diff --git a/dlls/ntoskrnl.exe/ntoskrnl.c b/dlls/ntoskrnl.exe/ntoskrnl.c
index 912d0840318..eac81f79276 100644
--- a/dlls/ntoskrnl.exe/ntoskrnl.c
+++ b/dlls/ntoskrnl.exe/ntoskrnl.c
@@ -744,31 +744,20 @@ PVOID WINAPI IoAllocateErrorLogEntry( PVOID IoObject, UCHAR EntrySize )
  */
 PMDL WINAPI IoAllocateMdl( PVOID va, ULONG length, BOOLEAN secondary, BOOLEAN charge_quota, IRP *irp )
 {
+    SIZE_T mdl_size;
     PMDL mdl;
-    ULONG_PTR address = (ULONG_PTR)va;
-    ULONG_PTR page_address;
-    SIZE_T nb_pages, mdl_size;
 
     TRACE("(%p, %u, %i, %i, %p)\n", va, length, secondary, charge_quota, irp);
 
     if (charge_quota)
         FIXME("Charge quota is not yet supported\n");
 
-    /* FIXME: We suppose that page size is 4096 */
-    page_address = address & ~(4096 - 1);
-    nb_pages = (((address + length - 1) & ~(4096 - 1)) - page_address) / 4096 + 1;
-
-    mdl_size = sizeof(MDL) + nb_pages * sizeof(PVOID);
-
-    mdl = HeapAlloc(GetProcessHeap(), HEAP_ZERO_MEMORY, mdl_size);
+    mdl_size = sizeof(MDL) + sizeof(PFN_NUMBER) * ADDRESS_AND_SIZE_TO_SPAN_PAGES(va, length);
+    mdl = HeapAlloc( GetProcessHeap(), HEAP_ZERO_MEMORY, mdl_size );
     if (!mdl)
         return NULL;
 
-    mdl->Size = mdl_size;
-    mdl->Process = NULL; /* FIXME: IoGetCurrentProcess */
-    mdl->StartVa = (PVOID)page_address;
-    mdl->ByteCount = length;
-    mdl->ByteOffset = address - page_address;
+    MmInitializeMdl( mdl, va, length );
 
     if (!irp) return mdl;
 
diff --git a/include/ddk/wdm.h b/include/ddk/wdm.h
index 3e934697961..3829e8b0554 100644
--- a/include/ddk/wdm.h
+++ b/include/ddk/wdm.h
@@ -28,6 +28,16 @@
 #define POINTER_ALIGNMENT
 #endif
 
+/* FIXME: We suppose that page size is 4096 */
+#undef PAGE_SIZE
+#define PAGE_SIZE   0x1000
+#define PAGE_SHIFT  12
+
+#define BYTE_OFFSET(va) ((ULONG)((ULONG_PTR)(va) & (PAGE_SIZE - 1)))
+#define PAGE_ALIGN(va)  ((PVOID)((ULONG_PTR)(va) & ~(PAGE_SIZE - 1)))
+#define ADDRESS_AND_SIZE_TO_SPAN_PAGES(va, length) \
+    ((BYTE_OFFSET(va) + ((SIZE_T)(length)) + (PAGE_SIZE - 1)) >> PAGE_SHIFT)
+
 typedef LONG KPRIORITY;
 
 typedef ULONG_PTR KSPIN_LOCK, *PKSPIN_LOCK;
@@ -1035,6 +1045,17 @@ typedef struct _MDL {
 } MDL, *PMDL;
 
 typedef MDL *PMDLX;
+typedef ULONG PFN_NUMBER, *PPFN_NUMBER;
+
+static inline void MmInitializeMdl(MDL *mdl, void *va, SIZE_T length)
+{
+    mdl->Next       = NULL;
+    mdl->Size       = sizeof(MDL) + sizeof(PFN_NUMBER) * ADDRESS_AND_SIZE_TO_SPAN_PAGES(va, length);
+    mdl->MdlFlags   = 0;
+    mdl->StartVa    = (void *)PAGE_ALIGN(va);
+    mdl->ByteOffset = BYTE_OFFSET(va);
+    mdl->ByteCount  = length;
+}
 
 typedef struct _KTIMER {
     DISPATCHER_HEADER Header;