TOMOYO Linux Cross Reference
Linux/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/delay.h>
  #include <linux/atomic.h>
  #include <linux/ctype.h>
  #include <linux/resume_user_mode.h>
  #include <linux/psi.h>
  #include <linux/part_stat.h>
  #include "blk.h"
  #include "blk-cgroup.h"
  #include "blk-ioprio.h"
  #include "blk-throttle.h"
  
  static void __blkcg_rstat_flush(struct blkcg *blkcg, int cpu);
  
  /*
   * 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;
  EXPORT_SYMBOL_GPL(blkcg_root_css);
  
  static struct blkcg_policy *blkcg_policy[BLKCG_MAX_POLS];
  
  static LIST_HEAD(all_blkcgs);           /* protected by blkcg_pol_mutex */
  
  bool blkcg_debug_stats = false;
  
  static DEFINE_RAW_SPINLOCK(blkg_stat_lock);
  
  #define BLKG_DESTROY_BATCH_SIZE  64
  
  /*
   * Lockless lists for tracking IO stats update
   *
   * New IO stats are stored in the percpu iostat_cpu within blkcg_gq (blkg).
   * There are multiple blkg's (one for each block device) attached to each
   * blkcg. The rstat code keeps track of which cpu has IO stats updated,
   * but it doesn't know which blkg has the updated stats. If there are many
   * block devices in a system, the cost of iterating all the blkg's to flush
   * out the IO stats can be high. To reduce such overhead, a set of percpu
   * lockless lists (lhead) per blkcg are used to track the set of recently
   * updated iostat_cpu's since the last flush. An iostat_cpu will be put
   * onto the lockless list on the update side [blk_cgroup_bio_start()] if
   * not there yet and then removed when being flushed [blkcg_rstat_flush()].
   * References to blkg are gotten and then put back in the process to
   * protect against blkg removal.
   *
   * Return: 0 if successful or -ENOMEM if allocation fails.
   */
  static int init_blkcg_llists(struct blkcg *blkcg)
  {
          int cpu;
  
          blkcg->lhead = alloc_percpu_gfp(struct llist_head, GFP_KERNEL);
          if (!blkcg->lhead)
                  return -ENOMEM;
  
          for_each_possible_cpu(cpu)
                  init_llist_head(per_cpu_ptr(blkcg->lhead, cpu));
          return 0;
  }
  
  /**
   * blkcg_css - find the current css
   *
   * Find the css associated with either the kthread or the current task.
  * This may return a dying css, so it is up to the caller to use tryget logic
  * to confirm it is alive and well.
  */
 static struct cgroup_subsys_state *blkcg_css(void)
 {
         struct cgroup_subsys_state *css;
 
         css = kthread_blkcg();
         if (css)
                 return css;
         return task_css(current, io_cgrp_id);
 }
 
 static bool blkcg_policy_enabled(struct request_queue *q,
                                  const struct blkcg_policy *pol)
 {
         return pol && test_bit(pol->plid, q->blkcg_pols);
 }
 
 static void blkg_free_workfn(struct work_struct *work)
 {
         struct blkcg_gq *blkg = container_of(work, struct blkcg_gq,
                                              free_work);
         struct request_queue *q = blkg->q;
         int i;
 
         /*
          * pd_free_fn() can also be called from blkcg_deactivate_policy(),
          * in order to make sure pd_free_fn() is called in order, the deletion
          * of the list blkg->q_node is delayed to here from blkg_destroy(), and
          * blkcg_mutex is used to synchronize blkg_free_workfn() and
          * blkcg_deactivate_policy().
          */
         mutex_lock(&q->blkcg_mutex);
         for (i = 0; i < BLKCG_MAX_POLS; i++)
                 if (blkg->pd[i])
                         blkcg_policy[i]->pd_free_fn(blkg->pd[i]);
         if (blkg->parent)
                 blkg_put(blkg->parent);
         spin_lock_irq(&q->queue_lock);
         list_del_init(&blkg->q_node);
         spin_unlock_irq(&q->queue_lock);
         mutex_unlock(&q->blkcg_mutex);
 
         blk_put_queue(q);
         free_percpu(blkg->iostat_cpu);
         percpu_ref_exit(&blkg->refcnt);
         kfree(blkg);
 }
 
 /**
  * blkg_free - free a blkg
  * @blkg: blkg to free
  *
  * Free @blkg which may be partially allocated.
  */
 static void blkg_free(struct blkcg_gq *blkg)
 {
         if (!blkg)
                 return;
 
