diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 6511012cbdcd86d971a6cdebcf6e1baad66ccded..a89cb52228c2ccf14d7df0317b646f30f407610e 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -292,8 +292,8 @@ static int mmap_kmem(struct file * file, struct vm_area_struct * vma)
 {
 	unsigned long pfn;
 
-	/* Turn a kernel-virtual address into a physical page frame */
-	pfn = __pa((u64)vma->vm_pgoff << PAGE_SHIFT) >> PAGE_SHIFT;
+	/* Turn a pfn offset into an absolute pfn */
+	pfn = PFN_DOWN(virt_to_phys((void *)PAGE_OFFSET)) + vma->vm_pgoff;
 
 	/*
 	 * RED-PEN: on some architectures there is more mapped memory