Re: [RFC PATCH 00/10] mm/swap: always use swap cache for synchronization

Ryan Roberts
2024-03-27T09:39:38Z

On 27/03/2024 08:32, Huang, Ying wrote:
> Ryan Roberts <ryan.roberts@arm.com> writes:
>
>> [...]
>>
>>>>> Test 1, sequential swapin/out of 30G zero page on ZRAM:
>>>>>
>>>>> Before (us) After (us)
>>>>> Swapout: 33619409 33886008
>>>>> Swapin: 32393771 32465441 (- 0.2%)
>>>>> Swapout (THP): 7817909 6899938 (+11.8%)
>>>>> Swapin (THP) : 32452387 33193479 (- 2.2%)
>>>>
>>>> If my understanding were correct, we don't have swapin (THP) support,
>>>> yet. Right?
>>>
>>> Yes, this series doesn't change how swapin/swapout works with THP in
>>> general, but now THP swapout will leave shadows with large order, so
>>> it needs to be splitted upon swapin, that will slow down later swapin
>>> by a little bit but I think that's worth it.
>>>
>>> If we can do THP swapin in the future, this split on swapin can be
>>> saved to make the performance even better.
>>
>> I'm confused by this (clearly my understanding of how this works is incorrect).
>> Perhaps you can help me understand:
>>
>> When you talk about "shadows" I assume you are referring to the swap cache? It
>> was my understanding that swapping out a THP would always leave the large folio
>> in the swap cache, so this is nothing new?
>>
>> And on swap-in, if the target page is in the swap cache, even if part of a large
>> folio, why does it need to be split? I assumed the single page would just be
>> mapped? (and if all the other pages subsequently fault, then you end up with a
>> fully mapped large folio back in the process)?
>>
>> Perhaps I'm misunderstanding what "shadows" are?
>
> Perhaps, shadow is used to support workingset protection/detection on
> the anonymous LRU list as in the following patchset (merged).
>
> https://lore.kernel.org/all/1595490560-15117-5-git-send-email-iamjoonsoo.kim@lge.com/T/#m962395eb5968c74b0c4c8e41d4b0dcdd3f28b2e6

Thanks! Although after reading the cover letter I still don't really understand
the need for splitting. The LRU applies to whole folios.

>
> --
> Best Regards,
> Huang, Ying