         /*
          * Both ->pd_free_fn() and request queue's release handler may
          * sleep, so free us by scheduling one work func
          */
         INIT_WORK(&blkg->free_work, blkg_free_workfn);
         schedule_work(&blkg->free_work);
 }
 
 static void __blkg_release(struct rcu_head *rcu)
 {
         struct blkcg_gq *blkg = container_of(rcu, struct blkcg_gq, rcu_head);
         struct blkcg *blkcg = blkg->blkcg;
         int cpu;
 
 #ifdef CONFIG_BLK_CGROUP_PUNT_BIO
         WARN_ON(!bio_list_empty(&blkg->async_bios));
 #endif
         /*
          * Flush all the non-empty percpu lockless lists before releasing
          * us, given these stat belongs to us.
          *
          * blkg_stat_lock is for serializing blkg stat update
          */
         for_each_possible_cpu(cpu)
                 __blkcg_rstat_flush(blkcg, cpu);
 
         /* release the blkcg and parent blkg refs this blkg has been holding */
         css_put(&blkg->blkcg->css);
         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);
 }
 
 #ifdef CONFIG_BLK_CGROUP_PUNT_BIO
 static struct workqueue_struct *blkcg_punt_bio_wq;
 
 static void blkg_async_bio_workfn(struct work_struct *work)
 {
         struct blkcg_gq *blkg = container_of(work, struct blkcg_gq,
                                              async_bio_work);
         struct bio_list bios = BIO_EMPTY_LIST;
         struct bio *bio;
         struct blk_plug plug;
         bool need_plug = false;
 
         /* as long as there are pending bios, @blkg can't go away */
         spin_lock(&blkg->async_bio_lock);
         bio_list_merge_init(&bios, &blkg->async_bios);
         spin_unlock(&blkg->async_bio_lock);
 
         /* start plug only when bio_list contains at least 2 bios */
         if (bios.head && bios.head->bi_next) {
                 need_plug = true;
                 blk_start_plug(&plug);
         }
         while ((bio = bio_list_pop(&bios)))
                 submit_bio(bio);
         if (need_plug)
                 blk_finish_plug(&plug);
 }
 
 /*
  * When a shared kthread issues a bio for a cgroup, doing so synchronously can
  * lead to priority inversions as the kthread can be trapped waiting for that
  * cgroup.  Use this helper instead of submit_bio to punt the actual issuing to
  * a dedicated per-blkcg work item to avoid such priority inversions.
  */
 void blkcg_punt_bio_submit(struct bio *bio)
 {
         struct blkcg_gq *blkg = bio->bi_blkg;
 
         if (blkg->parent) {
                 spin_lock(&blkg->async_bio_lock);
                 bio_list_add(&blkg->async_bios, bio);
                 spin_unlock(&blkg->async_bio_lock);
                 queue_work(blkcg_punt_bio_wq, &blkg->async_bio_work);
         } else {
                 /* never bounce for the root cgroup */
                 submit_bio(bio);
         }
 }
 EXPORT_SYMBOL_GPL(blkcg_punt_bio_submit);
 
 static int __init blkcg_punt_bio_init(void)
 {
         blkcg_punt_bio_wq = alloc_workqueue("blkcg_punt_bio",
                                             WQ_MEM_RECLAIM | WQ_FREEZABLE |
                                             WQ_UNBOUND | WQ_SYSFS, 0);
         if (!blkcg_punt_bio_wq)
                 return -ENOMEM;
         return 0;
 }
 subsys_initcall(blkcg_punt_bio_init);
 #endif /* CONFIG_BLK_CGROUP_PUNT_BIO */
 
 /**
  * bio_blkcg_css - return the blkcg CSS associated with a bio
  * @bio: target bio
  *
  * This returns the CSS for the blkcg associated with a bio, or %NULL if not
  * associated. Callers are expected to either handle %NULL or know association
  * has been done prior to calling this.
  */
 struct cgroup_subsys_state *bio_blkcg_css(struct bio *bio)
 {
         if (!bio || !bio->bi_blkg)
                 return NULL;
         return &bio->bi_blkg->blkcg->css;
 }
 EXPORT_SYMBOL_GPL(bio_blkcg_css);
 
 /**
  * blkcg_parent - get the parent of a blkcg
  * @blkcg: blkcg of interest
  *
  * Return the parent blkcg of @blkcg.  Can be called anytime.
  */
 static inline struct blkcg *blkcg_parent(struct blkcg *blkcg)
 {
         return css_to_blkcg(blkcg->css.parent);
 }
 
