mm: read page_type using READ_ONCE

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").

Link: https://lkml.kernel.org/r/20240531125616.2850153-1-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202405281431.c46a3be9-lkp@intel.com
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 1d441908b7266587b81619bcb0da4921a0301fb1..5769fe6e49509ac0731c317788c67cf36cff1c2c 100644 (file)
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -955,9 +955,9 @@ enum pagetype {
 };
 
 #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)
 {
@@ -966,7 +966,7 @@ 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)                                   \