[...] >>> 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?