 /**
  * blkg_alloc - allocate a blkg
  * @blkcg: block cgroup the new blkg is associated with
  * @disk: gendisk the new blkg is associated with
  * @gfp_mask: allocation mask to use
  *
  * Allocate a new blkg associating @blkcg and @disk.
  */
 static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct gendisk *disk,
                                    gfp_t gfp_mask)
 {
         struct blkcg_gq *blkg;
         int i, cpu;
 
         /* alloc and init base part */
         blkg = kzalloc_node(sizeof(*blkg), gfp_mask, disk->queue->node);
         if (!blkg)
                 return NULL;
         if (percpu_ref_init(&blkg->refcnt, blkg_release, 0, gfp_mask))
                 goto out_free_blkg;
         blkg->iostat_cpu = alloc_percpu_gfp(struct blkg_iostat_set, gfp_mask);
         if (!blkg->iostat_cpu)
                 goto out_exit_refcnt;
         if (!blk_get_queue(disk->queue))
                 goto out_free_iostat;
 
         blkg->q = disk->queue;
         INIT_LIST_HEAD(&blkg->q_node);
         blkg->blkcg = blkcg;
         blkg->iostat.blkg = blkg;
 #ifdef CONFIG_BLK_CGROUP_PUNT_BIO
         spin_lock_init(&blkg->async_bio_lock);
         bio_list_init(&blkg->async_bios);
         INIT_WORK(&blkg->async_bio_work, blkg_async_bio_workfn);
 #endif
 
         u64_stats_init(&blkg->iostat.sync);
         for_each_possible_cpu(cpu) {
                 u64_stats_init(&per_cpu_ptr(blkg->iostat_cpu, cpu)->sync);
                 per_cpu_ptr(blkg->iostat_cpu, cpu)->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(disk->queue, pol))
                         continue;
 
                 /* alloc per-policy data and attach it to blkg */
                 pd = pol->pd_alloc_fn(disk, blkcg, gfp_mask);
                 if (!pd)
                         goto out_free_pds;
                 blkg->pd[i] = pd;
                 pd->blkg = blkg;
                 pd->plid = i;
                 pd->online = false;
         }
 
         return blkg;
 
 out_free_pds:
         while (--i >= 0)
                 if (blkg->pd[i])
                         blkcg_policy[i]->pd_free_fn(blkg->pd[i]);
         blk_put_queue(disk->queue);
 out_free_iostat:
         free_percpu(blkg->iostat_cpu);
 out_exit_refcnt:
         percpu_ref_exit(&blkg->refcnt);
 out_free_blkg:
         kfree(blkg);
         return NULL;
 }
 
 /*
  * 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 gendisk *disk,
                                     struct blkcg_gq *new_blkg)
 {
         struct blkcg_gq *blkg;
         int i, ret;
 
         lockdep_assert_held(&disk->queue->queue_lock);
 
         /* request_queue is dying, do not create/recreate a blkg */
         if (blk_queue_dying(disk->queue)) {
                 ret = -ENODEV;
                 goto err_free_blkg;
         }
 
         /* blkg holds a reference to blkcg */
         if (!css_tryget_online(&blkcg->css)) {
                 ret = -ENODEV;
                 goto err_free_blkg;
         }
 
         /* allocate */
         if (!new_blkg) {
                 new_blkg = blkg_alloc(blkcg, disk, GFP_NOWAIT | __GFP_NOWARN);
                 if (unlikely(!new_blkg)) {
                         ret = -ENOMEM;
                         goto err_put_css;
                 }
         }
         blkg = new_blkg;
 
         /* link parent */
         if (blkcg_parent(blkcg)) {
                 blkg->parent = blkg_lookup(blkcg_parent(blkcg), disk->queue);
                 if (WARN_ON_ONCE(!blkg->parent)) {
                         ret = -ENODEV;
                         goto err_put_css;
                 }
                 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, disk->queue->id, blkg);
         if (likely(!ret)) {
                 hlist_add_head_rcu(&blkg->blkcg_node, &blkcg->blkg_list);
                 list_add(&blkg->q_node, &disk->queue->blkg_list);
 
                 for (i = 0; i < BLKCG_MAX_POLS; i++) {
                         struct blkcg_policy *pol = blkcg_policy[i];
 
