[LSF/MM/BPF TOPIC] Flexible orders for anonymous folios

[Yu Zhao <yuzhao@google.com>]

Currently anonymous folios only support two orders: 0 and the PMD
order. Flexible orders for anonymous folios can resist both internal
or external fragmentations when the PMD order is too underutilized or
costly to allocate. Flexible orders can also leverage the TLB
coalescing feature, .e.g., order 3 for AMD and order 4 for ARM CPUs.

Discussion points:
1. The page fault path: determining the best order and the fallback policy.
2. The reclaim path: detecting the utilization and the splitting policy.
3. The refcount and the mapcount models, e.g., reuse the PMD-mapped
THP model or not.
4. The splitting, and the collapsing if needed.
5. Other paths: COW, GUP, madvise(), mprotect(), page migration, etc.

A bit of history:
I discussed this topic at Matthew's THP Cabal meeting a couple of
years ago, as one of the major use cases of folios. Around the same
time, I tried to get Hugh to work with me on it, since he tried
something similar back in 2001 [1]. William's page clustering from
2003 [2] is also based on that, btw. Then I put the idea on the back
burner.

Now it seems to be a good time to revisit. Yang Shi and I have been
looking at some details, trying to scope out the work that would be
required. We don't have anything definitive yet, but we should have
enough to share by the time of the LSF/MM/BPF.

[1] https://lore.kernel.org/linux-mm/Pine.LNX.4.21.0107051737340.1577-100000@localhost.localdomain/
[2] https://lwn.net/Articles/23785/