(resent as plain text) On 9 Oct 2023, at 10:10, Ryan Roberts wrote: > On 09/10/2023 14:24, Zi Yan wrote: >> On 2 Oct 2023, at 8:32, Ryan Roberts wrote: >> >>> Hi Zi, >>> >>> On 12/09/2023 17:28, Zi Yan wrote: >>>> From: Zi Yan <ziy@xxxxxxxxxx> >>>> >>>> Hi all, >>>> >>>> This patchset enables >0 order folio memory compaction, which is one of >>>> the prerequisitions for large folio support[1]. It is on top of >>>> mm-everything-2023-09-11-22-56. >>> >>> I've taken a quick look at these and realize I'm not well equipped to provide >>> much in the way of meaningful review comments; All I can say is thanks for >>> putting this together, and yes, I think it will become even more important for >>> my work on anonymous large folios. >>> >>> >>>> >>>> Overview >>>> === >>>> >>>> To support >0 order folio compaction, the patchset changes how free pages used >>>> for migration are kept during compaction. Free pages used to be split into >>>> order-0 pages that are post allocation processed (i.e., PageBuddy flag cleared, >>>> page order stored in page->private is zeroed, and page reference is set to 1). >>>> Now all free pages are kept in a MAX_ORDER+1 array of page lists based >>>> on their order without post allocation process. When migrate_pages() asks for >>>> a new page, one of the free pages, based on the requested page order, is >>>> then processed and given out. >>>> >>>> >>>> Optimizations >>>> === >>>> >>>> 1. Free page split is added to increase migration success rate in case >>>> a source page does not have a matched free page in the free page lists. >>>> Free page merge is possible but not implemented, since existing >>>> PFN-based buddy page merge algorithm requires the identification of >>>> buddy pages, but free pages kept for memory compaction cannot have >>>> PageBuddy set to avoid confusing other PFN scanners. >>>> >>>> 2. Sort source pages in ascending order before migration is added to >>>> reduce free page split. Otherwise, high order free pages might be >>>> prematurely split, causing undesired high order folio migration failures. >>> >>> Not knowing much about how compaction actually works, naively I would imagine >>> that if you are just trying to free up a known amount of contiguous physical >>> space, then working through the pages in PFN order is more likely to yield the >>> result quicker? Unless all of the pages in the set must be successfully migrated >>> in order to free up the required amount of space... >> >> During compaction, pages are not freed, since that is the job of page reclaim. > > Sorry yes - my fault for using sloppy language. When I said "free up a known > amount of contiguous physical space", I really meant "move pages in order to > recover an amount of contiguous physical space". But I still think the rest of > what I said applies; wouldn't you be more likely to reach your goal quicker if > you sort by PFN? Not always. If the in-use folios on the left are order-2, order-2, order-4 (all contiguous in one pageblock) and free pages on the right are order-4 (pageblock N), order-2, order-2 (pageblock N-1) and it is not a single order-8, since there are in-use folios in the middle), going in PFN order will not get you an order-8 free page, since first order-4 free page will be split into two order-2 for the first two order-2 in-use folios. But if you migrate in the the descending order of in-use page orders, you can get an order-8 free page at the end. The patchset minimizes free page splits to avoid the situation described above, since once a high order free page is split, the opportunity of migrating a high order in-use folio into it is gone and hardly recoverable. >> The goal of compaction is to get a high order free page without freeing existing >> pages to avoid potential high cost IO operations. If compaction does not work, >> page reclaim would free pages to get us there (and potentially another follow-up >> compaction). So either pages are migrated or stay where they are during compaction. >> >> BTW compaction works by scanning in use pages from lower PFN to higher PFN, >> and free pages from higher PFN to lower PFN until two scanners meet in the middle. >> >> -- >> Best Regards, >> Yan, Zi Best Regards, Yan, Zi
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