mm, dax, pmem: introduce pfn_t

For the purpose of communicating the optional presence of a 'struct
page' for the pfn returned from ->direct_access(), introduce a type that
encapsulates a page-frame-number plus flags.  These flags contain the
historical "page_link" encoding for a scatterlist entry, but can also
denote "device memory".  Where "device memory" is a set of pfns that are
not part of the kernel's linear mapping by default, but are accessed via
the same memory controller as ram.

The motivation for this new type is large capacity persistent memory
that needs struct page entries in the 'memmap' to support 3rd party DMA
(i.e.  O_DIRECT I/O with a persistent memory source/target).  However,
we also need it in support of maintaining a list of mapped inodes which
need to be unmapped at driver teardown or freeze_bdev() time.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dave Hansen <dave@sr71.net>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/memremap.c

@@ -12,6 +12,7 @@
  */
 #include <linux/device.h>
 #include <linux/types.h>
+#include <linux/pfn_t.h>
 #include <linux/io.h>
 #include <linux/mm.h>
 #include <linux/memory_hotplug.h>
@@ -147,6 +148,12 @@ void devm_memunmap(struct device *dev, void *addr)
 }
 EXPORT_SYMBOL(devm_memunmap);
 
+pfn_t phys_to_pfn_t(dma_addr_t addr, unsigned long flags)
+{
+	return __pfn_to_pfn_t(addr >> PAGE_SHIFT, flags);
+}
+EXPORT_SYMBOL(phys_to_pfn_t);
+
 #ifdef CONFIG_ZONE_DEVICE
 struct page_map {
 	struct resource res;