Messages in this thread | | | Date | Tue, 27 Feb 2024 15:11:47 +0100 | Subject | Re: [PATCH] mm/huge_memory: fix swap entry values of tail pages of THP | From | David Hildenbrand <> |
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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")
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().
Ugly.
-- Cheers,
David / dhildenb
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