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:

quoted

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?

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR

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