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

[Huang, Ying]

Ryan Roberts <ryan.roberts@xxxxxxx> 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@xxxxxxx/T/#m962395eb5968c74b0c4c8e41d4b0dcdd3f28b2e6

--
Best Regards,
Huang, Ying