Pulling slabs out of struct page

At the risk of asking a horrible question: do we really need the ability to start with a _page_ (PFN mapped to userspace, for example) and find type information?

I don’t think we really need this. We already support, in a very limited way, non-struct-page user mappings. For lightweight operations on user memory, we can use the uaccess functions, and they inherently lock correctly against unmapping. For heavyweight operations, we can look up the VMA. This leaves things that don’t want to pay the full price of finding a VMA. Whether those really exist isn’t quite clear to me on a conceptual level, but there is certainly a lot of code that calls get_user_pages [0] and expects the result to be live until release. (And some FS code may want to do useful IO.)

I wonder if performance could be acceptable if GUP walked the VMA tree to find a refcountable object. Some interesting locking would be needed to compete with get_user_pages_fast.

[0] This is all kinds of messy. KVM does unspeakable and blatantly incorrect to host user memory. Even the normal pattern of GUPping a page interacts in unfortunate ways with COW.