Messages in this thread | | | Date | Thu, 3 Aug 2023 15:08:10 +0800 | Subject | Re: [PATCH 2/4] mm: migrate: convert numamigrate_isolate_page() to numamigrate_isolate_folio() | From | Kefeng Wang <> |
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On 2023/8/2 20:30, Matthew Wilcox wrote: > On Wed, Aug 02, 2023 at 05:53:44PM +0800, Kefeng Wang wrote: >> -static int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page) >> +static int numamigrate_isolate_folio(pg_data_t *pgdat, struct folio *folio) >> { >> - int nr_pages = thp_nr_pages(page); >> - int order = compound_order(page); >> + int nr_pages = folio_nr_pages(folio); >> + int order = folio_order(folio); >> >> - VM_BUG_ON_PAGE(order && !PageTransHuge(page), page); >> + VM_BUG_ON_FOLIO(order && !folio_test_pmd_mappable(folio), folio); > > I don't know why we have this assertion. I would be inclined to delete > it as part of generalising the migration code to handle arbitrary sizes > of folio, rather than assert that we only support PMD size folios.
Ok, will drop it. > >> /* Do not migrate THP mapped by multiple processes */ >> - if (PageTransHuge(page) && total_mapcount(page) > 1) >> + if (folio_test_pmd_mappable(folio) && folio_estimated_sharers(folio) > 1) >> return 0; > > I don't know if this is the right logic. We've willing to move folios > mapped by multiple processes, as long as they're smaller than PMD size, > but once they get to PMD size they're magical and can't be moved?
It seems that the logical is introduced by commit 04fa5d6a6547 ("mm: migrate: check page_count of THP before migrating") and refactor by 340ef3902cf2 ("mm: numa: cleanup flow of transhuge page migration"),
"Hugh Dickins pointed out that migrate_misplaced_transhuge_page() does not check page_count before migrating like base page migration and khugepage. He could not see why this was safe and he is right."
For now, there is no migrate_misplaced_transhuge_page() and base/thp page migrate's path is unified, there is a check(for old/new kernel) in migrate_misplaced_page(),
"Don't migrate file pages that are mapped in multiple processes with execute permissions as they are probably shared libraries."
We could drop the above check in numamigrate_isolate_page(), but according to 04fa5d6a6547, maybe disable migrate page shared by multi-process during numa balance for both base/thp page.
> >
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