[skissane]

How do people merge big new filesystems in practice though? Especially one with years of pre-existing out-of-tree development?

I guess one could start by merging a skeleton of the filesystem which supports mount/unmount but then returns an IO error on every operation? And then a patch to add directory traversal (you can view the files but not their contents), and then a patch to add file reading, and then a patch to add file writing, and then a patch to add mkdir/rmdir, and then a patch to add rename/delete of regular files.

Breaking down an existing filesystem into a sequence of patches like that, no doubt it is doable, but it is going to be a lot of work.

[wtallis]

My guess is that given the history of this filesystem implementation, most of the review effort will be focused on the interface between this FS and the rest of the kernel. It's typical for all the changes touching communal files or introducing generic helper functions or data structures to be broken out into separate commits. If any of those helpers are a reinvention of stuff that's already in the kernel, there will need to be a justification for why NTFS needs its own special versions. It's not typical for a large patch series adding genuinely new stuff to be broken up into absurdly tiny commits. For the stuff that's truly internal to the filesystem implementation, it looks like one patch per file will be an acceptable granularity.