[mirror_spl-debian.git] / 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/>.
 */

#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>
#include <linux/prefetch.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


/*
 * Linux 3.16 replaced smp_mb__{before,after}_{atomic,clear}_{dec,inc,bit}()
 * with smp_mb__{before,after}_atomic() because they were redundant. This is
 * only used inside our SLAB allocator, so we implement an internal wrapper
 * here to give us smp_mb__{before,after}_atomic() on older kernels.
 */
#ifndef smp_mb__before_atomic
#define smp_mb__before_atomic(x) smp_mb__before_clear_bit(x)
#endif

#ifndef smp_mb__after_atomic
#define smp_mb__after_atomic(x) smp_mb__after_clear_bit(x)
#endif

/*
 * 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)");

/*
 * Cache magazines are an optimization designed to minimize the cost of
 * allocating memory.  They do this by keeping a per-cpu cache of recently
 * freed objects, which can then be reallocated without taking a lock. This
 * can improve performance on highly contended caches.  However, because
 * objects in magazines will prevent otherwise empty slabs from being
 * immediately released this may not be ideal for low memory machines.
 *
 * For this reason spl_kmem_cache_magazine_size can be used to set a maximum
 * magazine size.  When this value is set to 0 the magazine size will be
 * automatically determined based on the object size.  Otherwise magazines
 * will be limited to 2-256 objects per magazine (i.e per cpu).  Magazines
 * may never be entirely disabled in this implementation.
 */
unsigned int spl_kmem_cache_magazine_size = 0;
module_param(spl_kmem_cache_magazine_size, uint, 0444);
MODULE_PARM_DESC(spl_kmem_cache_magazine_size,
        "Default magazine size (2-256), set automatically (0)\n");

/*
 * 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 = SPL_KMEM_CACHE_MAX_SIZE;
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");

/*
 * This value defaults to a threshold designed to avoid allocations which
 * have been deemed costly by the kernel.
 */
unsigned int spl_kmem_cache_kmem_limit =
    ((1 << (PAGE_ALLOC_COSTLY_ORDER - 1)) * PAGE_SIZE) /
    SPL_KMEM_CACHE_OBJ_PER_SLAB;
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");

/*
 * The number of threads available to allocate new slabs for caches.  This
 * should not need to be tuned but it is available for performance analysis.
 */
unsigned int spl_kmem_cache_kmem_threads = 4;
module_param(spl_kmem_cache_kmem_threads, uint, 0444);
MODULE_PARM_DESC(spl_kmem_cache_kmem_threads,
        "Number of spl_kmem_cache threads");

/*
 * 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 types 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.
 */

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)
{
        gfp_t lflags = kmem_flags_convert(flags);
        void *ptr;

        if (skc->skc_flags & KMC_KMEM) {
                ASSERT(ISP2(size));
                ptr = (void *)__get_free_pages(lflags, get_order(size));
        } else {
                ptr = __vmalloc(size, lflags | __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));

        /*
         * 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) {
                ASSERT(ISP2(size));
                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);
}

/*
 * Reclaim empty slabs at the end of the partial list.
 */
static void
spl_slab_reclaim(spl_kmem_cache_t *skc)
{
        spl_kmem_slab_t *sks, *m;
        spl_kmem_obj_t *sko, *n;
        LIST_HEAD(sks_list);
        LIST_HEAD(sko_list);
        uint32_t size = 0;

        /*
         * Empty slabs and objects must be moved to a private list so they
         * can be safely freed outside the spin lock.  All empty slabs are
         * at the end of skc->skc_partial_list, therefore once a non-empty
         * slab is found we can stop scanning.
         */
        spin_lock(&skc->skc_lock);
        list_for_each_entry_safe_reverse(sks, m,
            &skc->skc_partial_list, sks_list) {

                if (sks->sks_ref > 0)
                        break;

                spl_slab_free(sks, &sks_list, &sko_list);
        }
        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 < ske->ske_obj)
                        node = node->rb_left;
                else if (address > 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 = ske->ske_obj;

