[PATCH v1] mm: read page_type using READ_ONCE

[David Hildenbrand]

KCSAN complains about possible data races: while we check for a
page_type -- for example for sanity checks -- we might concurrently
modify the mapcount that overlays page_type.

Let's use READ_ONCE to avoid load tearing (shouldn't make a difference)
and to make KCSAN happy.

Likely, we might also want to use WRITE_ONCE for the writer side of
page_type, if KCSAN ever complains about that. But we'll not mess with
that for now.

Note: nothing should really be broken besides wrong KCSAN complaints.
The sanity check that triggers this was added in commit 68f0320824fa
("mm/rmap: convert folio_add_file_rmap_range() into
folio_add_file_rmap_[pte|ptes|pmd]()"). Even before that similar
races likely where possible, ever since we added page_type in
commit 6e292b9be7f4 ("mm: split page_type out from _mapcount").

Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-lkp/202405281431.c46a3be9-lkp@xxxxxxxxx
Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
Reviewed-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 include/linux/page-flags.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index f04fea86324d9..03eebb1aadbb1 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -961,9 +961,9 @@ PAGEFLAG_FALSE(HasHWPoisoned, has_hwpoisoned)
 #define PAGE_MAPCOUNT_RESERVE	(~0x0000ffff)
 
 #define PageType(page, flag)						\
-	((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
+	((READ_ONCE(page->page_type) & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
 #define folio_test_type(folio, flag)					\
-	((folio->page.page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
+	((READ_ONCE(folio->page.page_type) & (PAGE_TYPE_BASE | flag))  == PAGE_TYPE_BASE)
 
 static inline int page_type_has_type(unsigned int page_type)
 {
@@ -972,7 +972,7 @@ static inline int page_type_has_type(unsigned int page_type)
 
 static inline int page_has_type(const struct page *page)
 {
-	return page_type_has_type(page->page_type);
+	return page_type_has_type(READ_ONCE(page->page_type));
 }
 
 #define FOLIO_TYPE_OPS(lname, fname)					\
-- 
2.45.1