                         if (blkg->pd[i]) {
                                 if (pol->pd_online_fn)
                                         pol->pd_online_fn(blkg->pd[i]);
                                 blkg->pd[i]->online = true;
                         }
                 }
         }
         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_css:
         css_put(&blkcg->css);
 err_free_blkg:
         if (new_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
  * @disk: gendisk of interest
  *
  * Lookup blkg for the @blkcg - @disk 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 takes @disk->queue->queue_lock.
  *
  * Returns the blkg or the closest blkg if blkg_create() fails as it walks
  * down from root.
  */
 static struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
                 struct gendisk *disk)
 {
         struct request_queue *q = disk->queue;
         struct blkcg_gq *blkg;
         unsigned long flags;
 
         WARN_ON_ONCE(!rcu_read_lock_held());
 
         blkg = blkg_lookup(blkcg, q);
         if (blkg)
                 return blkg;
 
         spin_lock_irqsave(&q->queue_lock, flags);
         blkg = blkg_lookup(blkcg, q);
         if (blkg) {
                 if (blkcg != &blkcg_root &&
                     blkg != rcu_dereference(blkcg->blkg_hint))
                         rcu_assign_pointer(blkcg->blkg_hint, blkg);
                 goto found;
         }
 
         /*
          * 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);
                         if (blkg) {
                                 /* remember closest blkg */
                                 ret_blkg = blkg;
                                 break;
                         }
                         pos = parent;
                         parent = blkcg_parent(parent);
                 }
 
                 blkg = blkg_create(pos, disk, NULL);
                 if (IS_ERR(blkg)) {
                         blkg = ret_blkg;
                         break;
                 }
                 if (pos == blkcg)
                         break;
         }
 
 found:
         spin_unlock_irqrestore(&q->queue_lock, flags);
         return blkg;
 }
 
 static void blkg_destroy(struct blkcg_gq *blkg)
 {
         struct blkcg *blkcg = blkg->blkcg;
         int i;
 
         lockdep_assert_held(&blkg->q->queue_lock);
         lockdep_assert_held(&blkcg->lock);
 
         /*
          * blkg stays on the queue list until blkg_free_workfn(), see details in
          * blkg_free_workfn(), hence this function can be called from
          * blkcg_destroy_blkgs() first and again from blkg_destroy_all() before
          * blkg_free_workfn().
          */
         if (hlist_unhashed(&blkg->blkcg_node))
                 return;
 
         for (i = 0; i < BLKCG_MAX_POLS; i++) {
                 struct blkcg_policy *pol = blkcg_policy[i];
 
                 if (blkg->pd[i] && blkg->pd[i]->online) {
                         blkg->pd[i]->online = false;
                         if (pol->pd_offline_fn)
                                 pol->pd_offline_fn(blkg->pd[i]);
                 }
         }
 
         blkg->online = false;
 
         radix_tree_delete(&blkcg->blkg_tree, blkg->q->id);
         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);
 }
 
 static void blkg_destroy_all(struct gendisk *disk)
 {
         struct request_queue *q = disk->queue;
         struct blkcg_gq *blkg;
         int count = BLKG_DESTROY_BATCH_SIZE;
         int i;
 
 restart:
         spin_lock_irq(&q->queue_lock);
         list_for_each_entry(blkg, &q->blkg_list, q_node) {
                 struct blkcg *blkcg = blkg->blkcg;
 
                 if (hlist_unhashed(&blkg->blkcg_node))
                         continue;
 
                 spin_lock(&blkcg->lock);
                 blkg_destroy(blkg);
                 spin_unlock(&blkcg->lock);
 
                 /*
                  * in order to avoid holding the spin lock for too long, release
                  * it when a batch of blkgs are destroyed.
                  */
                 if (!(--count)) {
                         count = BLKG_DESTROY_BATCH_SIZE;
                         spin_unlock_irq(&q->queue_lock);
                         cond_resched();
                         goto restart;
                 }
         }
 
         /*
          * Mark policy deactivated since policy offline has been done, and
          * the free is scheduled, so future blkcg_deactivate_policy() can
          * be bypassed
          */
         for (i = 0; i < BLKCG_MAX_POLS; i++) {
                 struct blkcg_policy *pol = blkcg_policy[i];
 
                 if (pol)
                         __clear_bit(pol->plid, q->blkcg_pols);
         }
 
         q->root_blkg = NULL;
         spin_unlock_irq(&q->queue_lock);
 }
 
 static void blkg_iostat_set(struct blkg_iostat *dst, struct blkg_iostat *src)
 {
         int i;
 
