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

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Message-ID: <20160912180507.533b3549@roar.ozlabs.ibm.com>
Date:   Mon, 12 Sep 2016 18:05:07 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Oliver O'Halloran <oohall@...il.com>,
        Yumei Huang <yuhuang@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        KVM list <kvm@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        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>,
        Dave Hansen <dave.hansen@...el.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: DAX mapping detection (was: Re: [PATCH] Fix region lost in
 /proc/self/smaps)

On Mon, 12 Sep 2016 00:51:28 -0700
Christoph Hellwig <hch@...radead.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.