[PATCH mm-unstable v1] mm: allow set/clear page_type again

[Yu Zhao <yuzhao@google.com>]

Some page flags (page->flags) were converted to page types
(page->page_types). A recent example is PG_hugetlb.

From the exclusive writer's perspective, e.g., a thread doing
__folio_set_hugetlb(), there is a difference between the page flag and
type APIs: the former allows the same non-atomic operation to be
repeated whereas the latter does not. For example, calling
__folio_set_hugetlb() twice triggers VM_BUG_ON_FOLIO(), since the
second call expects the type (PG_hugetlb) not to be set previously.

Using add_hugetlb_folio() as an example, it calls
__folio_set_hugetlb() in the following error-handling path. And when
that happens, it triggers the aforementioned VM_BUG_ON_FOLIO().

  if (folio_test_hugetlb(folio)) {
    rc = hugetlb_vmemmap_restore_folio(h, folio);
    if (rc) {
      spin_lock_irq(&hugetlb_lock);
      add_hugetlb_folio(h, folio, false);
      ...

It is possible to make hugeTLB comply with the new requirements from
the page type API. However, a straightforward fix would be to just
allow the same page type to be set or cleared again inside the API,
to avoid any changes to its callers.

Fixes: d99e3140a4d3 ("mm: turn folio_test_hugetlb into a PageType")
Signed-off-by: Yu Zhao <yuzhao@google.com>
Cc: <stable@vger.kernel.org>
---
 include/linux/page-flags.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index ccf3c78faefc..e80665bc51fa 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -977,12 +977,16 @@ static __always_inline bool folio_test_##fname(const struct folio *folio) \
 }									\
 static __always_inline void __folio_set_##fname(struct folio *folio)	\
 {									\
+	if (folio_test_##fname(folio))					\
+		return;							\
 	VM_BUG_ON_FOLIO(data_race(folio->page.page_type) != UINT_MAX,	\
 			folio);						\
 	folio->page.page_type = (unsigned int)PGTY_##lname << 24;	\
 }									\
 static __always_inline void __folio_clear_##fname(struct folio *folio)	\
 {									\
+	if (folio->page.page_type == UINT_MAX)				\
+		return;							\
 	VM_BUG_ON_FOLIO(!folio_test_##fname(folio), folio);		\
 	folio->page.page_type = UINT_MAX;				\
 }
@@ -995,11 +999,15 @@ static __always_inline int Page##uname(const struct page *page)		\
 }									\
 static __always_inline void __SetPage##uname(struct page *page)		\
 {									\
+	if (Page##uname(page))						\
+		return;							\
 	VM_BUG_ON_PAGE(data_race(page->page_type) != UINT_MAX, page);	\
 	page->page_type = (unsigned int)PGTY_##lname << 24;		\
 }									\
 static __always_inline void __ClearPage##uname(struct page *page)	\
 {									\
+	if (page->page_type == UINT_MAX)				\
+		return;							\
 	VM_BUG_ON_PAGE(!Page##uname(page), page);			\
 	page->page_type = UINT_MAX;					\
 }
-- 
2.47.0.rc1.288.g06298d1525-goog