         for (i = 0; i < BLKG_IOSTAT_NR; i++) {
                 dst->bytes[i] = src->bytes[i];
                 dst->ios[i] = src->ios[i];
         }
 }
 
 static void __blkg_clear_stat(struct blkg_iostat_set *bis)
 {
         struct blkg_iostat cur = {0};
         unsigned long flags;
 
         flags = u64_stats_update_begin_irqsave(&bis->sync);
         blkg_iostat_set(&bis->cur, &cur);
         blkg_iostat_set(&bis->last, &cur);
         u64_stats_update_end_irqrestore(&bis->sync, flags);
 }
 
 static void blkg_clear_stat(struct blkcg_gq *blkg)
 {
         int cpu;
 
         for_each_possible_cpu(cpu) {
                 struct blkg_iostat_set *s = per_cpu_ptr(blkg->iostat_cpu, cpu);
 
                 __blkg_clear_stat(s);
         }
         __blkg_clear_stat(&blkg->iostat);
 }
 
 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_clear_stat(blkg);
                 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)
 {
         if (!blkg->q->disk)
                 return NULL;
         return bdi_dev_name(blkg->q->disk->bdi);
 }
 
 /**
  * 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 associated 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_conf_init - initialize a blkg_conf_ctx
  * @ctx: blkg_conf_ctx to initialize
  * @input: input string
  *
  * Initialize @ctx which can be used to parse blkg config input string @input.
  * Once initialized, @ctx can be used with blkg_conf_open_bdev() and
  * blkg_conf_prep(), and must be cleaned up with blkg_conf_exit().
  */
 void blkg_conf_init(struct blkg_conf_ctx *ctx, char *input)
 {
         *ctx = (struct blkg_conf_ctx){ .input = input };
 }
 EXPORT_SYMBOL_GPL(blkg_conf_init);
 
 /**
  * blkg_conf_open_bdev - parse and open bdev for per-blkg config update
  * @ctx: blkg_conf_ctx initialized with blkg_conf_init()
  *
  * Parse the device node prefix part, MAJ:MIN, of per-blkg config update from
  * @ctx->input and get and store the matching bdev in @ctx->bdev. @ctx->body is
  * set to point past the device node prefix.
  *
  * This function may be called multiple times on @ctx and the extra calls become
  * NOOPs. blkg_conf_prep() implicitly calls this function. Use this function
  * explicitly if bdev access is needed without resolving the blkcg / policy part
  * of @ctx->input. Returns -errno on error.
  */
 int blkg_conf_open_bdev(struct blkg_conf_ctx *ctx)
 {
         char *input = ctx->input;
         unsigned int major, minor;
         struct block_device *bdev;
         int key_len;
 
         if (ctx->bdev)
                 return 0;
 
         if (sscanf(input, "%u:%u%n", &major, &minor, &key_len) != 2)
                 return -EINVAL;
 
         input += key_len;
         if (!isspace(*input))
                 return -EINVAL;
         input = skip_spaces(input);
 
         bdev = blkdev_get_no_open(MKDEV(major, minor));
         if (!bdev)
                 return -ENODEV;
         if (bdev_is_partition(bdev)) {
                 blkdev_put_no_open(bdev);
                 return -ENODEV;
         }
 
         mutex_lock(&bdev->bd_queue->rq_qos_mutex);
         if (!disk_live(bdev->bd_disk)) {
                 blkdev_put_no_open(bdev);
                 mutex_unlock(&bdev->bd_queue->rq_qos_mutex);
                 return -ENODEV;
         }
 
         ctx->body = input;
         ctx->bdev = bdev;
         return 0;
 }
 
 /**
  * blkg_conf_prep - parse and prepare for per-blkg config update
  * @blkcg: target block cgroup
  * @pol: target policy
  * @ctx: blkg_conf_ctx initialized with blkg_conf_init()
  *
  * Parse per-blkg config update from @ctx->input and initialize @ctx
  * accordingly. On success, @ctx->body points to the part of @ctx->input
  * following MAJ:MIN, @ctx->bdev points to the target block device and
  * @ctx->blkg to the blkg being configured.
  *
  * blkg_conf_open_bdev() may be called on @ctx beforehand. On success, this
  * function returns with queue lock held and must be followed by
  * blkg_conf_exit().
  */
 int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
                    struct blkg_conf_ctx *ctx)
         __acquires(&bdev->bd_queue->queue_lock)
 {
         struct gendisk *disk;
         struct request_queue *q;
         struct blkcg_gq *blkg;
         int ret;
 
         ret = blkg_conf_open_bdev(ctx);
         if (ret)
                 return ret;
 
         disk = ctx->bdev->bd_disk;
         q = disk->queue;
 
