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