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

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Message-ID: <20160913154048.76b0e0bc@roar.ozlabs.ibm.com>
Date:   Tue, 13 Sep 2016 15:40:48 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Yumei Huang <yuhuang@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
        KVM list <kvm@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Gleb Natapov <gleb@...nel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...1.01.org>,
        mtosatti@...hat.com,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: 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@...el.com> wrote:

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