         /*
          * blkcg_deactivate_policy() requires queue to be frozen, we can grab
          * q_usage_counter to prevent concurrent with blkcg_deactivate_policy().
          */
         ret = blk_queue_enter(q, 0);
         if (ret)
                 goto fail;
 
         spin_lock_irq(&q->queue_lock);
 
         if (!blkcg_policy_enabled(q, pol)) {
                 ret = -EOPNOTSUPP;
                 goto fail_unlock;
         }
 
         blkg = blkg_lookup(blkcg, q);
         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)) {
                         pos = parent;
                         parent = blkcg_parent(parent);
                 }
 
                 /* Drop locks to do new blkg allocation with GFP_KERNEL. */
                 spin_unlock_irq(&q->queue_lock);
 
                 new_blkg = blkg_alloc(pos, disk, GFP_KERNEL);
                 if (unlikely(!new_blkg)) {
                         ret = -ENOMEM;
                         goto fail_exit_queue;
                 }
 
                 if (radix_tree_preload(GFP_KERNEL)) {
                         blkg_free(new_blkg);
                         ret = -ENOMEM;
                         goto fail_exit_queue;
                 }
 
                 spin_lock_irq(&q->queue_lock);
 
                 if (!blkcg_policy_enabled(q, pol)) {
                         blkg_free(new_blkg);
                         ret = -EOPNOTSUPP;
                         goto fail_preloaded;
                 }
 
                 blkg = blkg_lookup(pos, q);
                 if (blkg) {
                         blkg_free(new_blkg);
                 } else {
                         blkg = blkg_create(pos, disk, new_blkg);
                         if (IS_ERR(blkg)) {
                                 ret = PTR_ERR(blkg);
                                 goto fail_preloaded;
                         }
                 }
 
                 radix_tree_preload_end();
 
                 if (pos == blkcg)
                         goto success;
         }
 success:
         blk_queue_exit(q);
         ctx->blkg = blkg;
         return 0;
 
 fail_preloaded:
         radix_tree_preload_end();
 fail_unlock:
         spin_unlock_irq(&q->queue_lock);
 fail_exit_queue:
         blk_queue_exit(q);
 fail:
         /*
          * 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_exit - clean up per-blkg config update
  * @ctx: blkg_conf_ctx initialized with blkg_conf_init()
  *
  * Clean up after per-blkg config update. This function must be called on all
  * blkg_conf_ctx's initialized with blkg_conf_init().
  */
 void blkg_conf_exit(struct blkg_conf_ctx *ctx)
         __releases(&ctx->bdev->bd_queue->queue_lock)
         __releases(&ctx->bdev->bd_queue->rq_qos_mutex)
 {
         if (ctx->blkg) {
                 spin_unlock_irq(&bdev_get_queue(ctx->bdev)->queue_lock);
                 ctx->blkg = NULL;
         }
 
         if (ctx->bdev) {
                 mutex_unlock(&ctx->bdev->bd_queue->rq_qos_mutex);
                 blkdev_put_no_open(ctx->bdev);
                 ctx->body = NULL;
                 ctx->bdev = NULL;
         }
 }
 EXPORT_SYMBOL_GPL(blkg_conf_exit);
 
 static void blkg_iostat_add(struct blkg_iostat *dst, struct blkg_iostat *src)
 {
         int i;
 
         for (i = 0; i < BLKG_IOSTAT_NR; i++) {
                 dst->bytes[i] += src->bytes[i];
                 dst->ios[i] += src->ios[i];
         }
 }
 
 static void blkg_iostat_sub(struct blkg_iostat *dst, struct blkg_iostat *src)
 {
         int i;
 
         for (i = 0; i < BLKG_IOSTAT_NR; i++) {
                 dst->bytes[i] -= src->bytes[i];
                 dst->ios[i] -= src->ios[i];
         }
 }
 
 static void blkcg_iostat_update(struct blkcg_gq *blkg, struct blkg_iostat *cur,
                                 struct blkg_iostat *last)
 {
         struct blkg_iostat delta;
         unsigned long flags;
 
