[linux.git] / block / blk-cgroup.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Common Block IO controller cgroup interface
 *
 * Based on ideas and code from CFQ, CFS and BFQ:
 * Copyright (C) 2003 Jens Axboe <axboe@kernel.dk>
 *
 * Copyright (C) 2008 Fabio Checconi <fabio@gandalf.sssup.it>
 *                    Paolo Valente <paolo.valente@unimore.it>
 *
 * Copyright (C) 2009 Vivek Goyal <vgoyal@redhat.com>
 *                    Nauman Rafique <nauman@google.com>
 *
 * For policy-specific per-blkcg data:
 * Copyright (C) 2015 Paolo Valente <paolo.valente@unimore.it>
 *                    Arianna Avanzini <avanzini.arianna@gmail.com>
 */
#include <linux/ioprio.h>
#include <linux/kdev_t.h>
#include <linux/module.h>
#include <linux/sched/signal.h>
#include <linux/err.h>
#include <linux/blkdev.h>
#include <linux/backing-dev.h>
#include <linux/slab.h>
#include <linux/genhd.h>
#include <linux/delay.h>
#include <linux/atomic.h>
#include <linux/ctype.h>
#include <linux/blk-cgroup.h>
#include <linux/tracehook.h>
#include "blk.h"

#define MAX_KEY_LEN 100

/*
 * blkcg_pol_mutex protects blkcg_policy[] and policy [de]activation.
 * blkcg_pol_register_mutex nests outside of it and synchronizes entire
 * policy [un]register operations including cgroup file additions /
 * removals.  Putting cgroup file registration outside blkcg_pol_mutex
 * allows grabbing it from cgroup callbacks.
 */
static DEFINE_MUTEX(blkcg_pol_register_mutex);
static DEFINE_MUTEX(blkcg_pol_mutex);

struct blkcg blkcg_root;
EXPORT_SYMBOL_GPL(blkcg_root);

struct cgroup_subsys_state * const blkcg_root_css = &blkcg_root.css;

static struct blkcg_policy *blkcg_policy[BLKCG_MAX_POLS];

static LIST_HEAD(all_blkcgs);           /* protected by blkcg_pol_mutex */

static bool blkcg_debug_stats = false;

static bool blkcg_policy_enabled(struct request_queue *q,
                                 const struct blkcg_policy *pol)
{
        return pol && test_bit(pol->plid, q->blkcg_pols);
}

/**
 * blkg_free - free a blkg
 * @blkg: blkg to free
 *
 * Free @blkg which may be partially allocated.
 */
static void blkg_free(struct blkcg_gq *blkg)
{
        int i;

        if (!blkg)
                return;

        for (i = 0; i < BLKCG_MAX_POLS; i++)
                if (blkg->pd[i])
                        blkcg_policy[i]->pd_free_fn(blkg->pd[i]);

        blkg_rwstat_exit(&blkg->stat_ios);
        blkg_rwstat_exit(&blkg->stat_bytes);
        percpu_ref_exit(&blkg->refcnt);
        kfree(blkg);
}

static void __blkg_release(struct rcu_head *rcu)
{
        struct blkcg_gq *blkg = container_of(rcu, struct blkcg_gq, rcu_head);

        /* release the blkcg and parent blkg refs this blkg has been holding */
        css_put(&blkg->blkcg->css);
        if (blkg->parent)
                blkg_put(blkg->parent);

        wb_congested_put(blkg->wb_congested);

        blkg_free(blkg);
}

/*
 * A group is RCU protected, but having an rcu lock does not mean that one
 * can access all the fields of blkg and assume these are valid.  For
 * example, don't try to follow throtl_data and request queue links.
 *
 * Having a reference to blkg under an rcu allows accesses to only values
 * local to groups like group stats and group rate limits.
 */
static void blkg_release(struct percpu_ref *ref)
{
        struct blkcg_gq *blkg = container_of(ref, struct blkcg_gq, refcnt);

        call_rcu(&blkg->rcu_head, __blkg_release);
}

/**
 * blkg_alloc - allocate a blkg
 * @blkcg: block cgroup the new blkg is associated with
 * @q: request_queue the new blkg is associated with
 * @gfp_mask: allocation mask to use
 *
 * Allocate a new blkg assocating @blkcg and @q.
 */
static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct request_queue *q,
                                   gfp_t gfp_mask)
{
        struct blkcg_gq *blkg;
        int i;

        /* alloc and init base part */
        blkg = kzalloc_node(sizeof(*blkg), gfp_mask, q->node);
        if (!blkg)
                return NULL;

        if (percpu_ref_init(&blkg->refcnt, blkg_release, 0, gfp_mask))
                goto err_free;

        if (blkg_rwstat_init(&blkg->stat_bytes, gfp_mask) ||
            blkg_rwstat_init(&blkg->stat_ios, gfp_mask))
                goto err_free;

        blkg->q = q;
        INIT_LIST_HEAD(&blkg->q_node);
        blkg->blkcg = blkcg;

        for (i = 0; i < BLKCG_MAX_POLS; i++) {
                struct blkcg_policy *pol = blkcg_policy[i];
                struct blkg_policy_data *pd;

                if (!blkcg_policy_enabled(q, pol))
                        continue;

                /* alloc per-policy data and attach it to blkg */
                pd = pol->pd_alloc_fn(gfp_mask, q->node);
                if (!pd)
                        goto err_free;

                blkg->pd[i] = pd;
                pd->blkg = blkg;
                pd->plid = i;
        }

        return blkg;

err_free:
        blkg_free(blkg);
        return NULL;
}

struct blkcg_gq *blkg_lookup_slowpath(struct blkcg *blkcg,
                                      struct request_queue *q, bool update_hint)
{
        struct blkcg_gq *blkg;

        /*
         * Hint didn't match.  Look up from the radix tree.  Note that the
         * hint can only be updated under queue_lock as otherwise @blkg
         * could have already been removed from blkg_tree.  The caller is
         * responsible for grabbing queue_lock if @update_hint.
         */
        blkg = radix_tree_lookup(&blkcg->blkg_tree, q->id);
        if (blkg && blkg->q == q) {
                if (update_hint) {
                        lockdep_assert_held(&q->queue_lock);
                        rcu_assign_pointer(blkcg->blkg_hint, blkg);
                }
                return blkg;
        }

        return NULL;
}
EXPORT_SYMBOL_GPL(blkg_lookup_slowpath);

/*
 * If @new_blkg is %NULL, this function tries to allocate a new one as
 * necessary using %GFP_NOWAIT.  @new_blkg is always consumed on return.
 */
static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
                                    struct request_queue *q,
                                    struct blkcg_gq *new_blkg)
{
        struct blkcg_gq *blkg;
        struct bdi_writeback_congested *wb_congested;
        int i, ret;

        WARN_ON_ONCE(!rcu_read_lock_held());
        lockdep_assert_held(&q->queue_lock);

        /* request_queue is dying, do not create/recreate a blkg */
        if (blk_queue_dying(q)) {
                ret = -ENODEV;
                goto err_free_blkg;
        }

        /* blkg holds a reference to blkcg */
        if (!css_tryget_online(&blkcg->css)) {
                ret = -ENODEV;
                goto err_free_blkg;
        }

        wb_congested = wb_congested_get_create(q->backing_dev_info,
                                               blkcg->css.id,
                                               GFP_NOWAIT | __GFP_NOWARN);
        if (!wb_congested) {
                ret = -ENOMEM;
                goto err_put_css;
        }

        /* allocate */
        if (!new_blkg) {
                new_blkg = blkg_alloc(blkcg, q, GFP_NOWAIT | __GFP_NOWARN);
                if (unlikely(!new_blkg)) {
                        ret = -ENOMEM;
                        goto err_put_congested;
                }
        }
        blkg = new_blkg;
        blkg->wb_congested = wb_congested;

