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

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Date:   Mon, 12 Sep 2016 08:01:48 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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, Sep 12, 2016 at 06:05:07PM +1000, Nicholas Piggin wrote:
> It's not fundamentally broken, it just doesn't fit well existing
> filesystems.

Or the existing file system architecture for that matter.  Which makes
it a fundamentally broken model.

> 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.

Which doesn't really matter if your use case is manipulating
fully mapped files.

But back to the point: if you want to use a full blown Linux or Unix
filesystem you will always have to fsync (or variants of it like msync),
period.

If you want a volume manager on stereoids that hands out large chunks
of storage memory that can't ever be moved, truncated, shared, allocated
on demand, etc - implement it in your library on top of a device file.