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

The below referenced commit makes the same error as 1c56343258 ("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.

[akpm@linux-foundation.org: simplify comment, per John]
Link: https://lkml.kernel.org/r/166015037385.760108.16881097713975517242.stgit@omen
Link: https://lore.kernel.org/all/165490039431.944052.12458624139225785964.stgit@omen
Fixes: f25cbb7a95 ("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>
Tested-by: Slawomir Laba <slawomirx.laba@intel.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.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>

include/linux/mm.h

@@ -1544,9 +1544,16 @@ 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 may always be pinned */
+	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)