        /* link parent */
        if (blkcg_parent(blkcg)) {
                blkg->parent = __blkg_lookup(blkcg_parent(blkcg), q, false);
                if (WARN_ON_ONCE(!blkg->parent)) {
                        ret = -ENODEV;
                        goto err_put_congested;
                }
                blkg_get(blkg->parent);
        }

        /* invoke per-policy init */
        for (i = 0; i < BLKCG_MAX_POLS; i++) {
                struct blkcg_policy *pol = blkcg_policy[i];

                if (blkg->pd[i] && pol->pd_init_fn)
                        pol->pd_init_fn(blkg->pd[i]);
        }

        /* insert */
        spin_lock(&blkcg->lock);
        ret = radix_tree_insert(&blkcg->blkg_tree, q->id, blkg);
        if (likely(!ret)) {
                hlist_add_head_rcu(&blkg->blkcg_node, &blkcg->blkg_list);
                list_add(&blkg->q_node, &q->blkg_list);

                for (i = 0; i < BLKCG_MAX_POLS; i++) {
                        struct blkcg_policy *pol = blkcg_policy[i];

                        if (blkg->pd[i] && pol->pd_online_fn)
                                pol->pd_online_fn(blkg->pd[i]);
                }
        }
        blkg->online = true;
        spin_unlock(&blkcg->lock);

        if (!ret)
                return blkg;

        /* @blkg failed fully initialized, use the usual release path */
        blkg_put(blkg);
        return ERR_PTR(ret);

err_put_congested:
        wb_congested_put(wb_congested);
err_put_css:
        css_put(&blkcg->css);
err_free_blkg:
        blkg_free(new_blkg);
        return ERR_PTR(ret);
}

/**
 * __blkg_lookup_create - lookup blkg, try to create one if not there
 * @blkcg: blkcg of interest
 * @q: request_queue of interest
 *
 * Lookup blkg for the @blkcg - @q pair.  If it doesn't exist, try to
 * create one.  blkg creation is performed recursively from blkcg_root such
 * that all non-root blkg's have access to the parent blkg.  This function
 * should be called under RCU read lock and @q->queue_lock.
 *
 * Returns the blkg or the closest blkg if blkg_create() fails as it walks
 * down from root.
 */
struct blkcg_gq *__blkg_lookup_create(struct blkcg *blkcg,
                                      struct request_queue *q)
{
        struct blkcg_gq *blkg;

        WARN_ON_ONCE(!rcu_read_lock_held());
        lockdep_assert_held(&q->queue_lock);

        blkg = __blkg_lookup(blkcg, q, true);
        if (blkg)
                return blkg;

        /*
         * Create blkgs walking down from blkcg_root to @blkcg, so that all
         * non-root blkgs have access to their parents.  Returns the closest
         * blkg to the intended blkg should blkg_create() fail.
         */
        while (true) {
                struct blkcg *pos = blkcg;
                struct blkcg *parent = blkcg_parent(blkcg);
                struct blkcg_gq *ret_blkg = q->root_blkg;

                while (parent) {
                        blkg = __blkg_lookup(parent, q, false);
                        if (blkg) {
                                /* remember closest blkg */
                                ret_blkg = blkg;
                                break;
                        }
                        pos = parent;
                        parent = blkcg_parent(parent);
                }

                blkg = blkg_create(pos, q, NULL);
                if (IS_ERR(blkg))
                        return ret_blkg;
                if (pos == blkcg)
                        return blkg;
        }
}

/**
 * blkg_lookup_create - find or create a blkg
 * @blkcg: target block cgroup
 * @q: target request_queue
 *
 * This looks up or creates the blkg representing the unique pair
 * of the blkcg and the request_queue.
 */
struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
                                    struct request_queue *q)
{
        struct blkcg_gq *blkg = blkg_lookup(blkcg, q);

        if (unlikely(!blkg)) {
                unsigned long flags;

                spin_lock_irqsave(&q->queue_lock, flags);
                blkg = __blkg_lookup_create(blkcg, q);
                spin_unlock_irqrestore(&q->queue_lock, flags);
        }

        return blkg;
}

static void blkg_destroy(struct blkcg_gq *blkg)
{
        struct blkcg *blkcg = blkg->blkcg;
        struct blkcg_gq *parent = blkg->parent;
        int i;

        lockdep_assert_held(&blkg->q->queue_lock);
        lockdep_assert_held(&blkcg->lock);

        /* Something wrong if we are trying to remove same group twice */
        WARN_ON_ONCE(list_empty(&blkg->q_node));
        WARN_ON_ONCE(hlist_unhashed(&blkg->blkcg_node));

        for (i = 0; i < BLKCG_MAX_POLS; i++) {
                struct blkcg_policy *pol = blkcg_policy[i];

                if (blkg->pd[i] && pol->pd_offline_fn)
                        pol->pd_offline_fn(blkg->pd[i]);
        }

        if (parent) {
                blkg_rwstat_add_aux(&parent->stat_bytes, &blkg->stat_bytes);
                blkg_rwstat_add_aux(&parent->stat_ios, &blkg->stat_ios);
        }

        blkg->online = false;

        radix_tree_delete(&blkcg->blkg_tree, blkg->q->id);
        list_del_init(&blkg->q_node);
        hlist_del_init_rcu(&blkg->blkcg_node);

        /*
         * Both setting lookup hint to and clearing it from @blkg are done
         * under queue_lock.  If it's not pointing to @blkg now, it never
         * will.  Hint assignment itself can race safely.
         */
        if (rcu_access_pointer(blkcg->blkg_hint) == blkg)
                rcu_assign_pointer(blkcg->blkg_hint, NULL);

        /*
         * Put the reference taken at the time of creation so that when all
         * queues are gone, group can be destroyed.
         */
        percpu_ref_kill(&blkg->refcnt);
}

/**
 * blkg_destroy_all - destroy all blkgs associated with a request_queue
 * @q: request_queue of interest
 *
 * Destroy all blkgs associated with @q.
 */
static void blkg_destroy_all(struct request_queue *q)
{
        struct blkcg_gq *blkg, *n;

        spin_lock_irq(&q->queue_lock);
        list_for_each_entry_safe(blkg, n, &q->blkg_list, q_node) {
                struct blkcg *blkcg = blkg->blkcg;

                spin_lock(&blkcg->lock);
                blkg_destroy(blkg);
                spin_unlock(&blkcg->lock);
        }

        q->root_blkg = NULL;
        spin_unlock_irq(&q->queue_lock);
}

static int blkcg_reset_stats(struct cgroup_subsys_state *css,
                             struct cftype *cftype, u64 val)
{
        struct blkcg *blkcg = css_to_blkcg(css);
        struct blkcg_gq *blkg;
        int i;

        mutex_lock(&blkcg_pol_mutex);
        spin_lock_irq(&blkcg->lock);

        /*
         * Note that stat reset is racy - it doesn't synchronize against
         * stat updates.  This is a debug feature which shouldn't exist
         * anyway.  If you get hit by a race, retry.
         */
        hlist_for_each_entry(blkg, &blkcg->blkg_list, blkcg_node) {
                blkg_rwstat_reset(&blkg->stat_bytes);
                blkg_rwstat_reset(&blkg->stat_ios);

                for (i = 0; i < BLKCG_MAX_POLS; i++) {
                        struct blkcg_policy *pol = blkcg_policy[i];

