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-rw-r--r--mm/internal.h16
1 files changed, 15 insertions, 1 deletions
diff --git a/mm/internal.h b/mm/internal.h
index 4c1d604c396c..86ae964a25b0 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -164,7 +164,8 @@ isolate_migratepages_range(struct compact_control *cc,
* general, page_zone(page)->lock must be held by the caller to prevent the
* page from being allocated in parallel and returning garbage as the order.
* If a caller does not hold page_zone(page)->lock, it must guarantee that the
- * page cannot be allocated or merged in parallel.
+ * page cannot be allocated or merged in parallel. Alternatively, it must
+ * handle invalid values gracefully, and use page_order_unsafe() below.
*/
static inline unsigned long page_order(struct page *page)
{
@@ -172,6 +173,19 @@ static inline unsigned long page_order(struct page *page)
return page_private(page);
}
+/*
+ * Like page_order(), but for callers who cannot afford to hold the zone lock.
+ * PageBuddy() should be checked first by the caller to minimize race window,
+ * and invalid values must be handled gracefully.
+ *
+ * ACCESS_ONCE is used so that if the caller assigns the result into a local
+ * variable and e.g. tests it for valid range before using, the compiler cannot
+ * decide to remove the variable and inline the page_private(page) multiple
+ * times, potentially observing different values in the tests and the actual
+ * use of the result.
+ */
+#define page_order_unsafe(page) ACCESS_ONCE(page_private(page))
+
static inline bool is_cow_mapping(vm_flags_t flags)
{
return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;