[PATCH V9 37/45] memremap_pages: Introduce devmap_protected()

[ira . weiny]

From: Ira Weiny <ira.weiny@xxxxxxxxx>

Consumers of protected dev_pagemaps can check the PGMAP_PROTECTION flag to
see if the devmap is protected.  However, most contexts will have a struct
page not the pagemap structure directly.

Define devmap_protected() to determine if a page is part of a
dev_pagemap mapping and if the page is protected by additional
protections.

Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
---
 include/linux/mm.h | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 9ab799403004..4ca24329848a 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1151,6 +1151,23 @@ static inline bool pgmap_protection_available(void)
 	return pks_available();
 }
 
+DECLARE_STATIC_KEY_FALSE(dev_pgmap_protection_static_key);
+
+/*
+ * devmap_protected() requires a reference on the page to ensure there is no
+ * races with dev_pagemap tear down.
+ */
+static inline bool devmap_protected(struct page *page)
+{
+	if (!static_branch_unlikely(&dev_pgmap_protection_static_key))
+		return false;
+	if (!is_zone_device_page(page))
+		return false;
+	if (page->pgmap->flags & PGMAP_PROTECTION)
+		return true;
+	return false;
+}
+
 #else
 
 static inline bool pgmap_protection_available(void)
-- 
2.35.1