mirror_zfs/module/spl/spl-kmem-cache.c

/*****************************************************************************\
 *  Copyright (C) 2007-2010 Lawrence Livermore National Security, LLC.
 *  Copyright (C) 2007 The Regents of the University of California.
 *  Produced at Lawrence Livermore National Laboratory (cf, DISCLAIMER).
 *  Written by Brian Behlendorf <behlendorf1@llnl.gov>.
 *  UCRL-CODE-235197
 *
 *  This file is part of the SPL, Solaris Porting Layer.
 *  For details, see <http://zfsonlinux.org/>.
 *
 *  The SPL is free software; you can redistribute it and/or modify it
 *  under the terms of the GNU General Public License as published by the
 *  Free Software Foundation; either version 2 of the License, or (at your
 *  option) any later version.
 *
 *  The SPL is distributed in the hope that it will be useful, but WITHOUT
 *  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 *  FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 *  for more details.
 *
 *  You should have received a copy of the GNU General Public License along
 *  with the SPL.  If not, see <http://www.gnu.org/licenses/>.
 *****************************************************************************
 *  Solaris Porting Layer (SPL) Kmem Implementation.
\*****************************************************************************/

#include <sys/kmem.h>
#include <sys/kmem_cache.h>
#include <sys/taskq.h>
#include <sys/timer.h>
#include <sys/vmem.h>
#include <linux/slab.h>
#include <linux/swap.h>
#include <linux/mm_compat.h>
#include <linux/wait_compat.h>

/*
 * Within the scope of spl-kmem.c file the kmem_cache_* definitions
 * are removed to allow access to the real Linux slab allocator.
 */
#undef kmem_cache_destroy
#undef kmem_cache_create
#undef kmem_cache_alloc
#undef kmem_cache_free


/*
 * Cache expiration was implemented because it was part of the default Solaris
 * kmem_cache behavior.  The idea is that per-cpu objects which haven't been
 * accessed in several seconds should be returned to the cache.  On the other
 * hand Linux slabs never move objects back to the slabs unless there is
 * memory pressure on the system.  By default the Linux method is enabled
 * because it has been shown to improve responsiveness on low memory systems.
 * This policy may be changed by setting KMC_EXPIRE_AGE or KMC_EXPIRE_MEM.
 */
unsigned int spl_kmem_cache_expire = KMC_EXPIRE_MEM;
EXPORT_SYMBOL(spl_kmem_cache_expire);
module_param(spl_kmem_cache_expire, uint, 0644);
MODULE_PARM_DESC(spl_kmem_cache_expire, "By age (0x1) or low memory (0x2)");

/*
 * The default behavior is to report the number of objects remaining in the
 * cache.  This allows the Linux VM to repeatedly reclaim objects from the
 * cache when memory is low satisfy other memory allocations.  Alternately,
 * setting this value to KMC_RECLAIM_ONCE limits how aggressively the cache
 * is reclaimed.  This may increase the likelihood of out of memory events.
 */
unsigned int spl_kmem_cache_reclaim = 0 /* KMC_RECLAIM_ONCE */;
module_param(spl_kmem_cache_reclaim, uint, 0644);
MODULE_PARM_DESC(spl_kmem_cache_reclaim, "Single reclaim pass (0x1)");

unsigned int spl_kmem_cache_obj_per_slab = SPL_KMEM_CACHE_OBJ_PER_SLAB;
module_param(spl_kmem_cache_obj_per_slab, uint, 0644);
MODULE_PARM_DESC(spl_kmem_cache_obj_per_slab, "Number of objects per slab");

unsigned int spl_kmem_cache_obj_per_slab_min = SPL_KMEM_CACHE_OBJ_PER_SLAB_MIN;
module_param(spl_kmem_cache_obj_per_slab_min, uint, 0644);
MODULE_PARM_DESC(spl_kmem_cache_obj_per_slab_min,
    "Minimal number of objects per slab");

unsigned int spl_kmem_cache_max_size = 32;
module_param(spl_kmem_cache_max_size, uint, 0644);
MODULE_PARM_DESC(spl_kmem_cache_max_size, "Maximum size of slab in MB");

/*
 * For small objects the Linux slab allocator should be used to make the most
 * efficient use of the memory.  However, large objects are not supported by
 * the Linux slab and therefore the SPL implementation is preferred.  A cutoff
 * of 16K was determined to be optimal for architectures using 4K pages.
 */
#if PAGE_SIZE == 4096
unsigned int spl_kmem_cache_slab_limit = 16384;
#else
unsigned int spl_kmem_cache_slab_limit = 0;
#endif
module_param(spl_kmem_cache_slab_limit, uint, 0644);
MODULE_PARM_DESC(spl_kmem_cache_slab_limit,
    "Objects less than N bytes use the Linux slab");

unsigned int spl_kmem_cache_kmem_limit = (PAGE_SIZE / 4);
module_param(spl_kmem_cache_kmem_limit, uint, 0644);
MODULE_PARM_DESC(spl_kmem_cache_kmem_limit,
    "Objects less than N bytes use the kmalloc");

/*
 * Slab allocation interfaces
 *
 * While the Linux slab implementation was inspired by the Solaris
 * implementation I cannot use it to emulate the Solaris APIs.  I
 * require two features which are not provided by the Linux slab.
 *
 * 1) Constructors AND destructors.  Recent versions of the Linux
 *    kernel have removed support for destructors.  This is a deal
 *    breaker for the SPL which contains particularly expensive
 *    initializers for mutex's, condition variables, etc.  We also
 *    require a minimal level of cleanup for these data types unlike
 *    many Linux data type which do need to be explicitly destroyed.
 *
 * 2) Virtual address space backed slab.  Callers of the Solaris slab
 *    expect it to work well for both small are very large allocations.
 *    Because of memory fragmentation the Linux slab which is backed
 *    by kmalloc'ed memory performs very badly when confronted with
 *    large numbers of large allocations.  Basing the slab on the
 *    virtual address space removes the need for contiguous pages
 *    and greatly improve performance for large allocations.
 *
 * For these reasons, the SPL has its own slab implementation with
 * the needed features.  It is not as highly optimized as either the
 * Solaris or Linux slabs, but it should get me most of what is
 * needed until it can be optimized or obsoleted by another approach.
 *
 * One serious concern I do have about this method is the relatively
 * small virtual address space on 32bit arches.  This will seriously
 * constrain the size of the slab caches and their performance.
 *
 * XXX: Improve the partial slab list by carefully maintaining a
 *      strict ordering of fullest to emptiest slabs based on
 *      the slab reference count.  This guarantees the when freeing
 *      slabs back to the system we need only linearly traverse the
 *      last N slabs in the list to discover all the freeable slabs.
 *
 * XXX: NUMA awareness for optionally allocating memory close to a
 *      particular core.  This can be advantageous if you know the slab
 *      object will be short lived and primarily accessed from one core.
 *
 * XXX: Slab coloring may also yield performance improvements and would
 *      be desirable to implement.
 */

struct list_head spl_kmem_cache_list;   /* List of caches */
struct rw_semaphore spl_kmem_cache_sem; /* Cache list lock */
taskq_t *spl_kmem_cache_taskq;          /* Task queue for ageing / reclaim */

static void spl_cache_shrink(spl_kmem_cache_t *skc, void *obj);

SPL_SHRINKER_CALLBACK_FWD_DECLARE(spl_kmem_cache_generic_shrinker);
SPL_SHRINKER_DECLARE(spl_kmem_cache_shrinker,
	spl_kmem_cache_generic_shrinker, KMC_DEFAULT_SEEKS);

static void *
kv_alloc(spl_kmem_cache_t *skc, int size, int flags)
{
	void *ptr;

