On Mon, Mar 21, 2016 at 02:22:45PM +0100, Jan Kara wrote:
[Sorry for repost but I accidentally sent initial email without
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?
I typically use xfstests for regression testing. If we can come up with new
generally useful regression tests, especially ones concerning mmap races, that
would be awesome. I guess it's just a choice between adding them somewhere in
xfstests or somewhere else like with the unit tests in ndctl.
Changes since v1:
- handle wakeups of exclusive waiters properly
- fix cow fault races
- other minor stuff
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