mm, x86: get_user_pages() for dax mappings

A dax mapping establishes a pte with _PAGE_DEVMAP set when the driver
has established a devm_memremap_pages() mapping, i.e.  when the pfn_t
return from ->direct_access() has PFN_DEV and PFN_MAP set.  Later, when
encountering _PAGE_DEVMAP during a page table walk we lookup and pin a
struct dev_pagemap instance to keep the result of pfn_to_page() valid
until put_page().

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Tested-by: Logan Gunthorpe <logang@deltatee.com>
Cc: Dave Hansen <dave@sr71.net>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/memremap.c

@@ -169,6 +169,18 @@ struct page_map {
 	struct vmem_altmap altmap;
 };
 
+void get_zone_device_page(struct page *page)
+{
+	percpu_ref_get(page->pgmap->ref);
+}
+EXPORT_SYMBOL(get_zone_device_page);
+
+void put_zone_device_page(struct page *page)
+{
+	put_dev_pagemap(page->pgmap);
+}
+EXPORT_SYMBOL(put_zone_device_page);
+
 static void pgmap_radix_release(struct resource *res)
 {
 	resource_size_t key;