	ASSERT(ISP2(size));

	if (skc->skc_flags & KMC_KMEM)
		ptr = (void *)__get_free_pages(flags | __GFP_COMP,
		    get_order(size));
	else
		ptr = __vmalloc(size, flags | __GFP_HIGHMEM, PAGE_KERNEL);

	/* Resulting allocated memory will be page aligned */
	ASSERT(IS_P2ALIGNED(ptr, PAGE_SIZE));

	return ptr;
}

static void
kv_free(spl_kmem_cache_t *skc, void *ptr, int size)
{
	ASSERT(IS_P2ALIGNED(ptr, PAGE_SIZE));
	ASSERT(ISP2(size));

	/*
	 * The Linux direct reclaim path uses this out of band value to
	 * determine if forward progress is being made.  Normally this is
	 * incremented by kmem_freepages() which is part of the various
	 * Linux slab implementations.  However, since we are using none
	 * of that infrastructure we are responsible for incrementing it.
	 */
	if (current->reclaim_state)
		current->reclaim_state->reclaimed_slab += size >> PAGE_SHIFT;

	if (skc->skc_flags & KMC_KMEM)
		free_pages((unsigned long)ptr, get_order(size));
	else
		vfree(ptr);
}

/*
 * Required space for each aligned sks.
 */
static inline uint32_t
spl_sks_size(spl_kmem_cache_t *skc)
{
	return P2ROUNDUP_TYPED(sizeof(spl_kmem_slab_t),
	       skc->skc_obj_align, uint32_t);
}

/*
 * Required space for each aligned object.
 */
static inline uint32_t
spl_obj_size(spl_kmem_cache_t *skc)
{
	uint32_t align = skc->skc_obj_align;

	return P2ROUNDUP_TYPED(skc->skc_obj_size, align, uint32_t) +
	       P2ROUNDUP_TYPED(sizeof(spl_kmem_obj_t), align, uint32_t);
}

/*
 * Lookup the spl_kmem_object_t for an object given that object.
 */
static inline spl_kmem_obj_t *
spl_sko_from_obj(spl_kmem_cache_t *skc, void *obj)
{
	return obj + P2ROUNDUP_TYPED(skc->skc_obj_size,
	       skc->skc_obj_align, uint32_t);
}

/*
 * Required space for each offslab object taking in to account alignment
 * restrictions and the power-of-two requirement of kv_alloc().
 */
static inline uint32_t
spl_offslab_size(spl_kmem_cache_t *skc)
{
	return 1UL << (fls64(spl_obj_size(skc)) + 1);
}

/*
 * It's important that we pack the spl_kmem_obj_t structure and the
 * actual objects in to one large address space to minimize the number
 * of calls to the allocator.  It is far better to do a few large
 * allocations and then subdivide it ourselves.  Now which allocator
 * we use requires balancing a few trade offs.
 *
 * For small objects we use kmem_alloc() because as long as you are
 * only requesting a small number of pages (ideally just one) its cheap.
 * However, when you start requesting multiple pages with kmem_alloc()
 * it gets increasingly expensive since it requires contiguous pages.
 * For this reason we shift to vmem_alloc() for slabs of large objects
 * which removes the need for contiguous pages.  We do not use
 * vmem_alloc() in all cases because there is significant locking
 * overhead in __get_vm_area_node().  This function takes a single
 * global lock when acquiring an available virtual address range which
 * serializes all vmem_alloc()'s for all slab caches.  Using slightly
 * different allocation functions for small and large objects should
 * give us the best of both worlds.
 *
 * KMC_ONSLAB                       KMC_OFFSLAB
 *
 * +------------------------+       +-----------------+
 * | spl_kmem_slab_t --+-+  |       | spl_kmem_slab_t |---+-+
 * | skc_obj_size    <-+ |  |       +-----------------+   | |
 * | spl_kmem_obj_t      |  |                             | |
 * | skc_obj_size    <---+  |       +-----------------+   | |
 * | spl_kmem_obj_t      |  |       | skc_obj_size    | <-+ |
 * | ...                 v  |       | spl_kmem_obj_t  |     |
 * +------------------------+       +-----------------+     v
 */
static spl_kmem_slab_t *
spl_slab_alloc(spl_kmem_cache_t *skc, int flags)
{
	spl_kmem_slab_t *sks;
	spl_kmem_obj_t *sko, *n;
	void *base, *obj;
	uint32_t obj_size, offslab_size = 0;
	int i,  rc = 0;

	base = kv_alloc(skc, skc->skc_slab_size, flags);
	if (base == NULL)
		return (NULL);

	sks = (spl_kmem_slab_t *)base;
	sks->sks_magic = SKS_MAGIC;
	sks->sks_objs = skc->skc_slab_objs;
	sks->sks_age = jiffies;
	sks->sks_cache = skc;
	INIT_LIST_HEAD(&sks->sks_list);
	INIT_LIST_HEAD(&sks->sks_free_list);
	sks->sks_ref = 0;
	obj_size = spl_obj_size(skc);

	if (skc->skc_flags & KMC_OFFSLAB)
		offslab_size = spl_offslab_size(skc);

	for (i = 0; i < sks->sks_objs; i++) {
		if (skc->skc_flags & KMC_OFFSLAB) {
			obj = kv_alloc(skc, offslab_size, flags);
			if (!obj) {
				rc = -ENOMEM;
				goto out;
			}
		} else {
			obj = base + spl_sks_size(skc) + (i * obj_size);
		}

		ASSERT(IS_P2ALIGNED(obj, skc->skc_obj_align));
		sko = spl_sko_from_obj(skc, obj);
		sko->sko_addr = obj;
		sko->sko_magic = SKO_MAGIC;
		sko->sko_slab = sks;
		INIT_LIST_HEAD(&sko->sko_list);
		list_add_tail(&sko->sko_list, &sks->sks_free_list);
	}

out:
	if (rc) {
		if (skc->skc_flags & KMC_OFFSLAB)
			list_for_each_entry_safe(sko, n, &sks->sks_free_list,
						 sko_list)
				kv_free(skc, sko->sko_addr, offslab_size);

		kv_free(skc, base, skc->skc_slab_size);
		sks = NULL;
	}

	return (sks);
}

/*
 * Remove a slab from complete or partial list, it must be called with
 * the 'skc->skc_lock' held but the actual free must be performed
 * outside the lock to prevent deadlocking on vmem addresses.
 */
static void
spl_slab_free(spl_kmem_slab_t *sks,
	      struct list_head *sks_list, struct list_head *sko_list)
{
	spl_kmem_cache_t *skc;

	ASSERT(sks->sks_magic == SKS_MAGIC);
	ASSERT(sks->sks_ref == 0);

	skc = sks->sks_cache;
	ASSERT(skc->skc_magic == SKC_MAGIC);
	ASSERT(spin_is_locked(&skc->skc_lock));

	/*
	 * Update slab/objects counters in the cache, then remove the
	 * slab from the skc->skc_partial_list.  Finally add the slab
	 * and all its objects in to the private work lists where the
	 * destructors will be called and the memory freed to the system.
	 */
	skc->skc_obj_total -= sks->sks_objs;
	skc->skc_slab_total--;
	list_del(&sks->sks_list);
	list_add(&sks->sks_list, sks_list);
	list_splice_init(&sks->sks_free_list, sko_list);
}

