Re: [LSF/MM TOPIC] Sharing file backed pages

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

On Wed, Jan 23, 2019 at 03:54:34PM +0100, Jan Kara wrote:
> On Wed 23-01-19 10:48:58, Amir Goldstein wrote:
> > In his session about "reflink" in LSF/MM 2016 [1], Darrick Wong brought
> > up the subject of sharing pages between cloned files and the general vibe
> > in room was that it could be done.
> > 
> > In his talk about XFS subvolumes and snapshots [2], Dave Chinner said
> > that Matthew Willcox was "working on that problem".
> > 
> > I have started working on a new overlayfs address space implementation
> > that could also benefit from being able to share pages even for filesystems
> > that do not support clones (for copy up anticipation state).
> > 
> > To simplify the problem, we can start with sharing only uptodate clean
> > pages that map the same offset in respected files. While the same offset
> > requirement somewhat limits the use cases that benefit from shared file
> > pages, there is still a vast majority of use cases (i.e. clone full
> > image), where sharing pages of similar offset will bring a lot of
> > benefit.
> > 
> > At first glance, this requires dropping the assumption that a for an
> > uptodate clean page, vmf->vma->vm_file->f_inode == page->mapping->host.
> > Is there really such an assumption in common vfs/mm code?  and what will
> > it take to drop it?
> 
> There definitely is such assumption. Take for example page reclaim as one
> such place that will be non-trivial to deal with. You need to remove the
> page from page cache of all inodes that contain it without having any file
> context whatsoever. So you will need to create some way for this page->page
> caches mapping to happen.

We have it solved for anon pages where we need to find all VMA the page
might be mapped to. I think we should look into adopting anon_vma
approach[1] for files too. From the first look the problemspace looks very
similar.

[1] https://lwn.net/Articles/383162/

-- 
 Kirill A. Shutemov