                        if (blkg->pd[i] && pol->pd_reset_stats_fn)
                                pol->pd_reset_stats_fn(blkg->pd[i]);
                }
        }

        spin_unlock_irq(&blkcg->lock);
        mutex_unlock(&blkcg_pol_mutex);
        return 0;
}

const char *blkg_dev_name(struct blkcg_gq *blkg)
{
        /* some drivers (floppy) instantiate a queue w/o disk registered */
        if (blkg->q->backing_dev_info->dev)
                return dev_name(blkg->q->backing_dev_info->dev);
        return NULL;
}

/**
 * blkcg_print_blkgs - helper for printing per-blkg data
 * @sf: seq_file to print to
 * @blkcg: blkcg of interest
 * @prfill: fill function to print out a blkg
 * @pol: policy in question
 * @data: data to be passed to @prfill
 * @show_total: to print out sum of prfill return values or not
 *
 * This function invokes @prfill on each blkg of @blkcg if pd for the
 * policy specified by @pol exists.  @prfill is invoked with @sf, the
 * policy data and @data and the matching queue lock held.  If @show_total
 * is %true, the sum of the return values from @prfill is printed with
 * "Total" label at the end.
 *
 * This is to be used to construct print functions for
 * cftype->read_seq_string method.
 */
void blkcg_print_blkgs(struct seq_file *sf, struct blkcg *blkcg,
                       u64 (*prfill)(struct seq_file *,
                                     struct blkg_policy_data *, int),
                       const struct blkcg_policy *pol, int data,
                       bool show_total)
{
        struct blkcg_gq *blkg;
        u64 total = 0;

        rcu_read_lock();
        hlist_for_each_entry_rcu(blkg, &blkcg->blkg_list, blkcg_node) {
                spin_lock_irq(&blkg->q->queue_lock);
                if (blkcg_policy_enabled(blkg->q, pol))
                        total += prfill(sf, blkg->pd[pol->plid], data);
                spin_unlock_irq(&blkg->q->queue_lock);
        }
        rcu_read_unlock();

        if (show_total)
                seq_printf(sf, "Total %llu\n", (unsigned long long)total);
}
EXPORT_SYMBOL_GPL(blkcg_print_blkgs);

/**
 * __blkg_prfill_u64 - prfill helper for a single u64 value
 * @sf: seq_file to print to
 * @pd: policy private data of interest
 * @v: value to print
 *
 * Print @v to @sf for the device assocaited with @pd.
 */
u64 __blkg_prfill_u64(struct seq_file *sf, struct blkg_policy_data *pd, u64 v)
{
        const char *dname = blkg_dev_name(pd->blkg);

        if (!dname)
                return 0;

        seq_printf(sf, "%s %llu\n", dname, (unsigned long long)v);
        return v;
}
EXPORT_SYMBOL_GPL(__blkg_prfill_u64);

/**
 * __blkg_prfill_rwstat - prfill helper for a blkg_rwstat
 * @sf: seq_file to print to
 * @pd: policy private data of interest
 * @rwstat: rwstat to print
 *
 * Print @rwstat to @sf for the device assocaited with @pd.
 */
u64 __blkg_prfill_rwstat(struct seq_file *sf, struct blkg_policy_data *pd,
                         const struct blkg_rwstat *rwstat)
{
        static const char *rwstr[] = {
                [BLKG_RWSTAT_READ]      = "Read",
                [BLKG_RWSTAT_WRITE]     = "Write",
                [BLKG_RWSTAT_SYNC]      = "Sync",
                [BLKG_RWSTAT_ASYNC]     = "Async",
                [BLKG_RWSTAT_DISCARD]   = "Discard",
        };
        const char *dname = blkg_dev_name(pd->blkg);
        u64 v;
        int i;

        if (!dname)
                return 0;

        for (i = 0; i < BLKG_RWSTAT_NR; i++)
                seq_printf(sf, "%s %s %llu\n", dname, rwstr[i],
                           (unsigned long long)atomic64_read(&rwstat->aux_cnt[i]));

        v = atomic64_read(&rwstat->aux_cnt[BLKG_RWSTAT_READ]) +
                atomic64_read(&rwstat->aux_cnt[BLKG_RWSTAT_WRITE]) +
                atomic64_read(&rwstat->aux_cnt[BLKG_RWSTAT_DISCARD]);
        seq_printf(sf, "%s Total %llu\n", dname, (unsigned long long)v);
        return v;
}
EXPORT_SYMBOL_GPL(__blkg_prfill_rwstat);

/**
 * blkg_prfill_stat - prfill callback for blkg_stat
 * @sf: seq_file to print to
 * @pd: policy private data of interest
 * @off: offset to the blkg_stat in @pd
 *
 * prfill callback for printing a blkg_stat.
 */
u64 blkg_prfill_stat(struct seq_file *sf, struct blkg_policy_data *pd, int off)
{
        return __blkg_prfill_u64(sf, pd, blkg_stat_read((void *)pd + off));
}
EXPORT_SYMBOL_GPL(blkg_prfill_stat);

/**
 * blkg_prfill_rwstat - prfill callback for blkg_rwstat
 * @sf: seq_file to print to
 * @pd: policy private data of interest
 * @off: offset to the blkg_rwstat in @pd
 *
 * prfill callback for printing a blkg_rwstat.
 */
u64 blkg_prfill_rwstat(struct seq_file *sf, struct blkg_policy_data *pd,
                       int off)
{
        struct blkg_rwstat rwstat = blkg_rwstat_read((void *)pd + off);

        return __blkg_prfill_rwstat(sf, pd, &rwstat);
}
EXPORT_SYMBOL_GPL(blkg_prfill_rwstat);

static u64 blkg_prfill_rwstat_field(struct seq_file *sf,
                                    struct blkg_policy_data *pd, int off)
{
        struct blkg_rwstat rwstat = blkg_rwstat_read((void *)pd->blkg + off);

        return __blkg_prfill_rwstat(sf, pd, &rwstat);
}

/**
 * blkg_print_stat_bytes - seq_show callback for blkg->stat_bytes
 * @sf: seq_file to print to
 * @v: unused
 *
 * To be used as cftype->seq_show to print blkg->stat_bytes.
 * cftype->private must be set to the blkcg_policy.
 */
int blkg_print_stat_bytes(struct seq_file *sf, void *v)
{
        blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)),
                          blkg_prfill_rwstat_field, (void *)seq_cft(sf)->private,
                          offsetof(struct blkcg_gq, stat_bytes), true);
        return 0;
}
EXPORT_SYMBOL_GPL(blkg_print_stat_bytes);

/**
 * blkg_print_stat_bytes - seq_show callback for blkg->stat_ios
 * @sf: seq_file to print to
 * @v: unused
 *
 * To be used as cftype->seq_show to print blkg->stat_ios.  cftype->private
 * must be set to the blkcg_policy.
 */
int blkg_print_stat_ios(struct seq_file *sf, void *v)
{
        blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)),
                          blkg_prfill_rwstat_field, (void *)seq_cft(sf)->private,
                          offsetof(struct blkcg_gq, stat_ios), true);
        return 0;
}
EXPORT_SYMBOL_GPL(blkg_print_stat_ios);

static u64 blkg_prfill_rwstat_field_recursive(struct seq_file *sf,
                                              struct blkg_policy_data *pd,
                                              int off)
{
        struct blkg_rwstat rwstat = blkg_rwstat_recursive_sum(pd->blkg,
                                                              NULL, off);
        return __blkg_prfill_rwstat(sf, pd, &rwstat);
}

/**
 * blkg_print_stat_bytes_recursive - recursive version of blkg_print_stat_bytes
 * @sf: seq_file to print to
 * @v: unused
 */
int blkg_print_stat_bytes_recursive(struct seq_file *sf, void *v)
{
        blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)),
                          blkg_prfill_rwstat_field_recursive,
                          (void *)seq_cft(sf)->private,
                          offsetof(struct blkcg_gq, stat_bytes), true);
        return 0;
}
EXPORT_SYMBOL_GPL(blkg_print_stat_bytes_recursive);

