[MIPS] Fix aliasing bug in copy_user_highpage, take 2.

Turns out b868868a  wasn't quite right.
When called for a page that isn't marked dirty it would artificially
create an alias instead of doing the obvious thing and access the page
via KSEG0.

The same issue also exists in copy_to_user_page and copy_from_user_page
which was causing the machine to die under rare circumstances for example
when running ps if the BUG_ON() assertion added by the earlier fix was
getting triggered.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/mm/init.c

@@ -211,7 +211,7 @@ void copy_user_highpage(struct page *to, struct page *from,
 	void *vfrom, *vto;
 
 	vto = kmap_atomic(to, KM_USER1);
-	if (cpu_has_dc_aliases && !Page_dcache_dirty(from)) {
+	if (cpu_has_dc_aliases && page_mapped(from)) {
 		vfrom = kmap_coherent(from, vaddr);
 		copy_page(vto, vfrom);
 		kunmap_coherent();
@@ -234,12 +234,15 @@ void copy_to_user_page(struct vm_area_struct *vma,
 	struct page *page, unsigned long vaddr, void *dst, const void *src,
 	unsigned long len)
 {
-	if (cpu_has_dc_aliases) {
+	if (cpu_has_dc_aliases && page_mapped(page)) {
 		void *vto = kmap_coherent(page, vaddr) + (vaddr & ~PAGE_MASK);
 		memcpy(vto, src, len);
 		kunmap_coherent();
-	} else
+	} else {
 		memcpy(dst, src, len);
+		if (cpu_has_dc_aliases)
+			SetPageDcacheDirty(page);
+	}
 	if ((vma->vm_flags & VM_EXEC) && !cpu_has_ic_fills_f_dc)
 		flush_cache_page(vma, vaddr, page_to_pfn(page));
 }
@@ -250,13 +253,15 @@ void copy_from_user_page(struct vm_area_struct *vma,
 	struct page *page, unsigned long vaddr, void *dst, const void *src,
 	unsigned long len)
 {
-	if (cpu_has_dc_aliases) {
-		void *vfrom =
-			kmap_coherent(page, vaddr) + (vaddr & ~PAGE_MASK);
+	if (cpu_has_dc_aliases && page_mapped(page)) {
+		void *vfrom = kmap_coherent(page, vaddr) + (vaddr & ~PAGE_MASK);
 		memcpy(dst, vfrom, len);
 		kunmap_coherent();
-	} else
+	} else {
 		memcpy(dst, src, len);
+		if (cpu_has_dc_aliases)
+			SetPageDcacheDirty(page);
+	}
 }
 
 EXPORT_SYMBOL(copy_from_user_page);