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
