diff --git a/include/linux/idr.h b/include/linux/idr.h
index 871a213..f0db12b 100644
--- a/include/linux/idr.h
+++ b/include/linux/idr.h
@@ -16,6 +16,8 @@
 #include <linux/bitops.h>
 #include <linux/init.h>
 #include <linux/rcupdate.h>
+#include <linux/spinlock_types.h>
+#include <linux/wait.h>
 
 /*
  * We want shallower trees and thus more bits covered at each layer.  8
@@ -243,4 +245,55 @@ static inline int ida_get_new(struct ida *ida, int *p_id)
 
 void __init idr_init_cache(void);
 
+/* Percpu IDA/tag allocator */
+
+struct percpu_ida_cpu;
+
+struct percpu_ida {
+	/*
+	 * number of tags available to be allocated, as passed to
+	 * percpu_ida_init()
+	 */
+	unsigned			nr_tags;
+
+	struct percpu_ida_cpu __percpu	*tag_cpu;
+
+	/*
+	 * Bitmap of cpus that (may) have tags on their percpu freelists:
+	 * steal_tags() uses this to decide when to steal tags, and which cpus
+	 * to try stealing from.
+	 *
+	 * It's ok for a freelist to be empty when its bit is set - steal_tags()
+	 * will just keep looking - but the bitmap _must_ be set whenever a
+	 * percpu freelist does have tags.
+	 */
+	unsigned long			*cpus_have_tags;
+
+	struct {
+		spinlock_t		lock;
+		/*
+		 * When we go to steal tags from another cpu (see steal_tags()),
+		 * we want to pick a cpu at random. Cycling through them every
+		 * time we steal is a bit easier and more or less equivalent:
+		 */
+		unsigned		cpu_last_stolen;
+
+		/* For sleeping on allocation failure */
+		wait_queue_head_t	wait;
+
+		/*
+		 * Global freelist - it's a stack where nr_free points to the
+		 * top
+		 */
+		unsigned		nr_free;
+		unsigned		*freelist;
+	} ____cacheline_aligned_in_smp;
+};
+
+int percpu_ida_alloc(struct percpu_ida *pool, gfp_t gfp);
+void percpu_ida_free(struct percpu_ida *pool, unsigned tag);
+
+void percpu_ida_destroy(struct percpu_ida *pool);
+int percpu_ida_init(struct percpu_ida *pool, unsigned long nr_tags);
+
 #endif /* __IDR_H__ */
diff --git a/lib/idr.c b/lib/idr.c
index bfe4db4..57bfabe 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -26,17 +26,20 @@
  * with the slab allocator.
  */
 
-#ifndef TEST                        // to test in user space...
-#include <linux/slab.h>
-#include <linux/init.h>
-#include <linux/export.h>
-#endif
+#include <linux/bitmap.h>
+#include <linux/bitops.h>
+#include <linux/bug.h>
 #include <linux/err.h>
-#include <linux/string.h>
+#include <linux/export.h>
+#include <linux/hardirq.h>
 #include <linux/idr.h>
-#include <linux/spinlock.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
 #include <linux/percpu.h>
-#include <linux/hardirq.h>
+#include <linux/sched.h>
+#include <linux/slab.h>
+#include <linux/string.h>
+#include <linux/spinlock.h>
 
 #define MAX_IDR_SHIFT		(sizeof(int) * 8 - 1)
 #define MAX_IDR_BIT		(1U << MAX_IDR_SHIFT)
@@ -1159,3 +1162,300 @@ void ida_init(struct ida *ida)
 
