[PATCH] move_one_page kmap atomicity fix

move_one_page() is calling alloc_one_pte_map() while holding an atomic
kmap for the source pte's page.  But alloc_one_pte_map() can sleep in
the page allocator.

So change move_one_page() to take a peek at the destination pagetables
to work out whether the alloc_one_pte_map() will need to perform page
allocation.  If so, drop the atomic kmap and retake it after allocating
the pte.

mm/mremap.c

@@ -53,6 +53,22 @@ static inline pte_t *get_one_pte_map_nested(struct mm_struct *mm, unsigned long
 	return pte;
 }
 
+#ifdef CONFIG_HIGHPTE	/* Save a few cycles on the sane machines */
+static inline int page_table_present(struct mm_struct *mm, unsigned long addr)
+{
+	pgd_t *pgd;
+	pmd_t *pmd;
+
+	pgd = pgd_offset(mm, addr);
+	if (pgd_none(*pgd))
+		return 0;
+	pmd = pmd_offset(pgd, addr);
+	return pmd_present(*pmd);
+}
+#else
+#define page_table_present(mm, addr)	(1)
+#endif
+
 static inline pte_t *alloc_one_pte_map(struct mm_struct *mm, unsigned long addr)
 {
 	pmd_t * pmd;
@@ -98,7 +114,18 @@ static int move_one_page(struct vm_area_struct *vma, unsigned long old_addr, uns
 	spin_lock(&mm->page_table_lock);
 	src = get_one_pte_map_nested(mm, old_addr);
 	if (src) {
+		/*
+		 * Look to see whether alloc_one_pte_map needs to perform a
+		 * memory allocation.  If it does then we need to drop the
+		 * atomic kmap
+		 */
+		if (!page_table_present(mm, new_addr)) {
+			pte_unmap_nested(src);
+			src = NULL;
+		}
 		dst = alloc_one_pte_map(mm, new_addr);
+		if (src == NULL)
+			src = get_one_pte_map_nested(mm, old_addr);
 		error = copy_one_pte(mm, src, dst);
 		pte_unmap_nested(src);
 		pte_unmap(dst);