/*
 * Traverses all the partial slabs attached to a cache and free those
 * which which are currently empty, and have not been touched for
 * skc_delay seconds to  avoid thrashing.  The count argument is
 * passed to optionally cap the number of slabs reclaimed, a count
 * of zero means try and reclaim everything.  When flag is set we
 * always free an available slab regardless of age.
 */
static void
spl_slab_reclaim(spl_kmem_cache_t *skc, int count, int flag)
{
	spl_kmem_slab_t *sks, *m;
	spl_kmem_obj_t *sko, *n;
	LIST_HEAD(sks_list);
	LIST_HEAD(sko_list);
	uint32_t size = 0;
	int i = 0;

	/*
	 * Move empty slabs and objects which have not been touched in
	 * skc_delay seconds on to private lists to be freed outside
	 * the spin lock.  This delay time is important to avoid thrashing
	 * however when flag is set the delay will not be used.
	 */
	spin_lock(&skc->skc_lock);
	list_for_each_entry_safe_reverse(sks,m,&skc->skc_partial_list,sks_list){
		/*
		 * All empty slabs are at the end of skc->skc_partial_list,
		 * therefore once a non-empty slab is found we can stop
		 * scanning.  Additionally, stop when reaching the target
		 * reclaim 'count' if a non-zero threshold is given.
		 */
		if ((sks->sks_ref > 0) || (count && i >= count))
			break;

		if (time_after(jiffies,sks->sks_age+skc->skc_delay*HZ)||flag) {
			spl_slab_free(sks, &sks_list, &sko_list);
			i++;
		}
	}
	spin_unlock(&skc->skc_lock);

	/*
	 * The following two loops ensure all the object destructors are
	 * run, any offslab objects are freed, and the slabs themselves
	 * are freed.  This is all done outside the skc->skc_lock since
	 * this allows the destructor to sleep, and allows us to perform
	 * a conditional reschedule when a freeing a large number of
	 * objects and slabs back to the system.
	 */
	if (skc->skc_flags & KMC_OFFSLAB)
		size = spl_offslab_size(skc);

	list_for_each_entry_safe(sko, n, &sko_list, sko_list) {
		ASSERT(sko->sko_magic == SKO_MAGIC);

		if (skc->skc_flags & KMC_OFFSLAB)
			kv_free(skc, sko->sko_addr, size);
	}

	list_for_each_entry_safe(sks, m, &sks_list, sks_list) {
		ASSERT(sks->sks_magic == SKS_MAGIC);
		kv_free(skc, sks, skc->skc_slab_size);
	}
}

static spl_kmem_emergency_t *
spl_emergency_search(struct rb_root *root, void *obj)
{
	struct rb_node *node = root->rb_node;
	spl_kmem_emergency_t *ske;
	unsigned long address = (unsigned long)obj;

	while (node) {
		ske = container_of(node, spl_kmem_emergency_t, ske_node);

		if (address < (unsigned long)ske->ske_obj)
			node = node->rb_left;
		else if (address > (unsigned long)ske->ske_obj)
			node = node->rb_right;
		else
			return ske;
	}

	return NULL;
}

static int
spl_emergency_insert(struct rb_root *root, spl_kmem_emergency_t *ske)
{
	struct rb_node **new = &(root->rb_node), *parent = NULL;
	spl_kmem_emergency_t *ske_tmp;
	unsigned long address = (unsigned long)ske->ske_obj;

	while (*new) {
		ske_tmp = container_of(*new, spl_kmem_emergency_t, ske_node);

		parent = *new;
		if (address < (unsigned long)ske_tmp->ske_obj)
			new = &((*new)->rb_left);
		else if (address > (unsigned long)ske_tmp->ske_obj)
			new = &((*new)->rb_right);
		else
			return 0;
	}

	rb_link_node(&ske->ske_node, parent, new);
	rb_insert_color(&ske->ske_node, root);

	return 1;
}

/*
 * Allocate a single emergency object and track it in a red black tree.
 */
static int
spl_emergency_alloc(spl_kmem_cache_t *skc, int flags, void **obj)
{
	spl_kmem_emergency_t *ske;
	int empty;

	/* Last chance use a partial slab if one now exists */
	spin_lock(&skc->skc_lock);
	empty = list_empty(&skc->skc_partial_list);
	spin_unlock(&skc->skc_lock);
	if (!empty)
		return (-EEXIST);

	ske = kmalloc(sizeof(*ske), flags);
	if (ske == NULL)
		return (-ENOMEM);

	ske->ske_obj = kmalloc(skc->skc_obj_size, flags);
	if (ske->ske_obj == NULL) {
		kfree(ske);
		return (-ENOMEM);
	}

	spin_lock(&skc->skc_lock);
	empty = spl_emergency_insert(&skc->skc_emergency_tree, ske);
	if (likely(empty)) {
		skc->skc_obj_total++;
		skc->skc_obj_emergency++;
		if (skc->skc_obj_emergency > skc->skc_obj_emergency_max)
			skc->skc_obj_emergency_max = skc->skc_obj_emergency;
	}
	spin_unlock(&skc->skc_lock);

	if (unlikely(!empty)) {
		kfree(ske->ske_obj);
		kfree(ske);
		return (-EINVAL);
	}

	*obj = ske->ske_obj;

	return (0);
}

/*
 * Locate the passed object in the red black tree and free it.
 */
static int
spl_emergency_free(spl_kmem_cache_t *skc, void *obj)
{
	spl_kmem_emergency_t *ske;

	spin_lock(&skc->skc_lock);
	ske = spl_emergency_search(&skc->skc_emergency_tree, obj);
	if (likely(ske)) {
		rb_erase(&ske->ske_node, &skc->skc_emergency_tree);
		skc->skc_obj_emergency--;
		skc->skc_obj_total--;
	}
	spin_unlock(&skc->skc_lock);

	if (unlikely(ske == NULL))
		return (-ENOENT);

	kfree(ske->ske_obj);
	kfree(ske);

	return (0);
}

/*
 * Release objects from the per-cpu magazine back to their slab.  The flush
 * argument contains the max number of entries to remove from the magazine.
 */
static void
__spl_cache_flush(spl_kmem_cache_t *skc, spl_kmem_magazine_t *skm, int flush)
{
	int i, count = MIN(flush, skm->skm_avail);

	ASSERT(skc->skc_magic == SKC_MAGIC);
	ASSERT(skm->skm_magic == SKM_MAGIC);
	ASSERT(spin_is_locked(&skc->skc_lock));

	for (i = 0; i < count; i++)
		spl_cache_shrink(skc, skm->skm_objs[i]);

	skm->skm_avail -= count;
	memmove(skm->skm_objs, &(skm->skm_objs[count]),
	        sizeof(void *) * skm->skm_avail);
}

static void
spl_cache_flush(spl_kmem_cache_t *skc, spl_kmem_magazine_t *skm, int flush)
{
	spin_lock(&skc->skc_lock);
	__spl_cache_flush(skc, skm, flush);
	spin_unlock(&skc->skc_lock);
}

static void
spl_magazine_age(void *data)
{
	spl_kmem_cache_t *skc = (spl_kmem_cache_t *)data;
	spl_kmem_magazine_t *skm = skc->skc_mag[smp_processor_id()];

	ASSERT(skm->skm_magic == SKM_MAGIC);
	ASSERT(skm->skm_cpu == smp_processor_id());
	ASSERT(irqs_disabled());

	/* There are no available objects or they are too young to age out */
	if ((skm->skm_avail == 0) ||
	    time_before(jiffies, skm->skm_age + skc->skc_delay * HZ))
		return;

	/*
	 * Because we're executing in interrupt context we may have
	 * interrupted the holder of this lock.  To avoid a potential
	 * deadlock return if the lock is contended.
	 */
	if (!spin_trylock(&skc->skc_lock))
		return;

