12:43 a.m.
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.