diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
index e0ab173..21bc1f7 100644
--- a/arch/x86/mm/pat.c
+++ b/arch/x86/mm/pat.c
@@ -641,10 +641,11 @@
 	is_ram = pat_pagerange_is_ram(paddr, paddr + size);
 
 	/*
-	 * reserve_pfn_range() doesn't support RAM pages.
+	 * reserve_pfn_range() doesn't support RAM pages. Maintain the current
+	 * behavior with RAM pages by returning success.
 	 */
 	if (is_ram != 0)
-		return -EINVAL;
+		return 0;
 
 	ret = reserve_memtype(paddr, paddr + size, want_flags, &flags);
 	if (ret)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 065cdf8..3daa05f 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -98,7 +98,7 @@
 #define VM_HUGETLB	0x00400000	/* Huge TLB Page VM */
 #define VM_NONLINEAR	0x00800000	/* Is non-linear (remap_file_pages) */
 #define VM_MAPPED_COPY	0x01000000	/* T if mapped copy of data (nommu mmap) */
-#define VM_INSERTPAGE	0x02000000	/* The vma has had "vm_insert_page()" done on it */
+#define VM_INSERTPAGE	0x02000000	/* The vma has had "vm_insert_page()" done on it. Refer note in VM_PFNMAP_AT_MMAP below */
 #define VM_ALWAYSDUMP	0x04000000	/* Always include in core dumps */
 
 #define VM_CAN_NONLINEAR 0x08000000	/* Has ->fault & does nonlinear pages */
@@ -127,6 +127,17 @@
 #define VM_SPECIAL (VM_IO | VM_DONTEXPAND | VM_RESERVED | VM_PFNMAP)
 
 /*
+ * pfnmap vmas that are fully mapped at mmap time (not mapped on fault).
+ * Used by x86 PAT to identify such PFNMAP mappings and optimize their handling.
+ * Note VM_INSERTPAGE flag is overloaded here. i.e,
+ * VM_INSERTPAGE && !VM_PFNMAP implies
+ *     The vma has had "vm_insert_page()" done on it
+ * VM_INSERTPAGE && VM_PFNMAP implies
+ *     The vma is PFNMAP with full mapping at mmap time
+ */
+#define VM_PFNMAP_AT_MMAP (VM_INSERTPAGE | VM_PFNMAP)
+
+/*
  * mapping from the currently active vm_flags protection bits (the
  * low four bits) to a page protection mask..
  */
@@ -145,7 +156,7 @@
  */
 static inline int is_linear_pfn_mapping(struct vm_area_struct *vma)
 {
-	return ((vma->vm_flags & VM_PFNMAP) && vma->vm_pgoff);
+	return ((vma->vm_flags & VM_PFNMAP_AT_MMAP) == VM_PFNMAP_AT_MMAP);
 }
 
 static inline int is_pfn_mapping(struct vm_area_struct *vma)
diff --git a/mm/memory.c b/mm/memory.c
index baa999e..d7df5ba 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1665,9 +1665,10 @@
 	 * behaviour that some programs depend on. We mark the "original"
 	 * un-COW'ed pages by matching them up with "vma->vm_pgoff".
 	 */
-	if (addr == vma->vm_start && end == vma->vm_end)
+	if (addr == vma->vm_start && end == vma->vm_end) {
 		vma->vm_pgoff = pfn;
-	else if (is_cow_mapping(vma->vm_flags))
+		vma->vm_flags |= VM_PFNMAP_AT_MMAP;
+	} else if (is_cow_mapping(vma->vm_flags))
 		return -EINVAL;
 
 	vma->vm_flags |= VM_IO | VM_RESERVED | VM_PFNMAP;
@@ -1679,6 +1680,7 @@
 		 * needed from higher level routine calling unmap_vmas
 		 */
 		vma->vm_flags &= ~(VM_IO | VM_RESERVED | VM_PFNMAP);
+		vma->vm_flags &= ~VM_PFNMAP_AT_MMAP;
 		return -EINVAL;
 	}