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

From: Jerome Glisse
Date: Thu Feb 14 2019 - 15:34:17 EST

On Thu, Feb 14, 2019 at 12:20:11PM -0800, Dan Williams wrote:
> On Thu, Feb 14, 2019 at 12:09 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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@xxxxxxxxxx> 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.

Note that generic page write protection has such object, kind of like
stable_node in KSM. You overwritte page->mapping to point to this
generic struct which has a pointer to set of callback so that whatever
is protecting the page can offer API to break protection (break sharing

So instead of having struct proxy_page -> struct page you would have the
reverse: struct page -> struct proxy and so you do not have to change
much in all the file system beside removing the reliance on page->mapping
which is what most of my patches are about.