSF/MM TOPIC] The end of the DAX experiment

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Date:   Thu, 6 Jun 2019 23:00:35 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Goldwyn Rodrigues <rgoldwyn@...e.de>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Jerome Glisse <jglisse@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        lsf-pc@...ts.linux-foundation.org,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>
Subject: Re: [Lsf-pc] [LSF/MM TOPIC] The end of the DAX experiment

On Wed, Jun 5, 2019 at 9:22 AM Goldwyn Rodrigues <rgoldwyn@...e.de> wrote:
>
> Hi Dan/Jerome,
>
> On 12:20 14/02, Dan Williams wrote:
> > On Thu, Feb 14, 2019 at 12:09 PM Matthew Wilcox <willy@...radead.org> wrote:
> > >
> > > On Thu, Feb 14, 2019 at 11:31:24AM -0800, Dan Williams wrote:
> > > > On Thu, Feb 14, 2019 at 11:10 AM Jerome Glisse <jglisse@...hat.com> wrote:
> > > > > I am just again working on my struct page mapping patchset as well as
> > > > > the generic page write protection that sits on top. I hope to be able
> > > > > to post the v2 in couple weeks. You can always look at my posting last
> > > > > year to see more details.
> > > >
> > > > Yes, I have that in mind as one of the contenders. However, it's not
> > > > clear to me that its a suitable fit for filesystem-reflink. Others
> > > > have floated the 'page proxy' idea, so it would be good to discuss the
> > > > merits of the general approaches.
> > >
> > > ... and my preferred option of putting pfn entries in the page cache.
> >
> > Another option to include the discussion.
> >
> > > Or is that what you meant by "page proxy"?
> >
> > Page proxy would be an object that a filesystem could allocate to
> > point back to a single physical 'struct page *'. The proxy would
> > contain an override for page->index.
>
> Was there any outcome on this and its implementation? I am specifically
> interested in this for DAX support on btrfs/CoW: The TODO comment on
> top of dax_associate_entry() :)
>
> If there are patches/git tree I could use to base my patches on, it would
> be nice.

Half joking, but I was hoping that by the time I had circled back to
finally reviewing the brtfs dax patches that a solution to this
problem would be waiting. We spent more time on other DAX topics. I
recall that Jerome and I were mutually skeptical of each others
approaches at first glance, but we did not get into the details. So, I
think it will be a matter of getting the idea coded up. I'm finally
coming out from under a pile of mm work so I should have some time
this cycle to push the page proxy idea forward.