/**
 * blkg_print_stat_ios_recursive - recursive version of blkg_print_stat_ios
 * @sf: seq_file to print to
 * @v: unused
 */
int blkg_print_stat_ios_recursive(struct seq_file *sf, void *v)
{
        blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)),
                          blkg_prfill_rwstat_field_recursive,
                          (void *)seq_cft(sf)->private,
                          offsetof(struct blkcg_gq, stat_ios), true);
        return 0;
}
EXPORT_SYMBOL_GPL(blkg_print_stat_ios_recursive);

/**
 * blkg_stat_recursive_sum - collect hierarchical blkg_stat
 * @blkg: blkg of interest
 * @pol: blkcg_policy which contains the blkg_stat
 * @off: offset to the blkg_stat in blkg_policy_data or @blkg
 *
 * Collect the blkg_stat specified by @blkg, @pol and @off and all its
 * online descendants and their aux counts.  The caller must be holding the
 * queue lock for online tests.
 *
 * If @pol is NULL, blkg_stat is at @off bytes into @blkg; otherwise, it is
 * at @off bytes into @blkg's blkg_policy_data of the policy.
 */
u64 blkg_stat_recursive_sum(struct blkcg_gq *blkg,
                            struct blkcg_policy *pol, int off)
{
        struct blkcg_gq *pos_blkg;
        struct cgroup_subsys_state *pos_css;
        u64 sum = 0;

        lockdep_assert_held(&blkg->q->queue_lock);

        rcu_read_lock();
        blkg_for_each_descendant_pre(pos_blkg, pos_css, blkg) {
                struct blkg_stat *stat;

                if (!pos_blkg->online)
                        continue;

                if (pol)
                        stat = (void *)blkg_to_pd(pos_blkg, pol) + off;
                else
                        stat = (void *)blkg + off;

                sum += blkg_stat_read(stat) + atomic64_read(&stat->aux_cnt);
        }
        rcu_read_unlock();

        return sum;
}
EXPORT_SYMBOL_GPL(blkg_stat_recursive_sum);

/**
 * blkg_rwstat_recursive_sum - collect hierarchical blkg_rwstat
 * @blkg: blkg of interest
 * @pol: blkcg_policy which contains the blkg_rwstat
 * @off: offset to the blkg_rwstat in blkg_policy_data or @blkg
 *
 * Collect the blkg_rwstat specified by @blkg, @pol and @off and all its
 * online descendants and their aux counts.  The caller must be holding the
 * queue lock for online tests.
 *
 * If @pol is NULL, blkg_rwstat is at @off bytes into @blkg; otherwise, it
 * is at @off bytes into @blkg's blkg_policy_data of the policy.
 */
struct blkg_rwstat blkg_rwstat_recursive_sum(struct blkcg_gq *blkg,
                                             struct blkcg_policy *pol, int off)
{
        struct blkcg_gq *pos_blkg;
        struct cgroup_subsys_state *pos_css;
        struct blkg_rwstat sum = { };
        unsigned int i;

        lockdep_assert_held(&blkg->q->queue_lock);

        rcu_read_lock();
        blkg_for_each_descendant_pre(pos_blkg, pos_css, blkg) {
                struct blkg_rwstat *rwstat;

                if (!pos_blkg->online)
                        continue;

                if (pol)
                        rwstat = (void *)blkg_to_pd(pos_blkg, pol) + off;
                else
                        rwstat = (void *)pos_blkg + off;

                for (i = 0; i < BLKG_RWSTAT_NR; i++)
                        atomic64_add(blkg_rwstat_read_counter(rwstat, i),
                                &sum.aux_cnt[i]);
        }
        rcu_read_unlock();

        return sum;
}
EXPORT_SYMBOL_GPL(blkg_rwstat_recursive_sum);

/* Performs queue bypass and policy enabled checks then looks up blkg. */
static struct blkcg_gq *blkg_lookup_check(struct blkcg *blkcg,
                                          const struct blkcg_policy *pol,
                                          struct request_queue *q)
{
        WARN_ON_ONCE(!rcu_read_lock_held());
        lockdep_assert_held(&q->queue_lock);

        if (!blkcg_policy_enabled(q, pol))
                return ERR_PTR(-EOPNOTSUPP);
        return __blkg_lookup(blkcg, q, true /* update_hint */);
}

/**
 * blkg_conf_prep - parse and prepare for per-blkg config update
 * @blkcg: target block cgroup
 * @pol: target policy
 * @input: input string
 * @ctx: blkg_conf_ctx to be filled
 *
 * Parse per-blkg config update from @input and initialize @ctx with the
 * result.  @ctx->blkg points to the blkg to be updated and @ctx->body the
 * part of @input following MAJ:MIN.  This function returns with RCU read
 * lock and queue lock held and must be paired with blkg_conf_finish().
 */
int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
                   char *input, struct blkg_conf_ctx *ctx)
        __acquires(rcu) __acquires(&disk->queue->queue_lock)
{
        struct gendisk *disk;
        struct request_queue *q;
        struct blkcg_gq *blkg;
        unsigned int major, minor;
        int key_len, part, ret;
        char *body;

        if (sscanf(input, "%u:%u%n", &major, &minor, &key_len) != 2)
                return -EINVAL;

        body = input + key_len;
        if (!isspace(*body))
                return -EINVAL;
        body = skip_spaces(body);

        disk = get_gendisk(MKDEV(major, minor), &part);
        if (!disk)
                return -ENODEV;
        if (part) {
                ret = -ENODEV;
                goto fail;
        }

        q = disk->queue;

        rcu_read_lock();
        spin_lock_irq(&q->queue_lock);

        blkg = blkg_lookup_check(blkcg, pol, q);
        if (IS_ERR(blkg)) {
                ret = PTR_ERR(blkg);
                goto fail_unlock;
        }

        if (blkg)
                goto success;

        /*
         * Create blkgs walking down from blkcg_root to @blkcg, so that all
         * non-root blkgs have access to their parents.
         */
        while (true) {
                struct blkcg *pos = blkcg;
                struct blkcg *parent;
                struct blkcg_gq *new_blkg;

                parent = blkcg_parent(blkcg);
                while (parent && !__blkg_lookup(parent, q, false)) {
                        pos = parent;
                        parent = blkcg_parent(parent);
                }

                /* Drop locks to do new blkg allocation with GFP_KERNEL. */
                spin_unlock_irq(&q->queue_lock);
                rcu_read_unlock();

                new_blkg = blkg_alloc(pos, q, GFP_KERNEL);
                if (unlikely(!new_blkg)) {
                        ret = -ENOMEM;
                        goto fail;
                }

                rcu_read_lock();
                spin_lock_irq(&q->queue_lock);

                blkg = blkg_lookup_check(pos, pol, q);
                if (IS_ERR(blkg)) {
                        ret = PTR_ERR(blkg);
                        goto fail_unlock;
                }

                if (blkg) {
                        blkg_free(new_blkg);
                } else {
                        blkg = blkg_create(pos, q, new_blkg);
                        if (IS_ERR(blkg)) {
                                ret = PTR_ERR(blkg);
                                goto fail_unlock;
                        }
                }

                if (pos == blkcg)
                        goto success;
        }
success:
        ctx->disk = disk;
        ctx->blkg = blkg;
        ctx->body = body;
        return 0;

fail_unlock:
        spin_unlock_irq(&q->queue_lock);
        rcu_read_unlock();
fail:
        put_disk_and_module(disk);
        /*
         * If queue was bypassing, we should retry.  Do so after a
         * short msleep().  It isn't strictly necessary but queue
         * can be bypassing for some time and it's always nice to
         * avoid busy looping.
         */
        if (ret == -EBUSY) {
                msleep(10);
                ret = restart_syscall();
        }
        return ret;
}

