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SubjectRE: [PATCH 00/13] dax, pmem: move cpu cache maintenance to libnvdimm
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From: Christoph Hellwig [mailto:hch@lst.de]
> On Sat, Jan 21, 2017 at 04:28:52PM +0000, Matthew Wilcox wrote:
> > Of course, there may not be a backing device either!
>
> s/backing device/block device/ ? If so fully agreed. I like the dax_ops
> scheme, but we should go all the way and detangle it from the block
> device. I already brought up this issue with the fallback to direct I/O
> on I/O error series.

In the case of a network filesystem being used to communicate with a different VM on the same physical machine, there is no backing device, just a network protocol.

> And both of them are wrong. The write_begin/write_end mistake
> notwithstanding address_space ops are operations the VM can call without
> knowing things like fs locking contexts. The above on the other hand
> are device operations provided by the low-level driver, similar to
> block_device operations. So what we need is to have a way to mount
> a dax device as a file system, similar to how we support that for block
> or MTD devices and can then call methods on it. For now this will
> be a bit complicated because all current DAX-aware file systems also
> still need block device for the metadata path, so we can't just say
> you mount either a DAX or block device. But I think we should aim
> for mounting a DAX device as the primary use case, and then deal
> with block device emulation as a generic DAX layer thing, similarly
> how we implement (bad in the rw case) block devices on top of MTD.

I'm not terribly enthusiastic about creating a fake block device to sit on top of a network filesystem, but I suppose we could go that way if we had to.
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