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. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