/**
 * blkg_conf_finish - finish up per-blkg config update
 * @ctx: blkg_conf_ctx intiailized by blkg_conf_prep()
 *
 * Finish up after per-blkg config update.  This function must be paired
 * with blkg_conf_prep().
 */
void blkg_conf_finish(struct blkg_conf_ctx *ctx)
        __releases(&ctx->disk->queue->queue_lock) __releases(rcu)
{
        spin_unlock_irq(&ctx->disk->queue->queue_lock);
        rcu_read_unlock();
        put_disk_and_module(ctx->disk);
}

static int blkcg_print_stat(struct seq_file *sf, void *v)
{
        struct blkcg *blkcg = css_to_blkcg(seq_css(sf));
        struct blkcg_gq *blkg;

        rcu_read_lock();

        hlist_for_each_entry_rcu(blkg, &blkcg->blkg_list, blkcg_node) {
                const char *dname;
                char *buf;
                struct blkg_rwstat rwstat;
                u64 rbytes, wbytes, rios, wios, dbytes, dios;
                size_t size = seq_get_buf(sf, &buf), off = 0;
                int i;
                bool has_stats = false;

                dname = blkg_dev_name(blkg);
                if (!dname)
                        continue;

                /*
                 * Hooray string manipulation, count is the size written NOT
                 * INCLUDING THE \0, so size is now count+1 less than what we
                 * had before, but we want to start writing the next bit from
                 * the \0 so we only add count to buf.
                 */
                off += scnprintf(buf+off, size-off, "%s ", dname);

                spin_lock_irq(&blkg->q->queue_lock);

                rwstat = blkg_rwstat_recursive_sum(blkg, NULL,
                                        offsetof(struct blkcg_gq, stat_bytes));
                rbytes = atomic64_read(&rwstat.aux_cnt[BLKG_RWSTAT_READ]);
                wbytes = atomic64_read(&rwstat.aux_cnt[BLKG_RWSTAT_WRITE]);
                dbytes = atomic64_read(&rwstat.aux_cnt[BLKG_RWSTAT_DISCARD]);

                rwstat = blkg_rwstat_recursive_sum(blkg, NULL,
                                        offsetof(struct blkcg_gq, stat_ios));
                rios = atomic64_read(&rwstat.aux_cnt[BLKG_RWSTAT_READ]);
                wios = atomic64_read(&rwstat.aux_cnt[BLKG_RWSTAT_WRITE]);
                dios = atomic64_read(&rwstat.aux_cnt[BLKG_RWSTAT_DISCARD]);

                spin_unlock_irq(&blkg->q->queue_lock);

                if (rbytes || wbytes || rios || wios) {
                        has_stats = true;
                        off += scnprintf(buf+off, size-off,
                                         "rbytes=%llu wbytes=%llu rios=%llu wios=%llu dbytes=%llu dios=%llu",
                                         rbytes, wbytes, rios, wios,
                                         dbytes, dios);
                }

                if (!blkcg_debug_stats)
                        goto next;

                if (atomic_read(&blkg->use_delay)) {
                        has_stats = true;
                        off += scnprintf(buf+off, size-off,
                                         " use_delay=%d delay_nsec=%llu",
                                         atomic_read(&blkg->use_delay),
                                        (unsigned long long)atomic64_read(&blkg->delay_nsec));
                }

                for (i = 0; i < BLKCG_MAX_POLS; i++) {
                        struct blkcg_policy *pol = blkcg_policy[i];
                        size_t written;

                        if (!blkg->pd[i] || !pol->pd_stat_fn)
                                continue;

                        written = pol->pd_stat_fn(blkg->pd[i], buf+off, size-off);
                        if (written)
                                has_stats = true;
                        off += written;
                }
next:
                if (has_stats) {
                        if (off < size - 1) {
                                off += scnprintf(buf+off, size-off, "\n");
                                seq_commit(sf, off);
                        } else {
                                seq_commit(sf, -1);
                        }
                }
        }

        rcu_read_unlock();
        return 0;
}

static struct cftype blkcg_files[] = {
        {
                .name = "stat",
                .flags = CFTYPE_NOT_ON_ROOT,
                .seq_show = blkcg_print_stat,
        },
        { }     /* terminate */
};

static struct cftype blkcg_legacy_files[] = {
        {
                .name = "reset_stats",
                .write_u64 = blkcg_reset_stats,
        },
        { }     /* terminate */
};

/*
 * blkcg destruction is a three-stage process.
 *
 * 1. Destruction starts.  The blkcg_css_offline() callback is invoked
 *    which offlines writeback.  Here we tie the next stage of blkg destruction
 *    to the completion of writeback associated with the blkcg.  This lets us
 *    avoid punting potentially large amounts of outstanding writeback to root
 *    while maintaining any ongoing policies.  The next stage is triggered when
 *    the nr_cgwbs count goes to zero.
 *
 * 2. When the nr_cgwbs count goes to zero, blkcg_destroy_blkgs() is called
 *    and handles the destruction of blkgs.  Here the css reference held by
 *    the blkg is put back eventually allowing blkcg_css_free() to be called.
 *    This work may occur in cgwb_release_workfn() on the cgwb_release
 *    workqueue.  Any submitted ios that fail to get the blkg ref will be
 *    punted to the root_blkg.
 *
 * 3. Once the blkcg ref count goes to zero, blkcg_css_free() is called.
 *    This finally frees the blkcg.
 */

/**
 * blkcg_css_offline - cgroup css_offline callback
 * @css: css of interest
 *
 * This function is called when @css is about to go away.  Here the cgwbs are
 * offlined first and only once writeback associated with the blkcg has
 * finished do we start step 2 (see above).
 */
static void blkcg_css_offline(struct cgroup_subsys_state *css)
{
        struct blkcg *blkcg = css_to_blkcg(css);

        /* this prevents anyone from attaching or migrating to this blkcg */
        wb_blkcg_offline(blkcg);

        /* put the base cgwb reference allowing step 2 to be triggered */
        blkcg_cgwb_put(blkcg);
}

/**
 * blkcg_destroy_blkgs - responsible for shooting down blkgs
 * @blkcg: blkcg of interest
 *
 * blkgs should be removed while holding both q and blkcg locks.  As blkcg lock
 * is nested inside q lock, this function performs reverse double lock dancing.
 * Destroying the blkgs releases the reference held on the blkcg's css allowing
 * blkcg_css_free to eventually be called.
 *
 * This is the blkcg counterpart of ioc_release_fn().
 */
void blkcg_destroy_blkgs(struct blkcg *blkcg)
{
        spin_lock_irq(&blkcg->lock);

        while (!hlist_empty(&blkcg->blkg_list)) {
                struct blkcg_gq *blkg = hlist_entry(blkcg->blkg_list.first,
                                                struct blkcg_gq, blkcg_node);
                struct request_queue *q = blkg->q;

                if (spin_trylock(&q->queue_lock)) {
                        blkg_destroy(blkg);
                        spin_unlock(&q->queue_lock);
                } else {
                        spin_unlock_irq(&blkcg->lock);
                        cpu_relax();
                        spin_lock_irq(&blkcg->lock);
                }
        }

        spin_unlock_irq(&blkcg->lock);
}

static void blkcg_css_free(struct cgroup_subsys_state *css)
{
        struct blkcg *blkcg = css_to_blkcg(css);
        int i;

        mutex_lock(&blkcg_pol_mutex);

        list_del(&blkcg->all_blkcgs_node);

        for (i = 0; i < BLKCG_MAX_POLS; i++)
                if (blkcg->cpd[i])
                        blkcg_policy[i]->cpd_free_fn(blkcg->cpd[i]);

        mutex_unlock(&blkcg_pol_mutex);

        kfree(blkcg);
}

static struct cgroup_subsys_state *
blkcg_css_alloc(struct cgroup_subsys_state *parent_css)
{
        struct blkcg *blkcg;
        struct cgroup_subsys_state *ret;
        int i;

        mutex_lock(&blkcg_pol_mutex);

        if (!parent_css) {
                blkcg = &blkcg_root;
        } else {
                blkcg = kzalloc(sizeof(*blkcg), GFP_KERNEL);
                if (!blkcg) {
                        ret = ERR_PTR(-ENOMEM);
                        goto unlock;
                }
        }

        for (i = 0; i < BLKCG_MAX_POLS ; i++) {
                struct blkcg_policy *pol = blkcg_policy[i];
                struct blkcg_policy_data *cpd;

