On 26 Apr 2024, at 15:08, David Hildenbrand wrote: > On 26.04.24 21:02, Zi Yan wrote: >> From: Zi Yan >> >> In __folio_remove_rmap(), a large folio is added to deferred split list >> if any page in a folio loses its final mapping. But it is possible that >> the folio is fully unmapped and adding it to deferred split list is >> unnecessary. >> >> For PMD-mapped THPs, that was not really an issue, because removing the >> last PMD mapping in the absence of PTE mappings would not have added the >> folio to the deferred split queue. >> >> However, for PTE-mapped THPs, which are now more prominent due to mTHP, >> they are always added to the deferred split queue. One side effect >> is that the THP_DEFERRED_SPLIT_PAGE stat for a PTE-mapped folio can be >> unintentionally increased, making it look like there are many partially >> mapped folios -- although the whole folio is fully unmapped stepwise. >> >> Core-mm now tries batch-unmapping consecutive PTEs of PTE-mapped THPs >> where possible starting from commit b06dc281aa99 ("mm/rmap: introduce >> folio_remove_rmap_[pte|ptes|pmd]()"). When it happens, a whole PTE-mapped >> folio is unmapped in one go and can avoid being added to deferred split >> list, reducing the THP_DEFERRED_SPLIT_PAGE noise. But there will still be >> noise when we cannot batch-unmap a complete PTE-mapped folio in one go >> -- or where this type of batching is not implemented yet, e.g., migration. >> >> To avoid the unnecessary addition, folio->_nr_pages_mapped is checked >> to tell if the whole folio is unmapped. If the folio is already on >> deferred split list, it will be skipped, too. >> >> Note: 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(). >> >> Suggested-by: David Hildenbrand >> Signed-off-by: Zi Yan >> --- >> mm/rmap.c | 12 +++++++++--- >> 1 file changed, 9 insertions(+), 3 deletions(-) >> >> diff --git a/mm/rmap.c b/mm/rmap.c >> index 2608c40dffad..a9bd64ebdd9a 100644 >> --- a/mm/rmap.c >> +++ b/mm/rmap.c >> @@ -1495,6 +1495,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio, >> { >> atomic_t *mapped = &folio->_nr_pages_mapped; >> int last, nr = 0, nr_pmdmapped = 0; >> + bool partially_mapped = false; >> enum node_stat_item idx; >> __folio_rmap_sanity_checks(folio, page, nr_pages, level); >> @@ -1515,6 +1516,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio, >> nr++; >> } >> } while (page++, --nr_pages > 0); >> + >> + partially_mapped = !!nr && !!atomic_read(mapped); > > Nit: The && should remove the need for both !!. My impression was that !! is needed to convert from int to bool and I do find "!!int && !!int" use in the kernel. If this is unnecessary, Andrew can apply the fixup below. I can send a new version if it is really needed. diff --git a/mm/rmap.c b/mm/rmap.c index a9bd64ebdd9a..c1fd5828409b 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -1517,7 +1517,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio, } } while (page++, --nr_pages > 0); - partially_mapped = !!nr && !!atomic_read(mapped); + partially_mapped = nr && atomic_read(mapped); break; case RMAP_LEVEL_PMD: atomic_dec(&folio->_large_mapcount); > > Reviewed-by: David Hildenbrand Thanks. -- Best Regards, Yan, Zi