 }
 EXPORT_SYMBOL(ida_init);
+
+/* Percpu IDA */
+
+/*
+ * Number of tags we move between the percpu freelist and the global freelist at
+ * a time
+ */
+#define IDA_PCPU_BATCH_MOVE	32U
+
+/* Max size of percpu freelist, */
+#define IDA_PCPU_SIZE		((IDA_PCPU_BATCH_MOVE * 3) / 2)
+
+struct percpu_ida_cpu {
+	spinlock_t			lock;
+	unsigned			nr_free;
+	unsigned			freelist[];
+};
+
+static inline void move_tags(unsigned *dst, unsigned *dst_nr,
+			     unsigned *src, unsigned *src_nr,
+			     unsigned nr)
+{
+	*src_nr -= nr;
+	memcpy(dst + *dst_nr, src + *src_nr, sizeof(unsigned) * nr);
+	*dst_nr += nr;
+}
+
+/*
+ * Try to steal tags from a remote cpu's percpu freelist.
+ *
+ * We first check how many percpu freelists have tags - we don't steal tags
+ * unless enough percpu freelists have tags on them that it's possible more than
+ * half the total tags could be stuck on remote percpu freelists.
+ *
+ * Then we iterate through the cpus until we find some tags - we don't attempt
+ * to find the "best" cpu to steal from, to keep cacheline bouncing to a
+ * minimum.
+ */
+static inline void steal_tags(struct percpu_ida *pool,
+			      struct percpu_ida_cpu *tags)
+{
+	unsigned cpus_have_tags, cpu = pool->cpu_last_stolen;
+	struct percpu_ida_cpu *remote;
+
+	for (cpus_have_tags = bitmap_weight(pool->cpus_have_tags, nr_cpu_ids);
+	     cpus_have_tags * IDA_PCPU_SIZE > pool->nr_tags / 2;
+	     cpus_have_tags--) {
+		cpu = find_next_bit(pool->cpus_have_tags, nr_cpu_ids, cpu);
+
+		if (cpu == nr_cpu_ids)
+			cpu = find_first_bit(pool->cpus_have_tags, nr_cpu_ids);
+
+		if (cpu == nr_cpu_ids)
+			BUG();
+
+		pool->cpu_last_stolen = cpu;
+		remote = per_cpu_ptr(pool->tag_cpu, cpu);
+
+		clear_bit(cpu, pool->cpus_have_tags);
+
+		if (remote == tags)
+			continue;
+
+		spin_lock(&remote->lock);
+
+		if (remote->nr_free) {
+			memcpy(tags->freelist,
+			       remote->freelist,
+			       sizeof(unsigned) * remote->nr_free);
+
+			tags->nr_free = remote->nr_free;
+			remote->nr_free = 0;
+		}
+
+		spin_unlock(&remote->lock);
+
+		if (tags->nr_free)
+			break;
+	}
+}
+
+static inline void alloc_global_tags(struct percpu_ida *pool,
+				     struct percpu_ida_cpu *tags)
+{
+	move_tags(tags->freelist, &tags->nr_free,
+		  pool->freelist, &pool->nr_free,
+		  min(pool->nr_free, IDA_PCPU_BATCH_MOVE));
+}
+
+static inline unsigned alloc_local_tag(struct percpu_ida *pool,
+				       struct percpu_ida_cpu *tags)
+{
+	int tag = -ENOSPC;
+
+	spin_lock(&tags->lock);
+	if (tags->nr_free)
+		tag = tags->freelist[--tags->nr_free];
+	spin_unlock(&tags->lock);
+
+	return tag;
+}
+
+/**
+ * percpu_ida_alloc - allocate a tag
+ * @pool: pool to allocate from
+ * @gfp: gfp flags
+ *
+ * Returns a tag - an integer in the range [0..nr_tags) (passed to
+ * tag_pool_init()), or otherwise -ENOSPC on allocation failure.
+ *
+ * Safe to be called from interrupt context (assuming it isn't passed
+ * __GFP_WAIT, of course).
+ *
+ * Will not fail if passed __GFP_WAIT.
+ */
+int percpu_ida_alloc(struct percpu_ida *pool, gfp_t gfp)
+{
+	DEFINE_WAIT(wait);
+	struct percpu_ida_cpu *tags;
+	unsigned long flags;
+	int tag;
+
+	local_irq_save(flags);
+	tags = this_cpu_ptr(pool->tag_cpu);
+
+	/* Fastpath */
+	tag = alloc_local_tag(pool, tags);
+	if (likely(tag >= 0)) {
+		local_irq_restore(flags);
+		return tag;
+	}
+
+	while (1) {
+		spin_lock(&pool->lock);
+
+		/*
+		 * prepare_to_wait() must come before steal_tags(), in case
+		 * percpu_ida_free() on another cpu flips a bit in
+		 * cpus_have_tags
+		 *
+		 * global lock held and irqs disabled, don't need percpu lock
+		 */
+		prepare_to_wait(&pool->wait, &wait, TASK_UNINTERRUPTIBLE);
+
+		if (!tags->nr_free)
+			alloc_global_tags(pool, tags);
