Re: DAX mapping detection (was: Re: [PATCH] Fix region lost in /proc/self/smaps)

From: Dan Williams
Date: Tue Sep 13 2016 - 00:06:57 EST

On Mon, Sep 12, 2016 at 6:31 PM, Nicholas Piggin <npiggin@xxxxxxxxx> 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