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

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Feb 22, 2023 at 3:56 PM Yu Zhao <yuzhao@google.com> wrote:
>
> 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.

This might be useful for Android. I would like to participate in this
discussion.

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