On Tue, Jul 25, 2017 at 11:35:08AM +0200, Jan Kara wrote: > On Tue 25-07-17 10:01:58, Christoph Hellwig wrote: > > On Tue, Jul 25, 2017 at 01:14:00AM +0300, Kirill A. Shutemov wrote: > > > I guess it's up to filesystem if it wants to reuse the same spot to write > > > data or not. I think your assumptions works for ext4 and xfs. I wouldn't > > > be that sure for btrfs or other filesystems with CoW support. > > > > Or XFS with reflinks for that matter. Which currently can't be > > combined with DAX, but I had a somewhat working version a few month > > ago. > > But in cases like COW when the block mapping changes, the process > must run unmap_mapping_range() before installing the new PTE so that all > processes mapping this file offset actually refault and see the new > mapping. So this would go through pte_none() case. Am I missing something?
Yes, for DAX COW mappings we'd probably need something like this, unlike the pagecache COW handling for which only the underlying block change, but not the page.