	__spl_cache_flush(skc, skm, skm->skm_refill);
	spin_unlock(&skc->skc_lock);
}

/*
 * Called regularly to keep a downward pressure on the cache.
 *
 * Objects older than skc->skc_delay seconds in the per-cpu magazines will
 * be returned to the caches.  This is done to prevent idle magazines from
 * holding memory which could be better used elsewhere.  The delay is
 * present to prevent thrashing the magazine.
 *
 * The newly released objects may result in empty partial slabs.  Those
 * slabs should be released to the system.  Otherwise moving the objects
 * out of the magazines is just wasted work.
 */
static void
spl_cache_age(void *data)
{
	spl_kmem_cache_t *skc = (spl_kmem_cache_t *)data;
	taskqid_t id = 0;

	ASSERT(skc->skc_magic == SKC_MAGIC);

	/* Dynamically disabled at run time */
	if (!(spl_kmem_cache_expire & KMC_EXPIRE_AGE))
		return;

	atomic_inc(&skc->skc_ref);

	if (!(skc->skc_flags & KMC_NOMAGAZINE))
		on_each_cpu(spl_magazine_age, skc, 1);

	spl_slab_reclaim(skc, skc->skc_reap, 0);

	while (!test_bit(KMC_BIT_DESTROY, &skc->skc_flags) && !id) {
		id = taskq_dispatch_delay(
		    spl_kmem_cache_taskq, spl_cache_age, skc, TQ_SLEEP,
		    ddi_get_lbolt() + skc->skc_delay / 3 * HZ);

		/* Destroy issued after dispatch immediately cancel it */
		if (test_bit(KMC_BIT_DESTROY, &skc->skc_flags) && id)
			taskq_cancel_id(spl_kmem_cache_taskq, id);
	}

	spin_lock(&skc->skc_lock);
	skc->skc_taskqid = id;
	spin_unlock(&skc->skc_lock);

	atomic_dec(&skc->skc_ref);
}

/*
 * Size a slab based on the size of each aligned object plus spl_kmem_obj_t.
 * When on-slab we want to target spl_kmem_cache_obj_per_slab.  However,
 * for very small objects we may end up with more than this so as not
 * to waste space in the minimal allocation of a single page.  Also for
 * very large objects we may use as few as spl_kmem_cache_obj_per_slab_min,
 * lower than this and we will fail.
 */
static int
spl_slab_size(spl_kmem_cache_t *skc, uint32_t *objs, uint32_t *size)
{
	uint32_t sks_size, obj_size, max_size;

	if (skc->skc_flags & KMC_OFFSLAB) {
		*objs = spl_kmem_cache_obj_per_slab;
		*size = P2ROUNDUP(sizeof(spl_kmem_slab_t), PAGE_SIZE);
		return (0);
	} else {
		sks_size = spl_sks_size(skc);
		obj_size = spl_obj_size(skc);

		if (skc->skc_flags & KMC_KMEM)
			max_size = ((uint32_t)1 << (MAX_ORDER-3)) * PAGE_SIZE;
		else
			max_size = (spl_kmem_cache_max_size * 1024 * 1024);

		/* Power of two sized slab */
		for (*size = PAGE_SIZE; *size <= max_size; *size *= 2) {
			*objs = (*size - sks_size) / obj_size;
			if (*objs >= spl_kmem_cache_obj_per_slab)
				return (0);
		}

		/*
		 * Unable to satisfy target objects per slab, fall back to
		 * allocating a maximally sized slab and assuming it can
		 * contain the minimum objects count use it.  If not fail.
		 */
		*size = max_size;
		*objs = (*size - sks_size) / obj_size;
		if (*objs >= (spl_kmem_cache_obj_per_slab_min))
			return (0);
	}

	return (-ENOSPC);
}

/*
 * Make a guess at reasonable per-cpu magazine size based on the size of
 * each object and the cost of caching N of them in each magazine.  Long
 * term this should really adapt based on an observed usage heuristic.
 */
static int
spl_magazine_size(spl_kmem_cache_t *skc)
{
	uint32_t obj_size = spl_obj_size(skc);
	int size;

	/* Per-magazine sizes below assume a 4Kib page size */
	if (obj_size > (PAGE_SIZE * 256))
		size = 4;  /* Minimum 4Mib per-magazine */
	else if (obj_size > (PAGE_SIZE * 32))
		size = 16; /* Minimum 2Mib per-magazine */
	else if (obj_size > (PAGE_SIZE))
		size = 64; /* Minimum 256Kib per-magazine */
	else if (obj_size > (PAGE_SIZE / 4))
		size = 128; /* Minimum 128Kib per-magazine */
	else
		size = 256;

	return (size);
}

/*
 * Allocate a per-cpu magazine to associate with a specific core.
 */
static spl_kmem_magazine_t *
spl_magazine_alloc(spl_kmem_cache_t *skc, int cpu)
{
	spl_kmem_magazine_t *skm;
	int size = sizeof(spl_kmem_magazine_t) +
	           sizeof(void *) * skc->skc_mag_size;

	skm = kmem_alloc_node(size, KM_SLEEP, cpu_to_node(cpu));
	if (skm) {
		skm->skm_magic = SKM_MAGIC;
		skm->skm_avail = 0;
		skm->skm_size = skc->skc_mag_size;
		skm->skm_refill = skc->skc_mag_refill;
		skm->skm_cache = skc;
		skm->skm_age = jiffies;
		skm->skm_cpu = cpu;
	}

	return (skm);
}

/*
 * Free a per-cpu magazine associated with a specific core.
 */
static void
spl_magazine_free(spl_kmem_magazine_t *skm)
{
	int size = sizeof(spl_kmem_magazine_t) +
	           sizeof(void *) * skm->skm_size;

	ASSERT(skm->skm_magic == SKM_MAGIC);
	ASSERT(skm->skm_avail == 0);

	kmem_free(skm, size);
}

/*
 * Create all pre-cpu magazines of reasonable sizes.
 */
static int
spl_magazine_create(spl_kmem_cache_t *skc)
{
	int i;

	if (skc->skc_flags & KMC_NOMAGAZINE)
		return (0);

	skc->skc_mag_size = spl_magazine_size(skc);
	skc->skc_mag_refill = (skc->skc_mag_size + 1) / 2;

	for_each_online_cpu(i) {
		skc->skc_mag[i] = spl_magazine_alloc(skc, i);
		if (!skc->skc_mag[i]) {
			for (i--; i >= 0; i--)
				spl_magazine_free(skc->skc_mag[i]);

			return (-ENOMEM);
		}
	}

	return (0);
}

/*
 * Destroy all pre-cpu magazines.
 */
static void
spl_magazine_destroy(spl_kmem_cache_t *skc)
{
	spl_kmem_magazine_t *skm;
	int i;

	if (skc->skc_flags & KMC_NOMAGAZINE)
		return;

        for_each_online_cpu(i) {
		skm = skc->skc_mag[i];
		spl_cache_flush(skc, skm, skm->skm_avail);
		spl_magazine_free(skm);
        }
}

