Messages in this thread | | | Date | Tue, 27 Feb 2024 16:01:01 +0100 | Subject | Re: [PATCH] mm/huge_memory: fix swap entry values of tail pages of THP | From | David Hildenbrand <> |
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On 27.02.24 15:52, Zi Yan wrote: > On 27 Feb 2024, at 9:11, David Hildenbrand wrote: > >> On 14.02.24 15:34, David Hildenbrand wrote: >>> On 14.02.24 15:18, Matthew Wilcox wrote: >>>> On Wed, Feb 14, 2024 at 12:04:10PM +0530, Charan Teja Kalla wrote: >>>>>> 1) Is it broken in 5.15? Did you actually try to reproduce or is this >>>>>> just a guess? >>>>>> >>>>> >>>>> We didn't run the tests with THP enabled on 5.15, __so we didn't >>>>> encounter this issue__ on older to 6.1 kernels. >>>>> >>>>> I mentioned that issue exists is based on my understanding after code >>>>> walk through. To be specific, I just looked to the >>>>> migrate_pages()->..->migrate_page_move_mapping() & >>>>> __split_huge_page_tail() where the ->private field of thp sub-pages is >>>>> not filled with swap entry. If it could have set, I think these are the >>>>> only places where it would have done, per my understanding. CMIW. >>>> >>>> I think you have a misunderstanding. David's patch cfeed8ffe55b (part >>>> of 6.6) _stopped_ us using the tail ->private entries. So in 6.1, these >>>> tail pages should already have page->private set, and I don't understand >>>> what you're fixing. >>> >>> I think the issue is, that migrate_page_move_mapping() / >>> folio_migrate_mapping() would update ->private for a folio in the >>> swapcache (head page) >>> >>> newfolio->private = folio_get_private(folio); >>> >>> but not the ->private of the tail pages. >>> >>> So once you migrate a THP that is in the swapcache, ->private of the >>> tail pages would not be migrated and, therefore, be stale/wrong. >>> >>> Even before your patch that was the case. >>> >>> Looking at migrate_page_move_mapping(), we had: >>> >>> if (PageSwapBacked(page)) { >>> __SetPageSwapBacked(newpage); >>> if (PageSwapCache(page)) { >>> SetPageSwapCache(newpage); >>> set_page_private(newpage, page_private(page)); >>> } >>> } else { >>> VM_BUG_ON_PAGE(PageSwapCache(page), page); >>> } >>> >>> >>> I don't immediately see where the tail pages would similarly get updated >>> (via set_page_private). >>> >>> With my patch the problem is gone, because the tail page entries don't >>> have to be migrated, because they are unused. >>> >>> >>> Maybe this was an oversight from THP_SWAP -- 38d8b4e6bdc8 ("mm, THP, >>> swap: delay splitting THP during swap out"). >>> >>> It did update __add_to_swap_cache(): >>> >>> for (i = 0; i < nr; i++) { >>> set_page_private(page + i, entry.val + i); >>> error = radix_tree_insert(&address_space->page_tree, >>> idx + i, page + i); >>> if (unlikely(error)) >>> break; >>> } >>> >>> and similarly __delete_from_swap_cache(). >>> >>> But I don't see any updates to migration code. >>> >>> Now, it could be that THP migration was added later (post 2017), in that >>> case the introducing commit would not have been 38d8b4e6bdc8. >>> >> >> Let's continue: >> >> The introducing commit is likely either >> >> (1) 38d8b4e6bdc87 ("mm, THP, swap: delay splitting THP during swap out") >> >> That one added THP_SWAP, but THP migration wasn't supported yet AFAIKS. >> >> -> v4.13 >> >> (2) 616b8371539a6 ("mm: thp: enable thp migration in generic path") > > I think this is the one, since it makes THP entering migrate_page_move_mapping() > possible. > >> >> Or likely any of the following that actually allocate THP for migration: >> >> 8135d8926c08e mm: memory_hotplug: memory hotremove supports thp migration >> e8db67eb0ded3 mm: migrate: move_pages() supports thp migration >> c8633798497ce mm: mempolicy: mbind and migrate_pages support thp migration >> >> That actually enable THP migration. >> >> -> v4.14 >> >> >> So likely we'd have to fix the stable kernels: >> >> 4.19 >> 5.4 >> 5.10 >> 5.15 >> 6.1 >> >> That's a lot of pre-folio code. A backport of my series likely won't really make any sense. >> >> Staring at 4.19.307 code base, we likely have to perform a stable-only fix that properly handles the swapcache of compoud pages in migrate_page_move_mapping(). > > Something like (applies to v4.19.307): > > diff --git a/mm/migrate.c b/mm/migrate.c > index 171573613c39..59878459c28c 100644 > --- a/mm/migrate.c > +++ b/mm/migrate.c > @@ -514,8 +514,13 @@ int migrate_page_move_mapping(struct address_space *mapping, > if (PageSwapBacked(page)) { > __SetPageSwapBacked(newpage); > if (PageSwapCache(page)) { > + int i; > + > SetPageSwapCache(newpage); > - set_page_private(newpage, page_private(page)); > + for (i = 0; i < (1 << compound_order(page)); i++) { > + set_page_private(newpage + i, > + page_private(page + i)); > + } > } > } else { > VM_BUG_ON_PAGE(PageSwapCache(page), page);
I'm wondering if there is a swapcache update missing as well.
-- Cheers,
David / dhildenb
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