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Date: Mon, 12 Sep 2016 08:01:48 -0700
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: DAX mapping detection (was: Re: [PATCH] Fix region lost in
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To: Nicholas Piggin <npiggin@gmail.com>
Cc: Yumei Huang <yuhuang@redhat.com>, Michal Hocko <mhocko@suse.com>, Xiao Guangrong <guangrong.xiao@linux.intel.com>, KVM list <kvm@vger.kernel.org>, Dave Hansen <dave.hansen@intel.com>, Gleb Natapov <gleb@kernel.org>, "linux-nvdimm@lists.01.org" <linux-nvdimm@ml01.01.org>, mtosatti@redhat.com, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Christoph Hellwig <hch@infradead.org>, Linux MM <linux-mm@kvack.org>, Stefan Hajnoczi <stefanha@redhat.com>, linux-fsdevel <linux-fsdevel@vger.kernel.org>, Paolo Bonzini <pbonzini@redhat.com>, Andrew Morton <akpm@linux-foundation.org>
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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.
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