mm: re-allow pinning of zero pfns (again)

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.

Link: https://lkml.kernel.org/r/166015037385.760108.16881097713975517242.stgit@omen
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>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Suggested-by: Felix Kuehling <felix.kuehling@amd.com>
Cc: Alex Sierra <alex.sierra@amd.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 982f2607180b86bad07daaeb42b7a840b0c45e09..2fb203265e6ca8afaff93d6e75ca1732cb5e7435 100644 (file)
--- 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)