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

[Ryan Roberts]

On 28/02/2024 13:33, Matthew Wilcox wrote:
> 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.

Ahh yes, I'm familiar with this encoding scheme from other settings. Although
I've previously thought of it as having a bit to indicate whether the scheme is
enabled or not, and if it is enabled then the encoded PFN is:

PFNe = PFNd | (1 << (log2(n) - 1))

Where n is the power-of-2 page count.

Same thing, I think.

I think we would have to steal a bit from the offset to make this work, and it
looks like the size of that is bottlnecked on the arch's swp_entry PTE
representation. Looks like there is a MIPS config that only has 17 bits for
offset to begin with, so I doubt we would be able to spare a bit here? Although
it looks possible that there are some unused low bits that could be used...