On Wed, Feb 22, 2023 at 04:55:20PM -0700, Yu Zhao 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. 6. Swap out an entire folio instead of splitting it before swap. 7. At some point we're going to want Zi Yan's patches to split a folio into arbitrary order folios instead of just to order-0. 8. For file folios, folio->index % folio->nr_pages is 0. Do we want to maintain that invariant for anon folios? It helps tile the folios so we don't end up with say, order-0, order-4, order-2 folios in a sequence. 9. How do we ensure that two page faults do not overwrite each others folios, eg PF1 decides to allocate an order-0 folio at index 6 and PF2 decides to allocate an order-2 folio at index 4? Probably some other things left to decide. > 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. Excellent. There are other people also interested in this. eg: Ryan Roberts: https://lore.kernel.org/linux-mm/4c991dcb-c5bb-86bb-5a29-05df24429607@xxxxxxx/ Fengwei Yin: https://lore.kernel.org/linux-mm/20230109072232.2398464-1-fengwei.yin@xxxxxxxxx/ Obviously I'm keenly interested. https://lore.kernel.org/linux-mm/Y%2FU8bQd15aUO97vS@xxxxxxxxxxxxxxxxxxxx/ This is definitely worth its own session in the MM track.
