Re: How can we share page cache pages for reflinked files?

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

On Mon, Aug 14, 2017 at 01:14:57PM -0500, Christopher Lameter wrote:
> On Mon, 14 Aug 2017, Dave Chinner wrote:
> 
> > > Use XFS+reflink+DAX on top of this loop device.  Now there's only one
> > > copy of each page in RAM.
> >
> > Yes, I can see how that could work. Crazy, out of the box, abuses
> > DAX for non-DAX purposes and uses stuff we haven't enabled yet
> > because nobody has done the work to validate it. Full points for
> > creativity! :)
> 
> Another not so crazy solution is to break the 1-1 relation between page
> structs and pages. We already have issues with huge pages where one struct
> page may represent 2m of memmory using 512 or so page struct.
> 
> Therer are also constantly attempts to expand struct page.
> 
> So how about an m->n relationship? Any page (may it be 4k, 2m or 1G) has
> one page struct for each mapping that it is a member of?
> 
> Maybe a the page state could consist of a base struct that describes
> the page state and then 1..n  pieces of mapping information? In the future
> other state info could be added to the end if we allow dynamic sizing of
> page structs.
> 
> This would also allow the inevitable creeping page struct bloat to get
> completely out of control.

Nice wish list. Add pony. :)

Any attempt to replace struct page with something more complex will have
severe performance implications. I'll be glad proved otherwise.

-- 
 Kirill A. Shutemov

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