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

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

On 22/02/2024 10:20, David Hildenbrand wrote:
> On 22.02.24 11:19, David Hildenbrand wrote:
>> On 25.10.23 16:45, Ryan Roberts wrote:
>>> As preparation for supporting small-sized THP in the swap-out path,
>>> without first needing to split to order-0, Remove the CLUSTER_FLAG_HUGE,
>>> which, when present, always implies PMD-sized THP, which is the same as
>>> the cluster size.
>>>
>>> The only use of the flag was to determine whether a swap entry refers to
>>> a single page or a PMD-sized THP in swap_page_trans_huge_swapped().
>>> Instead of relying on the flag, we now pass in nr_pages, which
>>> originates from the folio's number of pages. This allows the logic to
>>> work for folios of any order.
>>>
>>> The one snag is that one of the swap_page_trans_huge_swapped() call
>>> sites does not have the folio. But it was only being called there to
>>> avoid bothering to call __try_to_reclaim_swap() in some cases.
>>> __try_to_reclaim_swap() gets the folio and (via some other functions)
>>> calls swap_page_trans_huge_swapped(). So I've removed the problematic
>>> call site and believe the new logic should be equivalent.
>>
>> That is the  __try_to_reclaim_swap() -> folio_free_swap() ->
>> folio_swapped() -> swap_page_trans_huge_swapped() call chain I assume.
>>
>> The "difference" is that you will now (1) get another temporary
>> reference on the folio and (2) (try)lock the folio every time you
>> discard a single PTE of a (possibly) large THP.
>>
> 
> Thinking about it, your change will not only affect THP, but any call to
> free_swap_and_cache().
> 
> Likely that's not what we want. :/
> 

Is folio_trylock() really that expensive given the code path is already locking
multiple spinlocks, and I don't think we would expect the folio lock to be very
contended?

I guess filemap_get_folio() could be a bit more expensive, but again, is this
really a deal-breaker?


I'm just trying to refamiliarize myself with this series, but I think I ended up
allocating a cluster per cpu per order. So one potential solution would be to
turn the flag into a size and store it in the cluster info. (In fact I think I
was doing that in an early version of this series - will have to look at why I
got rid of that). Then we could avoid needing to figure out nr_pages from the folio.