[PATCH v2] mm: re-allow pinning of zero pfns (again)

[Alex Williamson]

The below referenced commit makes the same error as 1c563432588d ("mm: fix
is_pinnable_page against a cma page"), re-interpreting the logic to exclude
pinning of the zero page, which breaks device assignment with vfio.

To avoid further subtle mistakes, split the logic into discrete tests.

Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Suggested-by: Felix Kuehling <felix.kuehling@xxxxxxx>
Link: https://lore.kernel.org/all/165490039431.944052.12458624139225785964.stgit@omen
Fixes: f25cbb7a95a2 ("mm: add zone device coherent type memory support")
Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
---
 include/linux/mm.h |   17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 18e01474cf6b..835106a9718f 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1544,9 +1544,20 @@ static inline bool is_longterm_pinnable_page(struct page *page)
 	if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
 		return false;
 #endif
-	return !(is_device_coherent_page(page) ||
-		 is_zone_movable_page(page) ||
-		 is_zero_pfn(page_to_pfn(page)));
+	/*
+	 * The zero page might reside in a movable zone, however it may not
+	 * be migrated and can therefore be pinned.  The vfio subsystem pins
+	 * user mappings including the zero page for IOMMU translation.
+	 */
+	if (is_zero_pfn(page_to_pfn(page)))
+		return true;
+
+	/* Coherent device memory must always allow eviction. */
+	if (is_device_coherent_page(page))
+		return false;
+
+	/* Otherwise, non-movable zone pages can be pinned. */
+	return !is_zone_movable_page(page);
 }
 #else
 static inline bool is_longterm_pinnable_page(struct page *page)