On Mon, Sep 12, 2016 at 6:31 PM, Nicholas Piggin [off-list ref] wrote:

On Mon, 12 Sep 2016 08:01:48 -0700

[..]

That said, a noop system call is on the order of 100 cycles nowadays,
so rushing to implement these APIs without seeing good numbers and
actual users ready to go seems premature. *This* is the real reason
not to implement new APIs yet.

Yes, and harvesting the current crop of low hanging performance fruit
in the filesystem-DAX I/O path remains on the todo list.

In the meantime we're pursuing this mm api, mincore+ or whatever we
end up with, to allow userspace to distinguish memory address ranges
that are backed by a filesystem requiring coordination of metadata
updates + flushes for updates, vs something like device-dax that does
not.

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