Re: [PATCH 2/2] dm log writes: add support for DAX

Now that we have the ability log filesystem writes using a flat buffer, add support for DAX. Unfortunately we can't easily track data that has been written via mmap() now that the dax_flush() abstraction was removed by this commit: commit c3ca015fab6d ("dax: remove the pmem_dax_ops->flush abstraction") Otherwise we could just treat each flush as a big write, and store the data that is being synced to media. It may be worthwhile to add the dax_flush() entry point back, just as a notifier so we can do this logging. The motivation for this support is the need for an xfstest that can test the new MAP_SYNC DAX flag. By logging the filesystem activity with dm-log-writes we can show that the MAP_SYNC page faults are writing out their metadata as they happen, instead of requiring an explicit msync/fsync. Signed-off-by: Ross Zwisler <ross.zwisler(a)linux.intel.com&gt;