From 4103b93b07bceac30bc83cbce81693bb2ea93c22 Mon Sep 17 00:00:00 2001 From: David Hildenbrand Date: Tue, 9 Apr 2024 21:22:49 +0200 Subject: [PATCH] mm: make folio_mapcount() return 0 for small typed folios We already handle it properly for large folios. Let's also return "0" for small typed folios, like page_mapcount() currently would. Consequently, folio_mapcount() will never return negative values for typed folios, but may return negative values for underflows. [david@redhat.com: make folio_mapcount() slightly more efficient] Link: https://lkml.kernel.org/r/c30fcda1-ed87-46f5-8297-cdedbddac009@redhat.com Link: https://lkml.kernel.org/r/20240409192301.907377-7-david@redhat.com Signed-off-by: David Hildenbrand Cc: Chris Zankel Cc: Hugh Dickins Cc: John Paul Adrian Glaubitz Cc: Jonathan Corbet Cc: Matthew Wilcox (Oracle) Cc: Max Filippov Cc: Miaohe Lin Cc: Muchun Song Cc: Naoya Horiguchi Cc: Peter Xu Cc: Richard Chang Cc: Rich Felker Cc: Ryan Roberts Cc: Yang Shi Cc: Yin Fengwei Cc: Yoshinori Sato Cc: Zi Yan Signed-off-by: Andrew Morton --- include/linux/mm.h | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/include/linux/mm.h b/include/linux/mm.h index c7470da5797f..78e583b50e42 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -1261,12 +1261,22 @@ static inline int folio_large_mapcount(const struct folio *folio) * references the entire folio counts exactly once, even when such special * page table entries are comprised of multiple ordinary page table entries. * + * Will report 0 for pages which cannot be mapped into userspace, such as + * slab, page tables and similar. + * * Return: The number of times this folio is mapped. */ static inline int folio_mapcount(const struct folio *folio) { - if (likely(!folio_test_large(folio))) - return atomic_read(&folio->_mapcount) + 1; + int mapcount; + + if (likely(!folio_test_large(folio))) { + mapcount = atomic_read(&folio->_mapcount) + 1; + /* Handle page_has_type() pages */ + if (mapcount < PAGE_MAPCOUNT_RESERVE + 1) + mapcount = 0; + return mapcount; + } return folio_large_mapcount(folio); }