/*
 * Create a object cache based on the following arguments:
 * name		cache name
 * size		cache object size
 * align	cache object alignment
 * ctor		cache object constructor
 * dtor		cache object destructor
 * reclaim	cache object reclaim
 * priv		cache private data for ctor/dtor/reclaim
 * vmp		unused must be NULL
 * flags
 *	KMC_NOTOUCH	Disable cache object aging (unsupported)
 *	KMC_NODEBUG	Disable debugging (unsupported)
 *	KMC_NOHASH      Disable hashing (unsupported)
 *	KMC_QCACHE	Disable qcache (unsupported)
 *	KMC_NOMAGAZINE	Enabled for kmem/vmem, Disabled for Linux slab
 *	KMC_KMEM	Force kmem backed cache
 *	KMC_VMEM        Force vmem backed cache
 *	KMC_SLAB        Force Linux slab backed cache
 *	KMC_OFFSLAB	Locate objects off the slab
 */
spl_kmem_cache_t *
spl_kmem_cache_create(char *name, size_t size, size_t align,
                      spl_kmem_ctor_t ctor,
                      spl_kmem_dtor_t dtor,
                      spl_kmem_reclaim_t reclaim,
                      void *priv, void *vmp, int flags)
{
        spl_kmem_cache_t *skc;
	int rc;

	/*
	 * Unsupported flags
	 */
	ASSERT0(flags & KMC_NOMAGAZINE);
	ASSERT0(flags & KMC_NOHASH);
	ASSERT0(flags & KMC_QCACHE);
	ASSERT(vmp == NULL);

	might_sleep();

	/*
	 * Allocate memory for a new cache an initialize it.  Unfortunately,
	 * this usually ends up being a large allocation of ~32k because
	 * we need to allocate enough memory for the worst case number of
	 * cpus in the magazine, skc_mag[NR_CPUS].  Because of this we
	 * explicitly pass KM_NODEBUG to suppress the kmem warning
	 */
	skc = kmem_zalloc(sizeof(*skc), KM_SLEEP| KM_NODEBUG);
	if (skc == NULL)
		return (NULL);

	skc->skc_magic = SKC_MAGIC;
	skc->skc_name_size = strlen(name) + 1;
	skc->skc_name = (char *)kmem_alloc(skc->skc_name_size, KM_SLEEP);
	if (skc->skc_name == NULL) {
		kmem_free(skc, sizeof(*skc));
		return (NULL);
	}
	strncpy(skc->skc_name, name, skc->skc_name_size);

	skc->skc_ctor = ctor;
	skc->skc_dtor = dtor;
	skc->skc_reclaim = reclaim;
	skc->skc_private = priv;
	skc->skc_vmp = vmp;
	skc->skc_linux_cache = NULL;
	skc->skc_flags = flags;
	skc->skc_obj_size = size;
	skc->skc_obj_align = SPL_KMEM_CACHE_ALIGN;
	skc->skc_delay = SPL_KMEM_CACHE_DELAY;
	skc->skc_reap = SPL_KMEM_CACHE_REAP;
	atomic_set(&skc->skc_ref, 0);

	INIT_LIST_HEAD(&skc->skc_list);
	INIT_LIST_HEAD(&skc->skc_complete_list);
	INIT_LIST_HEAD(&skc->skc_partial_list);
	skc->skc_emergency_tree = RB_ROOT;
	spin_lock_init(&skc->skc_lock);
	init_waitqueue_head(&skc->skc_waitq);
	skc->skc_slab_fail = 0;
	skc->skc_slab_create = 0;
	skc->skc_slab_destroy = 0;
	skc->skc_slab_total = 0;
	skc->skc_slab_alloc = 0;
	skc->skc_slab_max = 0;
	skc->skc_obj_total = 0;
	skc->skc_obj_alloc = 0;
	skc->skc_obj_max = 0;
	skc->skc_obj_deadlock = 0;
	skc->skc_obj_emergency = 0;
	skc->skc_obj_emergency_max = 0;

	/*
	 * Verify the requested alignment restriction is sane.
	 */
	if (align) {
		VERIFY(ISP2(align));
		VERIFY3U(align, >=, SPL_KMEM_CACHE_ALIGN);
		VERIFY3U(align, <=, PAGE_SIZE);
		skc->skc_obj_align = align;
	}

	/*
	 * When no specific type of slab is requested (kmem, vmem, or
	 * linuxslab) then select a cache type based on the object size
	 * and default tunables.
	 */
	if (!(skc->skc_flags & (KMC_KMEM | KMC_VMEM | KMC_SLAB))) {

		/*
		 * Objects smaller than spl_kmem_cache_slab_limit can
		 * use the Linux slab for better space-efficiency.  By
		 * default this functionality is disabled until its
		 * performance characters are fully understood.
		 */
		if (spl_kmem_cache_slab_limit &&
		    size <= (size_t)spl_kmem_cache_slab_limit)
			skc->skc_flags |= KMC_SLAB;

		/*
		 * Small objects, less than spl_kmem_cache_kmem_limit per
		 * object should use kmem because their slabs are small.
		 */
		else if (spl_obj_size(skc) <= spl_kmem_cache_kmem_limit)
			skc->skc_flags |= KMC_KMEM;

		/*
		 * All other objects are considered large and are placed
		 * on vmem backed slabs.
		 */
		else
			skc->skc_flags |= KMC_VMEM;
	}

	/*
	 * Given the type of slab allocate the required resources.
	 */
	if (skc->skc_flags & (KMC_KMEM | KMC_VMEM)) {
		rc = spl_slab_size(skc,
		    &skc->skc_slab_objs, &skc->skc_slab_size);
		if (rc)
			goto out;

		rc = spl_magazine_create(skc);
		if (rc)
			goto out;
	} else {
		skc->skc_linux_cache = kmem_cache_create(
		    skc->skc_name, size, align, 0, NULL);
		if (skc->skc_linux_cache == NULL) {
			rc = ENOMEM;
			goto out;
		}

		kmem_cache_set_allocflags(skc, __GFP_COMP);
		skc->skc_flags |= KMC_NOMAGAZINE;
	}

	if (spl_kmem_cache_expire & KMC_EXPIRE_AGE)
		skc->skc_taskqid = taskq_dispatch_delay(spl_kmem_cache_taskq,
		    spl_cache_age, skc, TQ_SLEEP,
		    ddi_get_lbolt() + skc->skc_delay / 3 * HZ);

	down_write(&spl_kmem_cache_sem);
	list_add_tail(&skc->skc_list, &spl_kmem_cache_list);
	up_write(&spl_kmem_cache_sem);

	return (skc);
out:
	kmem_free(skc->skc_name, skc->skc_name_size);
	kmem_free(skc, sizeof(*skc));
	return (NULL);
}
EXPORT_SYMBOL(spl_kmem_cache_create);

/*
 * Register a move callback to for cache defragmentation.
 * XXX: Unimplemented but harmless to stub out for now.
 */
void
spl_kmem_cache_set_move(spl_kmem_cache_t *skc,
    kmem_cbrc_t (move)(void *, void *, size_t, void *))
{
        ASSERT(move != NULL);
}
EXPORT_SYMBOL(spl_kmem_cache_set_move);

/*
 * Destroy a cache and all objects associated with the cache.
 */
void
spl_kmem_cache_destroy(spl_kmem_cache_t *skc)
{
	DECLARE_WAIT_QUEUE_HEAD(wq);
	taskqid_t id;

	ASSERT(skc->skc_magic == SKC_MAGIC);
	ASSERT(skc->skc_flags & (KMC_KMEM | KMC_VMEM | KMC_SLAB));

	down_write(&spl_kmem_cache_sem);
	list_del_init(&skc->skc_list);
	up_write(&spl_kmem_cache_sem);

	/* Cancel any and wait for any pending delayed tasks */
	VERIFY(!test_and_set_bit(KMC_BIT_DESTROY, &skc->skc_flags));

	spin_lock(&skc->skc_lock);
	id = skc->skc_taskqid;
	spin_unlock(&skc->skc_lock);

	taskq_cancel_id(spl_kmem_cache_taskq, id);