                /*
                 * If the policy hasn't been attached yet, wait for it
                 * to be attached before doing anything else. Otherwise,
                 * check if the policy requires any specific per-cgroup
                 * data: if it does, allocate and initialize it.
                 */
                if (!pol || !pol->cpd_alloc_fn)
                        continue;

                cpd = pol->cpd_alloc_fn(GFP_KERNEL);
                if (!cpd) {
                        ret = ERR_PTR(-ENOMEM);
                        goto free_pd_blkcg;
                }
                blkcg->cpd[i] = cpd;
                cpd->blkcg = blkcg;
                cpd->plid = i;
                if (pol->cpd_init_fn)
                        pol->cpd_init_fn(cpd);
        }

        spin_lock_init(&blkcg->lock);
        INIT_RADIX_TREE(&blkcg->blkg_tree, GFP_NOWAIT | __GFP_NOWARN);
        INIT_HLIST_HEAD(&blkcg->blkg_list);
#ifdef CONFIG_CGROUP_WRITEBACK
        INIT_LIST_HEAD(&blkcg->cgwb_list);
        refcount_set(&blkcg->cgwb_refcnt, 1);
#endif
        list_add_tail(&blkcg->all_blkcgs_node, &all_blkcgs);

        mutex_unlock(&blkcg_pol_mutex);
        return &blkcg->css;

free_pd_blkcg:
        for (i--; i >= 0; i--)
                if (blkcg->cpd[i])
                        blkcg_policy[i]->cpd_free_fn(blkcg->cpd[i]);

        if (blkcg != &blkcg_root)
                kfree(blkcg);
unlock:
        mutex_unlock(&blkcg_pol_mutex);
        return ret;
}

/**
 * blkcg_init_queue - initialize blkcg part of request queue
 * @q: request_queue to initialize
 *
 * Called from blk_alloc_queue_node(). Responsible for initializing blkcg
 * part of new request_queue @q.
 *
 * RETURNS:
 * 0 on success, -errno on failure.
 */
int blkcg_init_queue(struct request_queue *q)
{
        struct blkcg_gq *new_blkg, *blkg;
        bool preloaded;
        int ret;

        new_blkg = blkg_alloc(&blkcg_root, q, GFP_KERNEL);
        if (!new_blkg)
                return -ENOMEM;

        preloaded = !radix_tree_preload(GFP_KERNEL);

        /* Make sure the root blkg exists. */
        rcu_read_lock();
        spin_lock_irq(&q->queue_lock);
        blkg = blkg_create(&blkcg_root, q, new_blkg);
        if (IS_ERR(blkg))
                goto err_unlock;
        q->root_blkg = blkg;
        spin_unlock_irq(&q->queue_lock);
        rcu_read_unlock();

        if (preloaded)
                radix_tree_preload_end();

        ret = blk_iolatency_init(q);
        if (ret)
                goto err_destroy_all;

        ret = blk_throtl_init(q);
        if (ret)
                goto err_destroy_all;
        return 0;

err_destroy_all:
        blkg_destroy_all(q);
        return ret;
err_unlock:
        spin_unlock_irq(&q->queue_lock);
        rcu_read_unlock();
        if (preloaded)
                radix_tree_preload_end();
        return PTR_ERR(blkg);
}

/**
 * blkcg_drain_queue - drain blkcg part of request_queue
 * @q: request_queue to drain
 *
 * Called from blk_drain_queue().  Responsible for draining blkcg part.
 */
void blkcg_drain_queue(struct request_queue *q)
{
        lockdep_assert_held(&q->queue_lock);

        /*
         * @q could be exiting and already have destroyed all blkgs as
         * indicated by NULL root_blkg.  If so, don't confuse policies.
         */
        if (!q->root_blkg)
                return;

        blk_throtl_drain(q);
}

/**
 * blkcg_exit_queue - exit and release blkcg part of request_queue
 * @q: request_queue being released
 *
 * Called from blk_exit_queue().  Responsible for exiting blkcg part.
 */
void blkcg_exit_queue(struct request_queue *q)
{
        blkg_destroy_all(q);
        blk_throtl_exit(q);
}

/*
 * We cannot support shared io contexts, as we have no mean to support
 * two tasks with the same ioc in two different groups without major rework
 * of the main cic data structures.  For now we allow a task to change
 * its cgroup only if it's the only owner of its ioc.
 */
static int blkcg_can_attach(struct cgroup_taskset *tset)
{
        struct task_struct *task;
        struct cgroup_subsys_state *dst_css;
        struct io_context *ioc;
        int ret = 0;

        /* task_lock() is needed to avoid races with exit_io_context() */
        cgroup_taskset_for_each(task, dst_css, tset) {
                task_lock(task);
                ioc = task->io_context;
                if (ioc && atomic_read(&ioc->nr_tasks) > 1)
                        ret = -EINVAL;
                task_unlock(task);
                if (ret)
                        break;
        }
        return ret;
}

static void blkcg_bind(struct cgroup_subsys_state *root_css)
{
        int i;

        mutex_lock(&blkcg_pol_mutex);

        for (i = 0; i < BLKCG_MAX_POLS; i++) {
                struct blkcg_policy *pol = blkcg_policy[i];
                struct blkcg *blkcg;

                if (!pol || !pol->cpd_bind_fn)
                        continue;

                list_for_each_entry(blkcg, &all_blkcgs, all_blkcgs_node)
                        if (blkcg->cpd[pol->plid])
                                pol->cpd_bind_fn(blkcg->cpd[pol->plid]);
        }
        mutex_unlock(&blkcg_pol_mutex);
}

static void blkcg_exit(struct task_struct *tsk)
{
        if (tsk->throttle_queue)
                blk_put_queue(tsk->throttle_queue);
        tsk->throttle_queue = NULL;
}

struct cgroup_subsys io_cgrp_subsys = {
        .css_alloc = blkcg_css_alloc,
        .css_offline = blkcg_css_offline,
        .css_free = blkcg_css_free,
        .can_attach = blkcg_can_attach,
        .bind = blkcg_bind,
        .dfl_cftypes = blkcg_files,
        .legacy_cftypes = blkcg_legacy_files,
        .legacy_name = "blkio",
        .exit = blkcg_exit,
#ifdef CONFIG_MEMCG
        /*
         * This ensures that, if available, memcg is automatically enabled
         * together on the default hierarchy so that the owner cgroup can
         * be retrieved from writeback pages.
         */
        .depends_on = 1 << memory_cgrp_id,
#endif
};
EXPORT_SYMBOL_GPL(io_cgrp_subsys);

