Re: [PATCH v3 1/4] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags

[Matthew Wilcox]

On Wed, Feb 28, 2024 at 09:37:06AM +0000, Ryan Roberts wrote:
> Fundamentally, we would like to be able to figure out the size of the swap slot
> from the swap entry. Today swap supports 2 sizes; PAGE_SIZE and PMD_SIZE. For
> PMD_SIZE, it always uses a full cluster, so can easily add a flag to the cluster
> to mark it as PMD_SIZE.
> 
> Going forwards, we want to support all sizes (power-of-2). Most of the time, a
> cluster will contain only one size of THPs, but this is not the case when a THP
> in the swapcache gets split or when an order-0 slot gets stolen. We expect these
> cases to be rare.
> 
> 1) Keep the size of the smallest swap entry in the cluster header. Most of the
> time it will be the full size of the swap entry, but sometimes it will cover
> only a portion. In the latter case you may see a false negative for
> swap_page_trans_huge_swapped() meaning we take the slow path, but that is rare.
> There is one wrinkle: currently the HUGE flag is cleared in put_swap_folio(). We
> wouldn't want to do the equivalent in the new scheme (i.e. set the whole cluster
> to order-0). I think that is safe, but haven't completely convinced myself yet.
> 
> 2) allocate 4 bits per (small) swap slot to hold the order. This will give
> precise information and is conceptually simpler to understand, but will cost
> more memory (half as much as the initial swap_map[] again).
> 
> I still prefer to avoid this at all if we can (and would like to hear Huang's
> thoughts). But if its a choice between 1 and 2, I prefer 1 - I'll do some
> prototyping.

I can't quite bring myself to look up the encoding of swap entries
but as long as we're willing to restrict ourselves to naturally aligning
the clusters, there's an encoding (which I believe I invented) that lets
us encode arbitrary power-of-two sizes with a single bit.

I describe it here:
https://kernelnewbies.org/MatthewWilcox/NaturallyAlignedOrder

Let me know if it's not clear.