	/* Wait until all current callers complete, this is mainly
	 * to catch the case where a low memory situation triggers a
	 * cache reaping action which races with this destroy. */
	wait_event(wq, atomic_read(&skc->skc_ref) == 0);

	if (skc->skc_flags & (KMC_KMEM | KMC_VMEM)) {
		spl_magazine_destroy(skc);
		spl_slab_reclaim(skc, 0, 1);
	} else {
		ASSERT(skc->skc_flags & KMC_SLAB);
		kmem_cache_destroy(skc->skc_linux_cache);
	}

	spin_lock(&skc->skc_lock);

	/* Validate there are no objects in use and free all the
	 * spl_kmem_slab_t, spl_kmem_obj_t, and object buffers. */
	ASSERT3U(skc->skc_slab_alloc, ==, 0);
	ASSERT3U(skc->skc_obj_alloc, ==, 0);
	ASSERT3U(skc->skc_slab_total, ==, 0);
	ASSERT3U(skc->skc_obj_total, ==, 0);
	ASSERT3U(skc->skc_obj_emergency, ==, 0);
	ASSERT(list_empty(&skc->skc_complete_list));

	kmem_free(skc->skc_name, skc->skc_name_size);
	spin_unlock(&skc->skc_lock);

	kmem_free(skc, sizeof(*skc));
}
EXPORT_SYMBOL(spl_kmem_cache_destroy);

/*
 * Allocate an object from a slab attached to the cache.  This is used to
 * repopulate the per-cpu magazine caches in batches when they run low.
 */
static void *
spl_cache_obj(spl_kmem_cache_t *skc, spl_kmem_slab_t *sks)
{
	spl_kmem_obj_t *sko;

	ASSERT(skc->skc_magic == SKC_MAGIC);
	ASSERT(sks->sks_magic == SKS_MAGIC);
	ASSERT(spin_is_locked(&skc->skc_lock));

	sko = list_entry(sks->sks_free_list.next, spl_kmem_obj_t, sko_list);
	ASSERT(sko->sko_magic == SKO_MAGIC);
	ASSERT(sko->sko_addr != NULL);

	/* Remove from sks_free_list */
	list_del_init(&sko->sko_list);

	sks->sks_age = jiffies;
	sks->sks_ref++;
	skc->skc_obj_alloc++;

	/* Track max obj usage statistics */
	if (skc->skc_obj_alloc > skc->skc_obj_max)
		skc->skc_obj_max = skc->skc_obj_alloc;

	/* Track max slab usage statistics */
	if (sks->sks_ref == 1) {
		skc->skc_slab_alloc++;

		if (skc->skc_slab_alloc > skc->skc_slab_max)
			skc->skc_slab_max = skc->skc_slab_alloc;
	}

	return sko->sko_addr;
}

/*
 * Generic slab allocation function to run by the global work queues.
 * It is responsible for allocating a new slab, linking it in to the list
 * of partial slabs, and then waking any waiters.
 */
static void
spl_cache_grow_work(void *data)
{
	spl_kmem_alloc_t *ska = (spl_kmem_alloc_t *)data;
	spl_kmem_cache_t *skc = ska->ska_cache;
	spl_kmem_slab_t *sks;

	sks = spl_slab_alloc(skc, ska->ska_flags | __GFP_NORETRY | KM_NODEBUG);
	spin_lock(&skc->skc_lock);
	if (sks) {
		skc->skc_slab_total++;
		skc->skc_obj_total += sks->sks_objs;
		list_add_tail(&sks->sks_list, &skc->skc_partial_list);
	}

	atomic_dec(&skc->skc_ref);
	clear_bit(KMC_BIT_GROWING, &skc->skc_flags);
	clear_bit(KMC_BIT_DEADLOCKED, &skc->skc_flags);
	wake_up_all(&skc->skc_waitq);
	spin_unlock(&skc->skc_lock);

	kfree(ska);
}

/*
 * Returns non-zero when a new slab should be available.
 */
static int
spl_cache_grow_wait(spl_kmem_cache_t *skc)
{
	return !test_bit(KMC_BIT_GROWING, &skc->skc_flags);
}

/*
 * No available objects on any slabs, create a new slab.  Note that this
 * functionality is disabled for KMC_SLAB caches which are backed by the
 * Linux slab.
 */
static int
spl_cache_grow(spl_kmem_cache_t *skc, int flags, void **obj)
{
	int remaining, rc;

	ASSERT(skc->skc_magic == SKC_MAGIC);
	ASSERT((skc->skc_flags & KMC_SLAB) == 0);
	might_sleep();
	*obj = NULL;

	/*
	 * Before allocating a new slab wait for any reaping to complete and
	 * then return so the local magazine can be rechecked for new objects.
	 */
	if (test_bit(KMC_BIT_REAPING, &skc->skc_flags)) {
		rc = spl_wait_on_bit(&skc->skc_flags, KMC_BIT_REAPING,
		    TASK_UNINTERRUPTIBLE);
		return (rc ? rc : -EAGAIN);
	}

	/*
	 * This is handled by dispatching a work request to the global work
	 * queue.  This allows us to asynchronously allocate a new slab while
	 * retaining the ability to safely fall back to a smaller synchronous
	 * allocations to ensure forward progress is always maintained.
	 */
	if (test_and_set_bit(KMC_BIT_GROWING, &skc->skc_flags) == 0) {
		spl_kmem_alloc_t *ska;

		ska = kmalloc(sizeof(*ska), flags);
		if (ska == NULL) {
			clear_bit(KMC_BIT_GROWING, &skc->skc_flags);
			wake_up_all(&skc->skc_waitq);
			return (-ENOMEM);
		}

		atomic_inc(&skc->skc_ref);
		ska->ska_cache = skc;
		ska->ska_flags = flags & ~__GFP_FS;
		taskq_init_ent(&ska->ska_tqe);
		taskq_dispatch_ent(spl_kmem_cache_taskq,
		    spl_cache_grow_work, ska, 0, &ska->ska_tqe);
	}

	/*
	 * The goal here is to only detect the rare case where a virtual slab
	 * allocation has deadlocked.  We must be careful to minimize the use
	 * of emergency objects which are more expensive to track.  Therefore,
	 * we set a very long timeout for the asynchronous allocation and if
	 * the timeout is reached the cache is flagged as deadlocked.  From
	 * this point only new emergency objects will be allocated until the
	 * asynchronous allocation completes and clears the deadlocked flag.
	 */
	if (test_bit(KMC_BIT_DEADLOCKED, &skc->skc_flags)) {
		rc = spl_emergency_alloc(skc, flags, obj);
	} else {
		remaining = wait_event_timeout(skc->skc_waitq,
					       spl_cache_grow_wait(skc), HZ);

		if (!remaining && test_bit(KMC_BIT_VMEM, &skc->skc_flags)) {
			spin_lock(&skc->skc_lock);
			if (test_bit(KMC_BIT_GROWING, &skc->skc_flags)) {
				set_bit(KMC_BIT_DEADLOCKED, &skc->skc_flags);
				skc->skc_obj_deadlock++;
			}
			spin_unlock(&skc->skc_lock);
		}

		rc = -ENOMEM;
	}

	return (rc);
}

/*
 * Refill a per-cpu magazine with objects from the slabs for this cache.
 * Ideally the magazine can be repopulated using existing objects which have
 * been released, however if we are unable to locate enough free objects new
 * slabs of objects will be created.  On success NULL is returned, otherwise
 * the address of a single emergency object is returned for use by the caller.
 */
static void *
spl_cache_refill(spl_kmem_cache_t *skc, spl_kmem_magazine_t *skm, int flags)
{
	spl_kmem_slab_t *sks;
	int count = 0, rc, refill;
	void *obj = NULL;