/**
 * blkcg_activate_policy - activate a blkcg policy on a request_queue
 * @q: request_queue of interest
 * @pol: blkcg policy to activate
 *
 * Activate @pol on @q.  Requires %GFP_KERNEL context.  @q goes through
 * bypass mode to populate its blkgs with policy_data for @pol.
 *
 * Activation happens with @q bypassed, so nobody would be accessing blkgs
 * from IO path.  Update of each blkg is protected by both queue and blkcg
 * locks so that holding either lock and testing blkcg_policy_enabled() is
 * always enough for dereferencing policy data.
 *
 * The caller is responsible for synchronizing [de]activations and policy
 * [un]registerations.  Returns 0 on success, -errno on failure.
 */
int blkcg_activate_policy(struct request_queue *q,
                          const struct blkcg_policy *pol)
{
        struct blkg_policy_data *pd_prealloc = NULL;
        struct blkcg_gq *blkg;
        int ret;

        if (blkcg_policy_enabled(q, pol))
                return 0;

        if (queue_is_mq(q))
                blk_mq_freeze_queue(q);
pd_prealloc:
        if (!pd_prealloc) {
                pd_prealloc = pol->pd_alloc_fn(GFP_KERNEL, q->node);
                if (!pd_prealloc) {
                        ret = -ENOMEM;
                        goto out_bypass_end;
                }
        }

        spin_lock_irq(&q->queue_lock);

        /* blkg_list is pushed at the head, reverse walk to init parents first */
        list_for_each_entry_reverse(blkg, &q->blkg_list, q_node) {
                struct blkg_policy_data *pd;

                if (blkg->pd[pol->plid])
                        continue;

                pd = pol->pd_alloc_fn(GFP_NOWAIT | __GFP_NOWARN, q->node);
                if (!pd)
                        swap(pd, pd_prealloc);
                if (!pd) {
                        spin_unlock_irq(&q->queue_lock);
                        goto pd_prealloc;
                }

                blkg->pd[pol->plid] = pd;
                pd->blkg = blkg;
                pd->plid = pol->plid;
                if (pol->pd_init_fn)
                        pol->pd_init_fn(pd);
        }

        __set_bit(pol->plid, q->blkcg_pols);
        ret = 0;

        spin_unlock_irq(&q->queue_lock);
out_bypass_end:
        if (queue_is_mq(q))
                blk_mq_unfreeze_queue(q);
        if (pd_prealloc)
                pol->pd_free_fn(pd_prealloc);
        return ret;
}
EXPORT_SYMBOL_GPL(blkcg_activate_policy);

/**
 * blkcg_deactivate_policy - deactivate a blkcg policy on a request_queue
 * @q: request_queue of interest
 * @pol: blkcg policy to deactivate
 *
 * Deactivate @pol on @q.  Follows the same synchronization rules as
 * blkcg_activate_policy().
 */
void blkcg_deactivate_policy(struct request_queue *q,
                             const struct blkcg_policy *pol)
{
        struct blkcg_gq *blkg;

        if (!blkcg_policy_enabled(q, pol))
                return;

        if (queue_is_mq(q))
                blk_mq_freeze_queue(q);

        spin_lock_irq(&q->queue_lock);

        __clear_bit(pol->plid, q->blkcg_pols);

        list_for_each_entry(blkg, &q->blkg_list, q_node) {
                if (blkg->pd[pol->plid]) {
                        if (pol->pd_offline_fn)
                                pol->pd_offline_fn(blkg->pd[pol->plid]);
                        pol->pd_free_fn(blkg->pd[pol->plid]);
                        blkg->pd[pol->plid] = NULL;
                }
        }

        spin_unlock_irq(&q->queue_lock);

        if (queue_is_mq(q))
                blk_mq_unfreeze_queue(q);
}
EXPORT_SYMBOL_GPL(blkcg_deactivate_policy);

/**
 * blkcg_policy_register - register a blkcg policy
 * @pol: blkcg policy to register
 *
 * Register @pol with blkcg core.  Might sleep and @pol may be modified on
 * successful registration.  Returns 0 on success and -errno on failure.
 */
int blkcg_policy_register(struct blkcg_policy *pol)
{
        struct blkcg *blkcg;
        int i, ret;

        mutex_lock(&blkcg_pol_register_mutex);
        mutex_lock(&blkcg_pol_mutex);

        /* find an empty slot */
        ret = -ENOSPC;
        for (i = 0; i < BLKCG_MAX_POLS; i++)
                if (!blkcg_policy[i])
                        break;
        if (i >= BLKCG_MAX_POLS) {
                pr_warn("blkcg_policy_register: BLKCG_MAX_POLS too small\n");
                goto err_unlock;
        }

        /* Make sure cpd/pd_alloc_fn and cpd/pd_free_fn in pairs */
        if ((!pol->cpd_alloc_fn ^ !pol->cpd_free_fn) ||
                (!pol->pd_alloc_fn ^ !pol->pd_free_fn))
                goto err_unlock;

        /* register @pol */
        pol->plid = i;
        blkcg_policy[pol->plid] = pol;

        /* allocate and install cpd's */
        if (pol->cpd_alloc_fn) {
                list_for_each_entry(blkcg, &all_blkcgs, all_blkcgs_node) {
                        struct blkcg_policy_data *cpd;

                        cpd = pol->cpd_alloc_fn(GFP_KERNEL);
                        if (!cpd)
                                goto err_free_cpds;

                        blkcg->cpd[pol->plid] = cpd;
                        cpd->blkcg = blkcg;
                        cpd->plid = pol->plid;
                        pol->cpd_init_fn(cpd);
                }
        }

        mutex_unlock(&blkcg_pol_mutex);

        /* everything is in place, add intf files for the new policy */
        if (pol->dfl_cftypes)
                WARN_ON(cgroup_add_dfl_cftypes(&io_cgrp_subsys,
                                               pol->dfl_cftypes));
        if (pol->legacy_cftypes)
                WARN_ON(cgroup_add_legacy_cftypes(&io_cgrp_subsys,
                                                  pol->legacy_cftypes));
        mutex_unlock(&blkcg_pol_register_mutex);
        return 0;

err_free_cpds:
        if (pol->cpd_free_fn) {
                list_for_each_entry(blkcg, &all_blkcgs, all_blkcgs_node) {
                        if (blkcg->cpd[pol->plid]) {
                                pol->cpd_free_fn(blkcg->cpd[pol->plid]);
                                blkcg->cpd[pol->plid] = NULL;
                        }
                }
        }
        blkcg_policy[pol->plid] = NULL;
err_unlock:
        mutex_unlock(&blkcg_pol_mutex);
        mutex_unlock(&blkcg_pol_register_mutex);
        return ret;
}
EXPORT_SYMBOL_GPL(blkcg_policy_register);

/**
 * blkcg_policy_unregister - unregister a blkcg policy
 * @pol: blkcg policy to unregister
 *
 * Undo blkcg_policy_register(@pol).  Might sleep.
 */
void blkcg_policy_unregister(struct blkcg_policy *pol)
{
        struct blkcg *blkcg;

        mutex_lock(&blkcg_pol_register_mutex);

        if (WARN_ON(blkcg_policy[pol->plid] != pol))
                goto out_unlock;

        /* kill the intf files first */
        if (pol->dfl_cftypes)
                cgroup_rm_cftypes(pol->dfl_cftypes);
        if (pol->legacy_cftypes)
                cgroup_rm_cftypes(pol->legacy_cftypes);