         /* propagate percpu delta to global */
         flags = u64_stats_update_begin_irqsave(&blkg->iostat.sync);
         blkg_iostat_set(&delta, cur);
         blkg_iostat_sub(&delta, last);
         blkg_iostat_add(&blkg->iostat.cur, &delta);
         blkg_iostat_add(last, &delta);
         u64_stats_update_end_irqrestore(&blkg->iostat.sync, flags);
 }
 
 static void __blkcg_rstat_flush(struct blkcg *blkcg, int cpu)
 {
         struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu);
         struct llist_node *lnode;
         struct blkg_iostat_set *bisc, *next_bisc;
         unsigned long flags;
 
         rcu_read_lock();
 
         lnode = llist_del_all(lhead);
         if (!lnode)
                 goto out;
 
         /*
          * For covering concurrent parent blkg update from blkg_release().
          *
          * When flushing from cgroup, cgroup_rstat_lock is always held, so
          * this lock won't cause contention most of time.
          */
         raw_spin_lock_irqsave(&blkg_stat_lock, flags);
 
         /*
          * Iterate only the iostat_cpu's queued in the lockless list.
          */
         llist_for_each_entry_safe(bisc, next_bisc, lnode, lnode) {
                 struct blkcg_gq *blkg = bisc->blkg;
                 struct blkcg_gq *parent = blkg->parent;
                 struct blkg_iostat cur;
                 unsigned int seq;
 
                 /*
                  * Order assignment of `next_bisc` from `bisc->lnode.next` in
                  * llist_for_each_entry_safe and clearing `bisc->lqueued` for
                  * avoiding to assign `next_bisc` with new next pointer added
                  * in blk_cgroup_bio_start() in case of re-ordering.
                  *
                  * The pair barrier is implied in llist_add() in blk_cgroup_bio_start().
                  */
                 smp_mb();
 
                 WRITE_ONCE(bisc->lqueued, false);
                 if (bisc == &blkg->iostat)
                         goto propagate_up; /* propagate up to parent only */
 
                 /* fetch the current per-cpu values */
                 do {
                         seq = u64_stats_fetch_begin(&bisc->sync);
                         blkg_iostat_set(&cur, &bisc->cur);
                 } while (u64_stats_fetch_retry(&bisc->sync, seq));
 
                 blkcg_iostat_update(blkg, &cur, &bisc->last);
 
 propagate_up:
                 /* propagate global delta to parent (unless that's root) */
                 if (parent && parent->parent) {
                         blkcg_iostat_update(parent, &blkg->iostat.cur,
                                             &blkg->iostat.last);
                         /*
                          * Queue parent->iostat to its blkcg's lockless
                          * list to propagate up to the grandparent if the
                          * iostat hasn't been queued yet.
                          */
                         if (!parent->iostat.lqueued) {
                                 struct llist_head *plhead;
 
                                 plhead = per_cpu_ptr(parent->blkcg->lhead, cpu);
                                 llist_add(&parent->iostat.lnode, plhead);
                                 parent->iostat.lqueued = true;
                         }
                 }
         }
         raw_spin_unlock_irqrestore(&blkg_stat_lock, flags);
 out:
         rcu_read_unlock();
 }
 
 static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu)
 {
         /* Root-level stats are sourced from system-wide IO stats */
         if (cgroup_parent(css->cgroup))
                 __blkcg_rstat_flush(css_to_blkcg(css), cpu);
 }
 
 /*
  * We source root cgroup stats from the system-wide stats to avoid
  * tracking the same information twice and incurring overhead when no
  * cgroups are defined. For that reason, cgroup_rstat_flush in
  * blkcg_print_stat does not actually fill out the iostat in the root
  * cgroup's blkcg_gq.
  *
  * However, we would like to re-use the printing code between the root and
  * non-root cgroups to the extent possible. For that reason, we simulate
  * flushing the root cgroup's stats by explicitly filling in the iostat
  * with disk level statistics.
  */
 static void blkcg_fill_root_iostats(void)
 {
         struct class_dev_iter iter;
         struct device *dev;
 
         class_dev_iter_init(&iter, &block_class, NULL, &disk_type);
         while ((dev = class_dev_iter_next(&iter))) {
                 struct block_device *bdev = dev_to_bdev(dev);
                 struct blkcg_gq *blkg = bdev->bd_disk->queue->root_blkg;
                 struct blkg_iostat tmp;
                 int cpu;
                 unsigned long flags;
 
                 memset(&tmp, 0, sizeof(tmp));
                 for_each_possible_cpu(cpu) {
                         struct disk_stats *cpu_dkstats;
 