	ASSERT(skc->skc_magic == SKC_MAGIC);
	ASSERT(skm->skm_magic == SKM_MAGIC);

	refill = MIN(skm->skm_refill, skm->skm_size - skm->skm_avail);
	spin_lock(&skc->skc_lock);

	while (refill > 0) {
		/* No slabs available we may need to grow the cache */
		if (list_empty(&skc->skc_partial_list)) {
			spin_unlock(&skc->skc_lock);

			local_irq_enable();
			rc = spl_cache_grow(skc, flags, &obj);
			local_irq_disable();

			/* Emergency object for immediate use by caller */
			if (rc == 0 && obj != NULL)
				return (obj);

			if (rc)
				goto out;

			/* Rescheduled to different CPU skm is not local */
			if (skm != skc->skc_mag[smp_processor_id()])
				goto out;

			/* Potentially rescheduled to the same CPU but
			 * allocations may have occurred from this CPU while
			 * we were sleeping so recalculate max refill. */
			refill = MIN(refill, skm->skm_size - skm->skm_avail);

			spin_lock(&skc->skc_lock);
			continue;
		}

		/* Grab the next available slab */
		sks = list_entry((&skc->skc_partial_list)->next,
		                 spl_kmem_slab_t, sks_list);
		ASSERT(sks->sks_magic == SKS_MAGIC);
		ASSERT(sks->sks_ref < sks->sks_objs);
		ASSERT(!list_empty(&sks->sks_free_list));

		/* Consume as many objects as needed to refill the requested
		 * cache.  We must also be careful not to overfill it. */
		while (sks->sks_ref < sks->sks_objs && refill-- > 0 && ++count) {
			ASSERT(skm->skm_avail < skm->skm_size);
			ASSERT(count < skm->skm_size);
			skm->skm_objs[skm->skm_avail++]=spl_cache_obj(skc,sks);
		}

		/* Move slab to skc_complete_list when full */
		if (sks->sks_ref == sks->sks_objs) {
			list_del(&sks->sks_list);
			list_add(&sks->sks_list, &skc->skc_complete_list);
		}
	}

	spin_unlock(&skc->skc_lock);
out:
	return (NULL);
}

/*
 * Release an object back to the slab from which it came.
 */
static void
spl_cache_shrink(spl_kmem_cache_t *skc, void *obj)
{
	spl_kmem_slab_t *sks = NULL;
	spl_kmem_obj_t *sko = NULL;

	ASSERT(skc->skc_magic == SKC_MAGIC);
	ASSERT(spin_is_locked(&skc->skc_lock));

	sko = spl_sko_from_obj(skc, obj);
	ASSERT(sko->sko_magic == SKO_MAGIC);
	sks = sko->sko_slab;
	ASSERT(sks->sks_magic == SKS_MAGIC);
	ASSERT(sks->sks_cache == skc);
	list_add(&sko->sko_list, &sks->sks_free_list);

	sks->sks_age = jiffies;
	sks->sks_ref--;
	skc->skc_obj_alloc--;

	/* Move slab to skc_partial_list when no longer full.  Slabs
	 * are added to the head to keep the partial list is quasi-full
	 * sorted order.  Fuller at the head, emptier at the tail. */
	if (sks->sks_ref == (sks->sks_objs - 1)) {
		list_del(&sks->sks_list);
		list_add(&sks->sks_list, &skc->skc_partial_list);
	}

	/* Move empty slabs to the end of the partial list so
	 * they can be easily found and freed during reclamation. */
	if (sks->sks_ref == 0) {
		list_del(&sks->sks_list);
		list_add_tail(&sks->sks_list, &skc->skc_partial_list);
		skc->skc_slab_alloc--;
	}
}

/*
 * Allocate an object from the per-cpu magazine, or if the magazine
 * is empty directly allocate from a slab and repopulate the magazine.
 */
void *
spl_kmem_cache_alloc(spl_kmem_cache_t *skc, int flags)
{
	spl_kmem_magazine_t *skm;
	void *obj = NULL;

	ASSERT(skc->skc_magic == SKC_MAGIC);
	ASSERT(!test_bit(KMC_BIT_DESTROY, &skc->skc_flags));
	ASSERT(flags & KM_SLEEP);

	atomic_inc(&skc->skc_ref);

	/*
	 * Allocate directly from a Linux slab.  All optimizations are left
	 * to the underlying cache we only need to guarantee that KM_SLEEP
	 * callers will never fail.
	 */
	if (skc->skc_flags & KMC_SLAB) {
		struct kmem_cache *slc = skc->skc_linux_cache;

		do {
			obj = kmem_cache_alloc(slc, flags | __GFP_COMP);
		} while ((obj == NULL) && !(flags & KM_NOSLEEP));

		goto ret;
	}

	local_irq_disable();

restart:
	/* Safe to update per-cpu structure without lock, but
	 * in the restart case we must be careful to reacquire
	 * the local magazine since this may have changed
	 * when we need to grow the cache. */
	skm = skc->skc_mag[smp_processor_id()];
	ASSERT(skm->skm_magic == SKM_MAGIC);

	if (likely(skm->skm_avail)) {
		/* Object available in CPU cache, use it */
		obj = skm->skm_objs[--skm->skm_avail];
		skm->skm_age = jiffies;
	} else {
		obj = spl_cache_refill(skc, skm, flags);
		if (obj == NULL)
			goto restart;
	}

	local_irq_enable();
	ASSERT(obj);
	ASSERT(IS_P2ALIGNED(obj, skc->skc_obj_align));

ret:
	/* Pre-emptively migrate object to CPU L1 cache */
	if (obj) {
		if (obj && skc->skc_ctor)
			skc->skc_ctor(obj, skc->skc_private, flags);
		else
			prefetchw(obj);
	}

	atomic_dec(&skc->skc_ref);

	return (obj);
}

EXPORT_SYMBOL(spl_kmem_cache_alloc);

/*
 * Free an object back to the local per-cpu magazine, there is no
 * guarantee that this is the same magazine the object was originally
 * allocated from.  We may need to flush entire from the magazine
 * back to the slabs to make space.
 */
void
spl_kmem_cache_free(spl_kmem_cache_t *skc, void *obj)
{
	spl_kmem_magazine_t *skm;
	unsigned long flags;

	ASSERT(skc->skc_magic == SKC_MAGIC);
	ASSERT(!test_bit(KMC_BIT_DESTROY, &skc->skc_flags));
	atomic_inc(&skc->skc_ref);

	/*
	 * Run the destructor
	 */
	if (skc->skc_dtor)
		skc->skc_dtor(obj, skc->skc_private);

	/*
	 * Free the object from the Linux underlying Linux slab.
	 */
	if (skc->skc_flags & KMC_SLAB) {
		kmem_cache_free(skc->skc_linux_cache, obj);
		goto out;
	}

	/*
	 * Only virtual slabs may have emergency objects and these objects
	 * are guaranteed to have physical addresses.  They must be removed
	 * from the tree of emergency objects and the freed.
	 */
	if ((skc->skc_flags & KMC_VMEM) && !kmem_virt(obj)) {
		spl_emergency_free(skc, obj);
		goto out;
	}

	local_irq_save(flags);

	/* Safe to update per-cpu structure without lock, but
	 * no remote memory allocation tracking is being performed
	 * it is entirely possible to allocate an object from one
	 * CPU cache and return it to another. */
	skm = skc->skc_mag[smp_processor_id()];
	ASSERT(skm->skm_magic == SKM_MAGIC);