+		if (!tags->nr_free)
+			steal_tags(pool, tags);
+
+		if (tags->nr_free) {
+			tag = tags->freelist[--tags->nr_free];
+			if (tags->nr_free)
+				set_bit(smp_processor_id(),
+					pool->cpus_have_tags);
+		}
+
+		spin_unlock(&pool->lock);
+		local_irq_restore(flags);
+
+		if (tag >= 0 || !(gfp & __GFP_WAIT))
+			break;
+
+		schedule();
+
+		local_irq_save(flags);
+		tags = this_cpu_ptr(pool->tag_cpu);
+	}
+
+	finish_wait(&pool->wait, &wait);
+	return tag;
+}
+EXPORT_SYMBOL_GPL(percpu_ida_alloc);
+
+/**
+ * percpu_ida_free - free a tag
+ * @pool: pool @tag was allocated from
+ * @tag: a tag previously allocated with percpu_ida_alloc()
+ *
+ * Safe to be called from interrupt context.
+ */
+void percpu_ida_free(struct percpu_ida *pool, unsigned tag)
+{
+	struct percpu_ida_cpu *tags;
+	unsigned long flags;
+	unsigned nr_free;
+
+	BUG_ON(tag >= pool->nr_tags);
+
+	local_irq_save(flags);
+	tags = this_cpu_ptr(pool->tag_cpu);
+
+	spin_lock(&tags->lock);
+	tags->freelist[tags->nr_free++] = tag;
+
+	nr_free = tags->nr_free;
+	spin_unlock(&tags->lock);
+
+	if (nr_free == 1) {
+		set_bit(smp_processor_id(),
+			pool->cpus_have_tags);
+		wake_up(&pool->wait);
+	}
+
+	if (nr_free == IDA_PCPU_SIZE) {
+		spin_lock(&pool->lock);
+
+		/*
+		 * Global lock held and irqs disabled, don't need percpu
+		 * lock
+		 */
+		if (tags->nr_free == IDA_PCPU_SIZE) {
+			move_tags(pool->freelist, &pool->nr_free,
+				  tags->freelist, &tags->nr_free,
+				  IDA_PCPU_BATCH_MOVE);
+
+			wake_up(&pool->wait);
+		}
+		spin_unlock(&pool->lock);
+	}
+
+	local_irq_restore(flags);
+}
+EXPORT_SYMBOL_GPL(percpu_ida_free);
+
+/**
+ * percpu_ida_destroy - release a tag pool's resources
+ * @pool: pool to free
+ *
+ * Frees the resources allocated by percpu_ida_init().
+ */
+void percpu_ida_destroy(struct percpu_ida *pool)
+{
+	free_percpu(pool->tag_cpu);
+	kfree(pool->cpus_have_tags);
+	free_pages((unsigned long) pool->freelist,
+		   get_order(pool->nr_tags * sizeof(unsigned)));
+}
+EXPORT_SYMBOL_GPL(percpu_ida_destroy);
+
+/**
+ * percpu_ida_init - initialize a percpu tag pool
+ * @pool: pool to initialize
+ * @nr_tags: number of tags that will be available for allocation
+ *
+ * Initializes @pool so that it can be used to allocate tags - integers in the
+ * range [0, nr_tags). Typically, they'll be used by driver code to refer to a
+ * preallocated array of tag structures.
+ *
+ * Allocation is percpu, but sharding is limited by nr_tags - for best
+ * performance, the workload should not span more cpus than nr_tags / 128.
+ */
+int percpu_ida_init(struct percpu_ida *pool, unsigned long nr_tags)
+{
+	unsigned i, cpu, order;
+
+	memset(pool, 0, sizeof(*pool));
+
+	init_waitqueue_head(&pool->wait);
+	spin_lock_init(&pool->lock);
+	pool->nr_tags = nr_tags;
+
+	/* Guard against overflow */
+	if (nr_tags > (unsigned) INT_MAX + 1) {
+		pr_err("tags.c: nr_tags too large\n");
+		return -EINVAL;
+	}
+
+	order = get_order(nr_tags * sizeof(unsigned));
+	pool->freelist = (void *) __get_free_pages(GFP_KERNEL, order);
+	if (!pool->freelist)
+		return -ENOMEM;
+
+	for (i = 0; i < nr_tags; i++)
+		pool->freelist[i] = i;
+
+	pool->nr_free = nr_tags;
+
+	pool->cpus_have_tags = kzalloc(BITS_TO_LONGS(nr_cpu_ids) *
+				       sizeof(unsigned long), GFP_KERNEL);
+	if (!pool->cpus_have_tags)
+		goto err;
+
+	pool->tag_cpu = __alloc_percpu(sizeof(struct percpu_ida_cpu) +
+				       IDA_PCPU_SIZE * sizeof(unsigned),
+				       sizeof(unsigned));
+	if (!pool->tag_cpu)
+		goto err;
+
+	for_each_possible_cpu(cpu)
+		spin_lock_init(&per_cpu_ptr(pool->tag_cpu, cpu)->lock);
+
+	return 0;
+err:
+	percpu_ida_destroy(pool);
+	return -ENOMEM;
+}
+EXPORT_SYMBOL_GPL(percpu_ida_init);
