mm, devm_memremap_pages: kill mapping "System RAM" support

BugLink: https://bugs.launchpad.net/bugs/1811647

commit 06489cfb upstream.

Given the fact that devm_memremap_pages() requires a percpu_ref that is
torn down by devm_memremap_pages_release() the current support for mapping
RAM is broken.

Support for remapping "System RAM" has been broken since the beginning and
there is no existing user of this this code path, so just kill the support
and make it an explicit error.

This cleanup also simplifies a follow-on patch to fix the error path when
setting a devm release action for devm_memremap_pages_release() fails.

Link: http://lkml.kernel.org/r/154275557997.76910.14689813630968180480.stgit@dwillia2-desk3.amr.corp.intel.comSigned-off-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: "Jérôme Glisse" <jglisse@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

kernel/memremap.c

@@ -171,15 +171,12 @@ void *devm_memremap_pages(struct device *dev, struct resource *res)
 	struct page_map *page_map;
 	int error, nid;
 
-	if (is_ram == REGION_MIXED) {
-		WARN_ONCE(1, "%s attempted on mixed region %pr\n",
-				__func__, res);
+	if (is_ram != REGION_DISJOINT) {
+		WARN_ONCE(1, "%s attempted on %s region %pr\n", __func__,
+				is_ram == REGION_MIXED ? "mixed" : "ram", res);
 		return ERR_PTR(-ENXIO);
 	}
 
-	if (is_ram == REGION_INTERSECTS)
-		return __va(res->start);
-
 	page_map = devres_alloc_node(devm_memremap_pages_release,
 			sizeof(*page_map), GFP_KERNEL, dev_to_node(dev));
 	if (!page_map)