        /* remove cpds and unregister */
        mutex_lock(&blkcg_pol_mutex);

        if (pol->cpd_free_fn) {
                list_for_each_entry(blkcg, &all_blkcgs, all_blkcgs_node) {
                        if (blkcg->cpd[pol->plid]) {
                                pol->cpd_free_fn(blkcg->cpd[pol->plid]);
                                blkcg->cpd[pol->plid] = NULL;
                        }
                }
        }
        blkcg_policy[pol->plid] = NULL;

        mutex_unlock(&blkcg_pol_mutex);
out_unlock:
        mutex_unlock(&blkcg_pol_register_mutex);
}
EXPORT_SYMBOL_GPL(blkcg_policy_unregister);

/*
 * Scale the accumulated delay based on how long it has been since we updated
 * the delay.  We only call this when we are adding delay, in case it's been a
 * while since we added delay, and when we are checking to see if we need to
 * delay a task, to account for any delays that may have occurred.
 */
static void blkcg_scale_delay(struct blkcg_gq *blkg, u64 now)
{
        u64 old = atomic64_read(&blkg->delay_start);

        /*
         * We only want to scale down every second.  The idea here is that we
         * want to delay people for min(delay_nsec, NSEC_PER_SEC) in a certain
         * time window.  We only want to throttle tasks for recent delay that
         * has occurred, in 1 second time windows since that's the maximum
         * things can be throttled.  We save the current delay window in
         * blkg->last_delay so we know what amount is still left to be charged
         * to the blkg from this point onward.  blkg->last_use keeps track of
         * the use_delay counter.  The idea is if we're unthrottling the blkg we
         * are ok with whatever is happening now, and we can take away more of
         * the accumulated delay as we've already throttled enough that
         * everybody is happy with their IO latencies.
         */
        if (time_before64(old + NSEC_PER_SEC, now) &&
            atomic64_cmpxchg(&blkg->delay_start, old, now) == old) {
                u64 cur = atomic64_read(&blkg->delay_nsec);
                u64 sub = min_t(u64, blkg->last_delay, now - old);
                int cur_use = atomic_read(&blkg->use_delay);

                /*
                 * We've been unthrottled, subtract a larger chunk of our
                 * accumulated delay.
                 */
                if (cur_use < blkg->last_use)
                        sub = max_t(u64, sub, blkg->last_delay >> 1);

                /*
                 * This shouldn't happen, but handle it anyway.  Our delay_nsec
                 * should only ever be growing except here where we subtract out
                 * min(last_delay, 1 second), but lord knows bugs happen and I'd
                 * rather not end up with negative numbers.
                 */
                if (unlikely(cur < sub)) {
                        atomic64_set(&blkg->delay_nsec, 0);
                        blkg->last_delay = 0;
                } else {
                        atomic64_sub(sub, &blkg->delay_nsec);
                        blkg->last_delay = cur - sub;
                }
                blkg->last_use = cur_use;
        }
}

/*
 * This is called when we want to actually walk up the hierarchy and check to
 * see if we need to throttle, and then actually throttle if there is some
 * accumulated delay.  This should only be called upon return to user space so
 * we're not holding some lock that would induce a priority inversion.
 */
static void blkcg_maybe_throttle_blkg(struct blkcg_gq *blkg, bool use_memdelay)
{
        u64 now = ktime_to_ns(ktime_get());
        u64 exp;
        u64 delay_nsec = 0;
        int tok;

        while (blkg->parent) {
                if (atomic_read(&blkg->use_delay)) {
                        blkcg_scale_delay(blkg, now);
                        delay_nsec = max_t(u64, delay_nsec,
                                           atomic64_read(&blkg->delay_nsec));
                }
                blkg = blkg->parent;
        }

        if (!delay_nsec)
                return;

        /*
         * Let's not sleep for all eternity if we've amassed a huge delay.
         * Swapping or metadata IO can accumulate 10's of seconds worth of
         * delay, and we want userspace to be able to do _something_ so cap the
         * delays at 1 second.  If there's 10's of seconds worth of delay then
         * the tasks will be delayed for 1 second for every syscall.
         */
        delay_nsec = min_t(u64, delay_nsec, 250 * NSEC_PER_MSEC);

        /*
         * TODO: the use_memdelay flag is going to be for the upcoming psi stuff
         * that hasn't landed upstream yet.  Once that stuff is in place we need
         * to do a psi_memstall_enter/leave if memdelay is set.
         */

        exp = ktime_add_ns(now, delay_nsec);
        tok = io_schedule_prepare();
        do {
                __set_current_state(TASK_KILLABLE);
                if (!schedule_hrtimeout(&exp, HRTIMER_MODE_ABS))
                        break;
        } while (!fatal_signal_pending(current));
        io_schedule_finish(tok);
}

/**
 * blkcg_maybe_throttle_current - throttle the current task if it has been marked
 *
 * This is only called if we've been marked with set_notify_resume().  Obviously
 * we can be set_notify_resume() for reasons other than blkcg throttling, so we
 * check to see if current->throttle_queue is set and if not this doesn't do
 * anything.  This should only ever be called by the resume code, it's not meant
 * to be called by people willy-nilly as it will actually do the work to
 * throttle the task if it is setup for throttling.
 */
void blkcg_maybe_throttle_current(void)
{
        struct request_queue *q = current->throttle_queue;
        struct cgroup_subsys_state *css;
        struct blkcg *blkcg;
        struct blkcg_gq *blkg;
        bool use_memdelay = current->use_memdelay;

        if (!q)
                return;

        current->throttle_queue = NULL;
        current->use_memdelay = false;

        rcu_read_lock();
        css = kthread_blkcg();
        if (css)
                blkcg = css_to_blkcg(css);
        else
                blkcg = css_to_blkcg(task_css(current, io_cgrp_id));

        if (!blkcg)
                goto out;
        blkg = blkg_lookup(blkcg, q);
        if (!blkg)
                goto out;
        if (!blkg_tryget(blkg))
                goto out;
        rcu_read_unlock();

        blkcg_maybe_throttle_blkg(blkg, use_memdelay);
        blkg_put(blkg);
        blk_put_queue(q);
        return;
out:
        rcu_read_unlock();
        blk_put_queue(q);
}

/**
 * blkcg_schedule_throttle - this task needs to check for throttling
 * @q: the request queue IO was submitted on
 * @use_memdelay: do we charge this to memory delay for PSI
 *
 * This is called by the IO controller when we know there's delay accumulated
 * for the blkg for this task.  We do not pass the blkg because there are places
 * we call this that may not have that information, the swapping code for
 * instance will only have a request_queue at that point.  This set's the
 * notify_resume for the task to check and see if it requires throttling before
 * returning to user space.
 *
 * We will only schedule once per syscall.  You can call this over and over
 * again and it will only do the check once upon return to user space, and only
 * throttle once.  If the task needs to be throttled again it'll need to be
 * re-set at the next time we see the task.
 */
void blkcg_schedule_throttle(struct request_queue *q, bool use_memdelay)
{
        if (unlikely(current->flags & PF_KTHREAD))
                return;

        if (!blk_get_queue(q))
                return;

        if (current->throttle_queue)
                blk_put_queue(current->throttle_queue);
        current->throttle_queue = q;
        if (use_memdelay)
                current->use_memdelay = use_memdelay;
        set_notify_resume(current);
}

/**
 * blkcg_add_delay - add delay to this blkg
 * @blkg: blkg of interest
 * @now: the current time in nanoseconds
 * @delta: how many nanoseconds of delay to add
 *
 * Charge @delta to the blkg's current delay accumulation.  This is used to
 * throttle tasks if an IO controller thinks we need more throttling.
 */
void blkcg_add_delay(struct blkcg_gq *blkg, u64 now, u64 delta)
{
        blkcg_scale_delay(blkg, now);
        atomic64_add(delta, &blkg->delay_nsec);
}

module_param(blkcg_debug_stats, bool, 0644);
MODULE_PARM_DESC(blkcg_debug_stats, "True if you want debug stats, false if not");