On Tue 25-07-17 14:15:22, Christoph Hellwig wrote: > 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. Right. So again nothing where the WARN_ON should trigger. That being said I don't care about the WARN_ON too deeply but it can help to catch DAX bugs so if we can keep it I'd prefer to do so... Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
