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

Alex Williamson
2022-08-10T16:54:06Z

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@infradead.org>
Suggested-by: Felix Kuehling <felix.kuehling@amd.com>
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@redhat.com>
---
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)