Re: [PATCH 00/13] dax, pmem: move cpu cache maintenance to libnvdimm

[Dan Williams]

On Sat, Jan 21, 2017 at 9:52 AM, Christoph Hellwig <hch@xxxxxx> wrote:
> 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.
>
>> I see two possible routes here:
>>
>> 1. Add a new address_space_operation:
>>
>>       const struct dax_operations *(*get_dax_ops)(struct address_space *);
>>
>> 2. Add two of the dax_operations to address_space_operations:
>>
>>       size_t (*copy_from_iter)(struct address_space *, void *, size_t, struct iov_iter *);
>>       void (*flush)(struct address_space *, void *, size_t);
>> (we won't need ->direct_access as an address_space op because that'll be handled a different way in the brave new world that supports non-bdev-based filesystems)
>
> 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.

So are you saying we need a way to go from a block_device inode to a
dax_device inode and then look up the dax_operations from there?

A filesystem, if it so chooses, could mount on top of the dax_device
inode directly?

I did add a dax_superblock for the device-dax character device
representation I could refactor that so the block_device presentation
of a namespace and a character device presentation are just different
layers on top of the base-level dax inode.

...or am I not tracking what you are suggesting?