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

                parent = *new;
                if (address < ske_tmp->ske_obj)
                        new = &((*new)->rb_left);
                else if (address > 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)
{
        gfp_t lflags = kmem_flags_convert(flags);
        spl_kmem_emergency_t *ske;
        int order = get_order(skc->skc_obj_size);
        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), lflags);
        if (ske == NULL)
                return (-ENOMEM);

        ske->ske_obj = __get_free_pages(lflags, order);
        if (ske->ske_obj == 0) {
                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)) {
                free_pages(ske->ske_obj, order);
                kfree(ske);
                return (-EINVAL);
        }

        *obj = (void *)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;
        int order = get_order(skc->skc_obj_size);

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

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

        free_pages(ske->ske_obj, order);
        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);

        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, tgt_size, tgt_objs;

        if (skc->skc_flags & KMC_OFFSLAB) {
                tgt_objs = spl_kmem_cache_obj_per_slab;
                tgt_size = P2ROUNDUP(sizeof (spl_kmem_slab_t), PAGE_SIZE);

                if ((skc->skc_flags & KMC_KMEM) &&
                    (spl_obj_size(skc) > (SPL_MAX_ORDER_NR_PAGES * PAGE_SIZE)))
                        return (-ENOSPC);
        } else {
                sks_size = spl_sks_size(skc);
                obj_size = spl_obj_size(skc);
                max_size = (spl_kmem_cache_max_size * 1024 * 1024);
                tgt_size = (spl_kmem_cache_obj_per_slab * obj_size + sks_size);

                /*
                 * KMC_KMEM slabs are allocated by __get_free_pages() which
                 * rounds up to the nearest order.  Knowing this the size
                 * should be rounded up to the next power of two with a hard
                 * maximum defined by the maximum allowed allocation order.
                 */
                if (skc->skc_flags & KMC_KMEM) {
                        max_size = SPL_MAX_ORDER_NR_PAGES * PAGE_SIZE;
                        tgt_size = MIN(max_size,
                            PAGE_SIZE * (1 << MAX(get_order(tgt_size) - 1, 1)));
                }

                if (tgt_size <= max_size) {
                        tgt_objs = (tgt_size - sks_size) / obj_size;
                } else {
                        tgt_objs = (max_size - sks_size) / obj_size;
                        tgt_size = (tgt_objs * obj_size) + sks_size;
                }
        }

        if (tgt_objs == 0)
                return (-ENOSPC);

        *objs = tgt_objs;
        *size = tgt_size;

        return (0);
}

/*
 * 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;

        if (spl_kmem_cache_magazine_size > 0)
                return (MAX(MIN(spl_kmem_cache_magazine_size, 256), 2));

        /* 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 = kmalloc_node(size, GFP_KERNEL, 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)
{
        ASSERT(skm->skm_magic == SKM_MAGIC);
        ASSERT(skm->skm_avail == 0);
        kfree(skm);
}

/*
 * 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 = kzalloc(sizeof (spl_kmem_magazine_t *) *
            num_possible_cpus(), kmem_flags_convert(KM_SLEEP));
        skc->skc_mag_size = spl_magazine_size(skc);
        skc->skc_mag_refill = (skc->skc_mag_size + 1) / 2;

        for_each_possible_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]);

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

        kfree(skc->skc_mag);
}

/*
 * 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)
{
        gfp_t lflags = kmem_flags_convert(KM_SLEEP);
        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();

        skc = kzalloc(sizeof (*skc), lflags);
        if (skc == NULL)
                return (NULL);

        skc->skc_magic = SKC_MAGIC;
        skc->skc_name_size = strlen(name) + 1;
        skc->skc_name = (char *)kmalloc(skc->skc_name_size, lflags);
        if (skc->skc_name == NULL) {
                kfree(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 characteristics 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 {
                unsigned long slabflags = 0;

                if (size > (SPL_MAX_KMEM_ORDER_NR_PAGES * PAGE_SIZE)) {
                        rc = EINVAL;
                        goto out;
                }

