/*
   * 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]);

  	if (blkg->blkcg != &blkcg_root)

  		blk_exit_rl(blkg->q, &blkg->rl);

  
  	blkg_rwstat_exit(&blkg->stat_ios);
  	blkg_rwstat_exit(&blkg->stat_bytes);

  	kfree(blkg);

  }
  
  /**
   * 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 (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;

  	atomic_set(&blkg->refcnt, 1);

  	/* root blkg uses @q->root_rl, init rl only for !root blkgs */
  	if (blkcg != &blkcg_root) {
  		if (blk_init_rl(&blkg->rl, q, gfp_mask))
  			goto err_free;
  		blkg->rl.blkg = blkg;
  	}

  	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);

  	/* 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 pointer to the looked up or created blkg on success, ERR_PTR()
   * value on error.  If @q is dead, returns ERR_PTR(-EINVAL).  If @q is not
   * dead and bypassing, returns ERR_PTR(-EBUSY).
   */

  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);

  	/*
  	 * This could be the first entry point of blkcg implementation and
  	 * we shouldn't allow anything to go through for a bypassing queue.
  	 */
  	if (unlikely(blk_queue_bypass(q)))
  		return ERR_PTR(blk_queue_dying(q) ? -ENODEV : -EBUSY);

  	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.
  	 */
  	while (true) {
  		struct blkcg *pos = blkcg;
  		struct blkcg *parent = blkcg_parent(blkcg);
  
  		while (parent && !__blkg_lookup(parent, q, false)) {
  			pos = parent;
  			parent = blkcg_parent(parent);
  		}

  		blkg = blkg_create(pos, q, NULL);

  		if (pos == blkcg || IS_ERR(blkg))
  			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.
  	 */
  	blkg_put(blkg);
  }

  /**
   * 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;

  	lockdep_assert_held(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;
  	q->root_rl.blkg = NULL;

  }

  /*
   * 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.
   */
  void __blkg_release_rcu(struct rcu_head *rcu_head)

  {

  	struct blkcg_gq *blkg = container_of(rcu_head, 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);

  }

  EXPORT_SYMBOL_GPL(__blkg_release_rcu);

  /*
   * The next function used by blk_queue_for_each_rl().  It's a bit tricky
   * because the root blkg uses @q->root_rl instead of its own rl.
   */
  struct request_list *__blk_queue_next_rl(struct request_list *rl,
  					 struct request_queue *q)
  {
  	struct list_head *ent;
  	struct blkcg_gq *blkg;
  
  	/*
  	 * Determine the current blkg list_head.  The first entry is
  	 * root_rl which is off @q->blkg_list and mapped to the head.
  	 */
  	if (rl == &q->root_rl) {
  		ent = &q->blkg_list;

  		/* There are no more block groups, hence no request lists */
  		if (list_empty(ent))
  			return NULL;

  	} else {
  		blkg = container_of(rl, struct blkcg_gq, rl);
  		ent = &blkg->q_node;
  	}
  
  	/* walk to the next list_head, skip root blkcg */
  	ent = ent->next;
  	if (ent == &q->root_blkg->q_node)
  		ent = ent->next;
  	if (ent == &q->blkg_list)
  		return NULL;
  
  	blkg = container_of(ent, struct blkcg_gq, q_node);
  	return &blkg->rl;
  }

  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;

  }

  EXPORT_SYMBOL_GPL(blkg_dev_name);

  /**
   * 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
  ", (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
  ", 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
  ", 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
  ", 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 = { };

  	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(atomic64_read(&rwstat->aux_cnt[i]) +
  				percpu_counter_sum_positive(&rwstat->cpu_cnt[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);
  
  	/*
  	 * This could be the first entry point of blkcg implementation and
  	 * we shouldn't allow anything to go through for a bypassing queue.
  	 */
  	if (unlikely(blk_queue_bypass(q)))
  		return ERR_PTR(blk_queue_dying(q) ? -ENODEV : -EBUSY);
  
  	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 (unlikely(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;

  }

  EXPORT_SYMBOL_GPL(blkg_conf_prep);

  /**
   * 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);

  }

  EXPORT_SYMBOL_GPL(blkg_conf_finish);

  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) {
  			off += scnprintf(buf+off, size-off, "
  ");
  			seq_commit(sf, off);
  		}

  	}
  
  	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;
  	q->root_rl.blkg = blkg;

  	spin_unlock_irq(q->queue_lock);
  	rcu_read_unlock();

  	if (preloaded)
  		radix_tree_preload_end();

  	ret = blk_iolatency_init(q);
  	if (ret) {
  		spin_lock_irq(q->queue_lock);
  		blkg_destroy_all(q);
  		spin_unlock_irq(q->queue_lock);
  		return ret;
  	}

  	ret = blk_throtl_init(q);
  	if (ret) {
  		spin_lock_irq(q->queue_lock);
  		blkg_destroy_all(q);
  		spin_unlock_irq(q->queue_lock);
  	}
  	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_release_queue().  Responsible for exiting blkcg part.
   */
  void blkcg_exit_queue(struct request_queue *q)
  {

  	spin_lock_irq(q->queue_lock);

  	blkg_destroy_all(q);

  	spin_unlock_irq(q->queue_lock);

  	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 (q->mq_ops)

  		blk_mq_freeze_queue(q);

  	else

  		blk_queue_bypass_start(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);
  
  	list_for_each_entry(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 (q->mq_ops)

  		blk_mq_unfreeze_queue(q);

  	else

  		blk_queue_bypass_end(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 (q->mq_ops)

  		blk_mq_freeze_queue(q);

  	else

  		blk_queue_bypass_start(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 (q->mq_ops)

  		blk_mq_unfreeze_queue(q);

  	else

  		blk_queue_bypass_end(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
  ");

  		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;

  }