Hello, this is my second attempt at DAX page fault locking rewrite. Things now work reasonably well, it has survived full xfstests run on ext4. I guess I need to do more mmap targetted tests to unveil issues. Guys what do you used for DAX testing? Changes since v1: - handle wakeups of exclusive waiters properly - fix cow fault races - other minor stuff General description The basic idea is that we use a bit in an exceptional radix tree entry as a lock bit and use it similarly to how page lock is used for normal faults. That way we fix races between hole instantiation and read faults of the same index. For now I have disabled PMD faults since there the issues with page fault locking are even worse. Now that Matthew's multi-order radix tree has landed, I can have a look into using that for proper locking of PMD faults but first I want normal pages sorted out. In the end I have decided to implement the bit locking directly in the DAX code. Originally I was thinking we could provide something generic directly in the radix tree code but the functions DAX needs are rather specific. Maybe someone else will have a good idea how to distill some generally useful functions out of what I've implemented for DAX but for now I didn't bother with that. Honza