#if defined(SLAB_USERCOPY)
                /*
                 * Required for PAX-enabled kernels if the slab is to be
                 * used for coping between user and kernel space.
                 */
                slabflags |= SLAB_USERCOPY;
#endif

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

#if defined(HAVE_KMEM_CACHE_ALLOCFLAGS)
                skc->skc_linux_cache->allocflags |= __GFP_COMP;
#elif defined(HAVE_KMEM_CACHE_GFPFLAGS)
                skc->skc_linux_cache->gfpflags |= __GFP_COMP;
#endif
                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:
        kfree(skc->skc_name);
        kfree(skc);
        return (NULL);
}
EXPORT_SYMBOL(spl_kmem_cache_create);

/*
 * Register a move callback 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);
        } 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));

        spin_unlock(&skc->skc_lock);

        kfree(skc->skc_name);
        kfree(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;

        fstrans_cookie_t cookie = spl_fstrans_mark();
        sks = spl_slab_alloc(skc, ska->ska_flags);
        spl_fstrans_unmark(cookie);

        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);
        smp_mb__before_atomic();
        clear_bit(KMC_BIT_GROWING, &skc->skc_flags);
        clear_bit(KMC_BIT_DEADLOCKED, &skc->skc_flags);
        smp_mb__after_atomic();
        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 = 0;

        ASSERT0(flags & ~KM_PUBLIC_MASK);
        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), kmem_flags_convert(flags));
                if (ska == NULL) {
                        clear_bit_unlock(KMC_BIT_GROWING, &skc->skc_flags);
                        smp_mb__after_atomic();
                        wake_up_all(&skc->skc_waitq);
                        return (-ENOMEM);
                }

                atomic_inc(&skc->skc_ref);
                ska->ska_cache = skc;
                ska->ska_flags = flags;
                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 / 10);

                if (!remaining) {
                        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;

        ASSERT0(flags & ~KM_PUBLIC_MASK);
        ASSERT(skc->skc_magic == SKC_MAGIC);
        ASSERT(!test_bit(KMC_BIT_DESTROY, &skc->skc_flags));

        /*
         * 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, kmem_flags_convert(flags));
                } 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) && !(flags & KM_NOSLEEP))
                        goto restart;

                local_irq_enable();
                goto ret;
        }

        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);
        }

        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;
        int do_reclaim = 0;
        int do_emergency = 0;

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

        /*
         * 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);
                return;
        }

        /*
         * While a cache has outstanding emergency objects all freed objects
         * must be checked.  However, since emergency objects will never use
         * a virtual address these objects can be safely excluded as an
         * optimization.
         */
        if (!is_vmalloc_addr(obj)) {
                spin_lock(&skc->skc_lock);
                do_emergency = (skc->skc_obj_emergency > 0);
                spin_unlock(&skc->skc_lock);

                if (do_emergency && (spl_emergency_free(skc, obj) == 0))
                        return;
        }

        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 for this object,
         * this may result in an empty slab which can be reclaimed once
         * interrupts are re-enabled.
         */
        if (unlikely(skm->skm_avail >= skm->skm_size)) {
                spl_cache_flush(skc, skm, skm->skm_refill);
                do_reclaim = 1;
        }

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

        local_irq_restore(flags);

        if (do_reclaim)
                spl_slab_reclaim(skc);
}
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;

        /*
         * No shrinking in a transaction context.  Can cause deadlocks.
         */
        if (sc->nr_to_scan && spl_fstrans_check())
                return (SHRINK_STOP);

        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.
         */
        if (skc->skc_flags & KMC_SLAB) {
                if (skc->skc_reclaim)
                        skc->skc_reclaim(skc->skc_private);
                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 and free all now empty slabs. */
        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);
        clear_bit_unlock(KMC_BIT_REAPING, &skc->skc_flags);
        smp_mb__after_atomic();
        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",
            spl_kmem_cache_kmem_threads, maxclsyspri,
            spl_kmem_cache_kmem_threads * 8, INT_MAX,
            TASKQ_PREPOPULATE | TASKQ_DYNAMIC);
        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);
}