diff --git a/lib/idr.c b/lib/idr.c
index 2d016f5c410e..63dda62131b3 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -43,6 +43,14 @@ static DEFINE_PER_CPU(struct idr_layer *, idr_preload_head);
 static DEFINE_PER_CPU(int, idr_preload_cnt);
 static DEFINE_SPINLOCK(simple_ida_lock);
 
+/* the maximum ID which can be allocated given idr->layers */
+static int idr_max(int layers)
+{
+	int bits = min_t(int, layers * IDR_BITS, MAX_IDR_SHIFT);
+
+	return (1 << bits) - 1;
+}
+
 static struct idr_layer *get_from_free_list(struct idr *idp)
 {
 	struct idr_layer *p;
@@ -290,7 +298,7 @@ build_up:
 	 * Add a new layer to the top of the tree if the requested
 	 * id is larger than the currently allocated space.
 	 */
-	while ((layers < (MAX_IDR_LEVEL - 1)) && (id >= (1 << (layers*IDR_BITS)))) {
+	while (id > idr_max(layers)) {
 		layers++;
 		if (!p->count) {
 			/* special case: if the tree is currently empty,
@@ -361,7 +369,7 @@ static void idr_fill_slot(void *ptr, int id, struct idr_layer **pa)
  */
 int idr_get_new_above(struct idr *idp, void *ptr, int starting_id, int *id)
 {
-	struct idr_layer *pa[MAX_IDR_LEVEL];
+	struct idr_layer *pa[MAX_IDR_LEVEL + 1];
 	int rv;
 
 	rv = idr_get_empty_slot(idp, starting_id, pa, 0, idp);
@@ -457,7 +465,7 @@ EXPORT_SYMBOL(idr_preload);
 int idr_alloc(struct idr *idr, void *ptr, int start, int end, gfp_t gfp_mask)
 {
 	int max = end > 0 ? end - 1 : INT_MAX;	/* inclusive upper limit */
-	struct idr_layer *pa[MAX_IDR_LEVEL];
+	struct idr_layer *pa[MAX_IDR_LEVEL + 1];
 	int id;
 
 	might_sleep_if(gfp_mask & __GFP_WAIT);
@@ -490,7 +498,7 @@ static void idr_remove_warning(int id)
 static void sub_remove(struct idr *idp, int shift, int id)
 {
 	struct idr_layer *p = idp->top;
-	struct idr_layer **pa[MAX_IDR_LEVEL];
+	struct idr_layer **pa[MAX_IDR_LEVEL + 1];
 	struct idr_layer ***paa = &pa[0];
 	struct idr_layer *to_free;
 	int n;
@@ -571,16 +579,16 @@ void __idr_remove_all(struct idr *idp)
 	int n, id, max;
 	int bt_mask;
 	struct idr_layer *p;
-	struct idr_layer *pa[MAX_IDR_LEVEL];
+	struct idr_layer *pa[MAX_IDR_LEVEL + 1];
 	struct idr_layer **paa = &pa[0];
 
 	n = idp->layers * IDR_BITS;
 	p = idp->top;
 	rcu_assign_pointer(idp->top, NULL);
-	max = 1 << n;
+	max = idr_max(idp->layers);
 
 	id = 0;
-	while (id < max) {
+	while (id >= 0 && id <= max) {
 		while (n > IDR_BITS && p) {
 			n -= IDR_BITS;
 			*paa++ = p;
@@ -650,7 +658,7 @@ void *idr_find(struct idr *idp, int id)
 	/* Mask off upper bits we don't use for the search. */
 	id &= MAX_IDR_MASK;
 
-	if (id >= (1 << n))
+	if (id > idr_max(p->layer + 1))
 		return NULL;
 	BUG_ON(n == 0);
 
@@ -686,15 +694,15 @@ int idr_for_each(struct idr *idp,
 {
 	int n, id, max, error = 0;
 	struct idr_layer *p;
-	struct idr_layer *pa[MAX_IDR_LEVEL];
+	struct idr_layer *pa[MAX_IDR_LEVEL + 1];
 	struct idr_layer **paa = &pa[0];
 
 	n = idp->layers * IDR_BITS;
 	p = rcu_dereference_raw(idp->top);
-	max = 1 << n;
+	max = idr_max(idp->layers);
 
 	id = 0;
-	while (id < max) {
+	while (id >= 0 && id <= max) {
 		while (n > 0 && p) {
 			n -= IDR_BITS;
 			*paa++ = p;
@@ -732,7 +740,7 @@ EXPORT_SYMBOL(idr_for_each);
  */
 void *idr_get_next(struct idr *idp, int *nextidp)
 {
-	struct idr_layer *p, *pa[MAX_IDR_LEVEL];
+	struct idr_layer *p, *pa[MAX_IDR_LEVEL + 1];
 	struct idr_layer **paa = &pa[0];
 	int id = *nextidp;
 	int n, max;
@@ -742,9 +750,9 @@ void *idr_get_next(struct idr *idp, int *nextidp)
 	if (!p)
 		return NULL;
 	n = (p->layer + 1) * IDR_BITS;
-	max = 1 << n;
+	max = idr_max(p->layer + 1);
 
-	while (id < max) {
+	while (id >= 0 && id <= max) {
 		while (n > 0 && p) {
 			n -= IDR_BITS;
 			*paa++ = p;
@@ -918,7 +926,7 @@ EXPORT_SYMBOL(ida_pre_get);
  */
 int ida_get_new_above(struct ida *ida, int starting_id, int *p_id)
 {
-	struct idr_layer *pa[MAX_IDR_LEVEL];
+	struct idr_layer *pa[MAX_IDR_LEVEL + 1];
 	struct ida_bitmap *bitmap;
 	unsigned long flags;
 	int idr_id = starting_id / IDA_BITMAP_BITS;