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

[Christoph Hellwig]

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.