[PATCH v3 2/9] mm: Avoid using set_page_count() in set_page_recounted()

[Pasha Tatashin <pasha.tatashin@soleen.com>]

set_page_refcounted() converts a non-refcounted page that has
(page->_refcount == 0) into a refcounted page by setting _refcount to
1.

The current apporach uses the following logic:

VM_BUG_ON_PAGE(page_ref_count(page), page);
set_page_count(page, 1);

However, if _refcount changes from 0 to 1 between the VM_BUG_ON_PAGE()
and set_page_count() we can break _refcount, which can cause other
problems such as memory corruptions.

Instead, use a safer method: increment _refcount first and verify
that at increment time it was indeed 1.

refcnt = page_ref_inc_return(page);
VM_BUG_ON_PAGE(refcnt != 1, page);

Use page_ref_inc_return() to avoid unconditionally overwriting
the _refcount value with set_page_count(), and check the return value.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
 mm/internal.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 4c2d06a2f50b..6b74f7f32613 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -141,9 +141,11 @@ static inline bool page_evictable(struct page *page)
  */
 static inline void set_page_refcounted(struct page *page)
 {
+	int refcnt;
+
 	VM_BUG_ON_PAGE(PageTail(page), page);
-	VM_BUG_ON_PAGE(page_ref_count(page), page);
-	set_page_count(page, 1);
+	refcnt = page_ref_inc_return(page);
+	VM_BUG_ON_PAGE(refcnt != 1, page);
 }
 
 extern unsigned long highest_memmap_pfn;
-- 
2.35.0.rc0.227.g00780c9af4-goog