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

[Ryan Roberts <ryan.roberts@arm.com>]

On 23/02/2023 04:01, Matthew Wilcox wrote:
> 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@arm.com/

Yes, I'm very interested, and would be keen to join the discussion. I have some
thoughts in this mail, if you want to have a look:
https://lore.kernel.org/linux-mm/a7cd938e-a86f-e3af-f56c-433c92ac69c2@arm.com/

Feel free to CC me on any discussion.

> Fengwei Yin: https://lore.kernel.org/linux-mm/20230109072232.2398464-1-fengwei.yin@intel.com/
> 
> Obviously I'm keenly interested.
> https://lore.kernel.org/linux-mm/Y%2FU8bQd15aUO97vS@casper.infradead.org/
> 
> This is definitely worth its own session in the MM track.
> 
>