futher decouple DAX from block devices

[Dan Williams]

On Thu, Nov 4, 2021 at 10:36 AM Christoph Hellwig <hch at lst.de> wrote:
>
> On Thu, Nov 04, 2021 at 10:34:17AM -0700, Darrick J. Wong wrote:
> > /me wonders, are block devices going away?  Will mkfs.xfs have to learn
> > how to talk to certain chardevs?  I guess jffs2 and others already do
> > that kind of thing... but I suppose I can wait for the real draft to
> > show up to ramble further. ;)
>
> Right now I've mostly been looking into the kernel side.  An no, I
> do not expect /dev/pmem* to go away as you'll still need it for a
> not DAX aware file system and/or application (such as mkfs initially).
>
> But yes, just pointing mkfs to the chardev should be doable with very
> little work.  We can point it to a regular file after all.

Note that I've avoided implementing read/write fops for dax devices
partly out of concern for not wanting to figure out shared-mmap vs
write coherence issues, but also because of a bet with Dave Hansen
that device-dax not grow features like what happened to hugetlbfs. So
it would seem mkfs would need to switch to mmap I/O, or bite the
bullet and implement read/write fops in the driver.