Re: DAX mapping detection (was: Re: [PATCH] Fix region lost in /proc/self/smaps)
On Mon, 12 Sep 2016 21:06:49 -0700
Dan Williams <dan.j.williams(a)intel.com> wrote:
On Mon, Sep 12, 2016 at 6:31 PM, Nicholas Piggin
<npiggin(a)gmail.com> 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.
Yes, that's reasonable.
Do you need page/block granularity? Do you need a way to advise/request
the fs for a particular capability? Is it enough to request and check
success? Would the capability be likely to change, and if so, how would
you notify the app asynchronously?