Messages in this thread | | | Date | Thu, 25 Apr 2024 09:29:34 +0200 | Subject | Re: [PATCH v3] mm/rmap: do not add fully unmapped large folio to deferred split list | From | David Hildenbrand <> |
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On 25.04.24 09:27, Lance Yang wrote: > On Thu, Apr 25, 2024 at 3:21 PM David Hildenbrand <david@redhat.com> wrote: >> >> On 25.04.24 05:45, Lance Yang wrote: >>> Hey Zi, >>> >>> On Thu, Apr 25, 2024 at 6:46 AM Zi Yan <zi.yan@sent.com> wrote: >>>> >>>> From: Zi Yan <ziy@nvidia.com> >>>> >>>> In __folio_remove_rmap(), a large folio is added to deferred split list >>>> if any page in a folio loses its final mapping. It is possible that >>>> the folio is unmapped fully, but it is unnecessary to add the folio >>> >>> Agreed. If a folio is fully unmapped, then that's unnecessary to add >>> to the deferred split list. >>> >>>> to deferred split list at all. Fix it by checking folio->_nr_pages_mapped >>>> before adding a folio to deferred split list. If the folio is already >>>> on the deferred split list, it will be skipped. This issue applies to >>>> both PTE-mapped THP and mTHP. >>>> >>>> Commit 98046944a159 ("mm: huge_memory: add the missing >>>> folio_test_pmd_mappable() for THP split statistics") tried to exclude >>>> mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does not >>>> fix the above issue. A fully unmapped PTE-mapped order-9 THP was still >>>> added to deferred split list and counted as THP_DEFERRED_SPLIT_PAGE, >>>> since nr is 512 (non zero), level is RMAP_LEVEL_PTE, and inside >>>> deferred_split_folio() the order-9 folio is folio_test_pmd_mappable(). >>>> However, this miscount was present even earlier due to implementation, >>>> since PTEs are unmapped individually and first PTE unmapping adds the THP >>>> into the deferred split list. >>>> >>>> With commit b06dc281aa99 ("mm/rmap: introduce >>>> folio_remove_rmap_[pte|ptes|pmd]()"), kernel is able to unmap PTE-mapped >>>> folios in one shot without causing the miscount, hence this patch. >>>> >>>> Signed-off-by: Zi Yan <ziy@nvidia.com> >>>> Reviewed-by: Yang Shi <shy828301@gmail.com> >>>> --- >>>> mm/rmap.c | 7 ++++--- >>>> 1 file changed, 4 insertions(+), 3 deletions(-) >>>> >>>> diff --git a/mm/rmap.c b/mm/rmap.c >>>> index a7913a454028..2809348add7b 100644 >>>> --- a/mm/rmap.c >>>> +++ b/mm/rmap.c >>>> @@ -1553,9 +1553,10 @@ static __always_inline void __folio_remove_rmap(struct folio *folio, >>>> * page of the folio is unmapped and at least one page >>>> * is still mapped. >>>> */ >>>> - if (folio_test_large(folio) && folio_test_anon(folio)) >>>> - if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped) >>>> - deferred_split_folio(folio); >>>> + if (folio_test_large(folio) && folio_test_anon(folio) && >>>> + ((level == RMAP_LEVEL_PTE && atomic_read(mapped)) || >>>> + (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped))) >>> >>> Perhaps we only need to check the mapcount? >>> >>> IIUC, if a large folio that was PMD/PTE mapped is fully unmapped here, >>> then folio_mapcount() will return 0. >> >> See discussion on v1. folio_large_mapcount() would achieve the same >> without another folio_test_large() check, but in the context of this >> patch it doesn't really matter. > > Got it. Thanks for pointing that out! > I'll take a closer look at the discussion in v1.
Forgot to add: as long as the large mapcount patches are not upstream, folio_large_mapcount() would be expensive. So this patch can be added independent of the other stuff.
-- Cheers,
David / dhildenb
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