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

The below referenced commit makes the same error as 1c56343 ("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.

[[email protected]: 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: f25cbb7 ("mm: add zone device coherent type memory support")
Signed-off-by: Alex Williamson <[email protected]>
Suggested-by: Matthew Wilcox <[email protected]>
Suggested-by: Felix Kuehling <[email protected]>
Tested-by: Slawomir Laba <[email protected]>
Reviewed-by: John Hubbard <[email protected]>
Cc: Alex Sierra <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Alistair Popple <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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)