Re: [PATCH v5 1/5] mm: add vm_insert_mixed_mkwrite()

[Christoph Hellwig]

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.