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Date: Mon, 12 Sep 2016 18:05:07 +1000
From: Nicholas Piggin <npiggin@gmail.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: "Oliver O'Halloran" <oohall@gmail.com>,
        Yumei Huang <yuhuang@redhat.com>, Michal Hocko <mhocko@suse.com>,
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Subject: Re: DAX mapping detection (was: Re: [PATCH] Fix region lost in
 /proc/self/smaps)
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On Mon, 12 Sep 2016 00:51:28 -0700
Christoph Hellwig <hch@infradead.org> wrote:

> On Mon, Sep 12, 2016 at 05:25:15PM +1000, Oliver O'Halloran wrote:
> > What are the problems here? Is this a matter of existing filesystems
> > being unable/unwilling to support this or is it just fundamentally
> > broken?  
> 
> It's a fundamentally broken model.  See Dave's post that actually was
> sent slightly earlier then mine for the list of required items, which
> is fairly unrealistic.  You could probably try to architect a file
> system for it, but I doubt it would gain much traction.

It's not fundamentally broken, it just doesn't fit well existing
filesystems.

Dave's post of requirements is also wrong. A filesystem does not have
to guarantee all that, it only has to guarantee that is the case for
a given block after it has a mapping and page fault returns, other
operations can be supported by invalidating mappings, etc.