Re: [RFC PATCH 2/2] mm, fs: daxfile, an interface for byte-addressable updates to pmem

On Sun, Jun 18, 2017 at 1:18 AM, Christoph Hellwig [off-list ref] wrote:

On Sat, Jun 17, 2017 at 08:15:05PM -0700, Dan Williams wrote:

quoted

The hang up is that it requires per-fs enabling as it needs to be
careful to manage mmap_sem vs fs journal locks for example. I know the
in-development NOVA [1] filesystem is planning to support this out of
the gate. ext4 would be open to implementing it, but I think xfs is
cold on the idea. Christoph originally proposed it here [2], before
Dave went on to propose immutable semantics.

[1]: https://github.com/NVSL/NOVA
[2]: https://lists.01.org/pipermail/linux-nvdimm/2016-February/004609.html

And I stand to that statement.  Let's get DAX stable first, and
properly cleaned up (e.g. follow on work with separating it entirely
from the block device).  Then think hard about how most of the
persistent memory technologies actually work, including the point that
for a lot of workloads page cache will be required at least on the
write side.   And then come up with actual real use cases and we can
look into it.

I see it differently. We're already at a good point in time to start
iterating on a fix for this issue. Ross and Jan have done a lot of
good work on the dax stability front, and the block-device separation
of dax is well underway.

And stop trying to shoe-horn crap like this in.

The kernel shoe-horning all pmem+filesystem-dax applications into
abiding page-cache semantics is a problem, and this RFC has already
helped move the needle on a couple fronts. 1/ Swapfiles are subtly
broken which is something worth fixing, and if it gets us a
synchronous-dax mode without major filesystem surgery then that's all
for the better. 2/ There's an appetite for just fixing this
incrementally in each filesystem's fault handler, so if ext4 was able
to prove out an interface / implementation for synchronous faults we
could go with that instead of a pre-allocated + immutable interface
and let other filesystems set their own timelines.