	/* Per-CPU cache full, flush it to make space */
	if (unlikely(skm->skm_avail >= skm->skm_size))
		spl_cache_flush(skc, skm, skm->skm_refill);

	/* Available space in cache, use it */
	skm->skm_objs[skm->skm_avail++] = obj;

	local_irq_restore(flags);
out:
	atomic_dec(&skc->skc_ref);
}
EXPORT_SYMBOL(spl_kmem_cache_free);

/*
 * The generic shrinker function for all caches.  Under Linux a shrinker
 * may not be tightly coupled with a slab cache.  In fact Linux always
 * systematically tries calling all registered shrinker callbacks which
 * report that they contain unused objects.  Because of this we only
 * register one shrinker function in the shim layer for all slab caches.
 * We always attempt to shrink all caches when this generic shrinker
 * is called.
 *
 * If sc->nr_to_scan is zero, the caller is requesting a query of the
 * number of objects which can potentially be freed.  If it is nonzero,
 * the request is to free that many objects.
 *
 * Linux kernels >= 3.12 have the count_objects and scan_objects callbacks
 * in struct shrinker and also require the shrinker to return the number
 * of objects freed.
 *
 * Older kernels require the shrinker to return the number of freeable
 * objects following the freeing of nr_to_free.
 *
 * Linux semantics differ from those under Solaris, which are to
 * free all available objects which may (and probably will) be more
 * objects than the requested nr_to_scan.
 */
static spl_shrinker_t
__spl_kmem_cache_generic_shrinker(struct shrinker *shrink,
    struct shrink_control *sc)
{
	spl_kmem_cache_t *skc;
	int alloc = 0;

	down_read(&spl_kmem_cache_sem);
	list_for_each_entry(skc, &spl_kmem_cache_list, skc_list) {
		if (sc->nr_to_scan) {
#ifdef HAVE_SPLIT_SHRINKER_CALLBACK
			uint64_t oldalloc = skc->skc_obj_alloc;
			spl_kmem_cache_reap_now(skc,
			   MAX(sc->nr_to_scan >> fls64(skc->skc_slab_objs), 1));
			if (oldalloc > skc->skc_obj_alloc)
				alloc += oldalloc - skc->skc_obj_alloc;
#else
			spl_kmem_cache_reap_now(skc,
			   MAX(sc->nr_to_scan >> fls64(skc->skc_slab_objs), 1));
			alloc += skc->skc_obj_alloc;
#endif /* HAVE_SPLIT_SHRINKER_CALLBACK */
		} else {
			/* Request to query number of freeable objects */
			alloc += skc->skc_obj_alloc;
		}
	}
	up_read(&spl_kmem_cache_sem);

	/*
	 * When KMC_RECLAIM_ONCE is set allow only a single reclaim pass.
	 * This functionality only exists to work around a rare issue where
	 * shrink_slabs() is repeatedly invoked by many cores causing the
	 * system to thrash.
	 */
	if ((spl_kmem_cache_reclaim & KMC_RECLAIM_ONCE) && sc->nr_to_scan)
		return (SHRINK_STOP);

	return (MAX(alloc, 0));
}

SPL_SHRINKER_CALLBACK_WRAPPER(spl_kmem_cache_generic_shrinker);

/*
 * Call the registered reclaim function for a cache.  Depending on how
 * many and which objects are released it may simply repopulate the
 * local magazine which will then need to age-out.  Objects which cannot
 * fit in the magazine we will be released back to their slabs which will
 * also need to age out before being release.  This is all just best
 * effort and we do not want to thrash creating and destroying slabs.
 */
void
spl_kmem_cache_reap_now(spl_kmem_cache_t *skc, int count)
{
	ASSERT(skc->skc_magic == SKC_MAGIC);
	ASSERT(!test_bit(KMC_BIT_DESTROY, &skc->skc_flags));

	atomic_inc(&skc->skc_ref);

	/*
	 * Execute the registered reclaim callback if it exists.  The
	 * per-cpu caches will be drained when is set KMC_EXPIRE_MEM.
	 */
	if (skc->skc_flags & KMC_SLAB) {
		if (skc->skc_reclaim)
			skc->skc_reclaim(skc->skc_private);

		if (spl_kmem_cache_expire & KMC_EXPIRE_MEM)
			kmem_cache_shrink(skc->skc_linux_cache);

		goto out;
	}

	/*
	 * Prevent concurrent cache reaping when contended.
	 */
	if (test_and_set_bit(KMC_BIT_REAPING, &skc->skc_flags))
		goto out;

	/*
	 * When a reclaim function is available it may be invoked repeatedly
	 * until at least a single slab can be freed.  This ensures that we
	 * do free memory back to the system.  This helps minimize the chance
	 * of an OOM event when the bulk of memory is used by the slab.
	 *
	 * When free slabs are already available the reclaim callback will be
	 * skipped.  Additionally, if no forward progress is detected despite
	 * a reclaim function the cache will be skipped to avoid deadlock.
	 *
	 * Longer term this would be the correct place to add the code which
	 * repacks the slabs in order minimize fragmentation.
	 */
	if (skc->skc_reclaim) {
		uint64_t objects = UINT64_MAX;
		int do_reclaim;

		do {
			spin_lock(&skc->skc_lock);
			do_reclaim =
			    (skc->skc_slab_total > 0) &&
			    ((skc->skc_slab_total - skc->skc_slab_alloc) == 0) &&
			    (skc->skc_obj_alloc < objects);

			objects = skc->skc_obj_alloc;
			spin_unlock(&skc->skc_lock);

			if (do_reclaim)
				skc->skc_reclaim(skc->skc_private);

		} while (do_reclaim);
	}

	/* Reclaim from the magazine then the slabs ignoring age and delay. */
	if (spl_kmem_cache_expire & KMC_EXPIRE_MEM) {
		spl_kmem_magazine_t *skm;
		unsigned long irq_flags;

		local_irq_save(irq_flags);
		skm = skc->skc_mag[smp_processor_id()];
		spl_cache_flush(skc, skm, skm->skm_avail);
		local_irq_restore(irq_flags);
	}

	spl_slab_reclaim(skc, count, 1);
	clear_bit(KMC_BIT_REAPING, &skc->skc_flags);
	smp_wmb();
	wake_up_bit(&skc->skc_flags, KMC_BIT_REAPING);
out:
	atomic_dec(&skc->skc_ref);
}
EXPORT_SYMBOL(spl_kmem_cache_reap_now);

/*
 * Reap all free slabs from all registered caches.
 */
void
spl_kmem_reap(void)
{
	struct shrink_control sc;

	sc.nr_to_scan = KMC_REAP_CHUNK;
	sc.gfp_mask = GFP_KERNEL;

	(void) __spl_kmem_cache_generic_shrinker(NULL, &sc);
}
EXPORT_SYMBOL(spl_kmem_reap);

int
spl_kmem_cache_init(void)
{
	init_rwsem(&spl_kmem_cache_sem);
	INIT_LIST_HEAD(&spl_kmem_cache_list);
	spl_kmem_cache_taskq = taskq_create("spl_kmem_cache",
	    1, maxclsyspri, 1, 32, TASKQ_PREPOPULATE);
	spl_register_shrinker(&spl_kmem_cache_shrinker);

	return (0);
}

void
spl_kmem_cache_fini(void)
{
	spl_unregister_shrinker(&spl_kmem_cache_shrinker);
	taskq_destroy(spl_kmem_cache_taskq);
}