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    SubjectRe: 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@intel.com> wrote:

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

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