mm/migrate.c: add missing flush_dcache_page for non-mapped page migrate

Our MIPS 1004Kc SoCs were seeing random userspace crashes with SIGILL
and SIGSEGV that could not be traced back to a userspace code bug.  They
had all the magic signs of an I/D cache coherency issue.

Now recently we noticed that the /proc/sys/vm/compact_memory interface
was quite efficient at provoking this class of userspace crashes.

Studying the code in mm/migrate.c there is a distinction made between
migrating a page that is mapped at the instant of migration and one that
is not mapped.  Our problem turned out to be the non-mapped pages.

For the non-mapped page the code performs a copy of the page content and
all relevant meta-data of the page without doing the required D-cache
maintenance.  This leaves dirty data in the D-cache of the CPU and on
the 1004K cores this data is not visible to the I-cache.  A subsequent
page-fault that triggers a mapping of the page will happily serve the
process with potentially stale code.

What about ARM then, this bug should have seen greater exposure? Well
ARM became immune to this flaw back in 2010, see commit c0177800
("ARM: 6379/1: Assume new page cache pages have dirty D-cache").

My proposed fix moves the D-cache maintenance inside move_to_new_page to
make it common for both cases.

Link: http://lkml.kernel.org/r/20190315083502.11849-1-larper@axis.com
Fixes: 97ee0524 ("flush cache before installing new page at migraton")
Signed-off-by: Lars Persson <larper@axis.com>
Reviewed-by: Paul Burton <paul.burton@mips.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/migrate.c

@@ -248,10 +248,8 @@ static bool remove_migration_pte(struct page *page, struct vm_area_struct *vma,
 				pte = swp_entry_to_pte(entry);
 			} else if (is_device_public_page(new)) {
 				pte = pte_mkdevmap(pte);
-				flush_dcache_page(new);
 			}
-		} else
-			flush_dcache_page(new);
+		}
 
 #ifdef CONFIG_HUGETLB_PAGE
 		if (PageHuge(new)) {
@@ -995,6 +993,13 @@ static int move_to_new_page(struct page *newpage, struct page *page,
 		 */
 		if (!PageMappingFlags(page))
 			page->mapping = NULL;
+
+		if (unlikely(is_zone_device_page(newpage))) {
+			if (is_device_public_page(newpage))
+				flush_dcache_page(newpage);
+		} else
+			flush_dcache_page(newpage);
+
 	}
 out:
 	return rc;