Messages in this thread | ![/](/images/icornerl.gif) | | Date | Thu, 25 Apr 2024 09:21:07 +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 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.
-- Cheers,
David / dhildenb
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