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Date:   Mon, 12 Sep 2016 21:06:49 -0700
From:   Dan Williams <>
To:     Nicholas Piggin <>
Cc:     Christoph Hellwig <>,
        Yumei Huang <>,
        Michal Hocko <>,
        Xiao Guangrong <>,
        KVM list <>,
        Dave Hansen <>,
        Gleb Natapov <>,
        "" <>,,
        "" <>,
        Linux MM <>,
        Stefan Hajnoczi <>,
        linux-fsdevel <>,
        Paolo Bonzini <>,
        Andrew Morton <>
Subject: Re: DAX mapping detection (was: Re: [PATCH] Fix region lost in /proc/self/smaps)

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

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