[PATCH] s390: endless loop in follow_page.

From: Martin Schwidefsky <schwidefsky@de.ibm.com>

Fix endless loop in get_user_pages() on s390.  It happens only on s/390
because pte_dirty always returns 0.  For all other architectures this is an
optimization.

In the case of "write && !pte_dirty(pte)" follow_page() returns NULL.  On all
architectures except s390 handle_pte_fault() will then create a pte with
pte_dirty(pte)==1 because write_access==1.  In the following, second call to
follow_page() all is fine.  With the physical dirty bit patch pte_dirty() is
always 0 for s/390 because the dirty bit doesn't live in the pte.

mm/memory.c

@@ -651,7 +651,13 @@ follow_page(struct mm_struct *mm, unsigned long address, int write)
 	pte = *ptep;
 	pte_unmap(ptep);
 	if (pte_present(pte)) {
-		if (!write || (pte_write(pte) && pte_dirty(pte))) {
+		if (write && !pte_write(pte))
+			goto out;
+		if (write && !pte_dirty(pte)) {
+			struct page *page = pte_page(pte);
+			if (!PageDirty(page))
+				set_page_dirty(page);
+		}
 		pfn = pte_pfn(pte);
 		if (pfn_valid(pfn)) {
 			struct page *page = pfn_to_page(pfn);
@@ -660,7 +666,6 @@ follow_page(struct mm_struct *mm, unsigned long address, int write)
 			return page;
 		}
 	}
-	}
 
 out:
 	return NULL;