                         cpu_dkstats = per_cpu_ptr(bdev->bd_stats, cpu);
                         tmp.ios[BLKG_IOSTAT_READ] +=
                                 cpu_dkstats->ios[STAT_READ];
                         tmp.ios[BLKG_IOSTAT_WRITE] +=
                                 cpu_dkstats->ios[STAT_WRITE];
                         tmp.ios[BLKG_IOSTAT_DISCARD] +=
                                 cpu_dkstats->ios[STAT_DISCARD];
                         // convert sectors to bytes
                         tmp.bytes[BLKG_IOSTAT_READ] +=
                                 cpu_dkstats->sectors[STAT_READ] << 9;
                         tmp.bytes[BLKG_IOSTAT_WRITE] +=
                                 cpu_dkstats->sectors[STAT_WRITE] << 9;
                         tmp.bytes[BLKG_IOSTAT_DISCARD] +=
                                 cpu_dkstats->sectors[STAT_DISCARD] << 9;
                 }
 
                 flags = u64_stats_update_begin_irqsave(&blkg->iostat.sync);
                 blkg_iostat_set(&blkg->iostat.cur, &tmp);
                 u64_stats_update_end_irqrestore(&blkg->iostat.sync, flags);
         }
 }
 
 static void blkcg_print_one_stat(struct blkcg_gq *blkg, struct seq_file *s)
 {
         struct blkg_iostat_set *bis = &blkg->iostat;
         u64 rbytes, wbytes, rios, wios, dbytes, dios;
         const char *dname;
         unsigned seq;
         int i;
 
         if (!blkg->online)
                 return;
 
         dname = blkg_dev_name(blkg);
         if (!dname)
                 return;
 
         seq_printf(s, "%s ", dname);
 
         do {
                 seq = u64_stats_fetch_begin(&bis->sync);
 
                 rbytes = bis->cur.bytes[BLKG_IOSTAT_READ];
                 wbytes = bis->cur.bytes[BLKG_IOSTAT_WRITE];
                 dbytes = bis->cur.bytes[BLKG_IOSTAT_DISCARD];
                 rios = bis->cur.ios[BLKG_IOSTAT_READ];
                 wios = bis->cur.ios[BLKG_IOSTAT_WRITE];
                 dios = bis->cur.ios[BLKG_IOSTAT_DISCARD];
         } while (u64_stats_fetch_retry(&bis->sync, seq));
 
         if (rbytes || wbytes || rios || wios) {
                 seq_printf(s, "rbytes=%llu wbytes=%llu rios=%llu wios=%llu dbytes=%llu dios=%llu",
                         rbytes, wbytes, rios, wios,
                         dbytes, dios);
         }
 
         if (blkcg_debug_stats && atomic_read(&blkg->use_delay)) {
                 seq_printf(s, " use_delay=%d delay_nsec=%llu",
                         atomic_read(&blkg->use_delay),
                         atomic64_read(&blkg->delay_nsec));
         }
 
         for (i = 0; i < BLKCG_MAX_POLS; i++) {
                 struct blkcg_policy *pol = blkcg_policy[i];
 
                 if (!blkg->pd[i] || !pol->pd_stat_fn)
                         continue;
 
                 pol->pd_stat_fn(blkg->pd[i], s);
         }
 
         seq_puts(s, "\n");
 }
 
 static int blkcg_print_stat(struct seq_file *sf, void *v)
 {
         struct blkcg *blkcg = css_to_blkcg(seq_css(sf));
         struct blkcg_gq *blkg;
 
         if (!seq_css(sf)->parent)
                 blkcg_fill_root_iostats();
         else
                 cgroup_rstat_flush(blkcg->css.cgroup);
 
         rcu_read_lock();
         hlist_for_each_entry_rcu(blkg, &blkcg->blkg_list, blkcg_node) {
                 spin_lock_irq(&blkg->q->queue_lock);
                 blkcg_print_one_stat(blkg, sf);
                 spin_unlock_irq(&blkg->q->queue_lock);
         }
         rcu_read_unlock();
         return 0;
 }
 
 static struct cftype blkcg_files[] = {
         {
                 .name = "stat",
                 .seq_show = blkcg_print_stat,
         },
         { }     /* terminate */
 };
 
 static struct cftype blkcg_legacy_files[] = {
         {
                 .name = "reset_stats",
                 .write_u64 = blkcg_reset_stats,
         },
         { }     /* terminate */
 };
 
 #ifdef CONFIG_CGROUP_WRITEBACK
 struct list_head *blkcg_get_cgwb_list(struct cgroup_subsys_state *css)
 {
         return &css_to_blkcg(css)->cgwb_list;
 }
 #endif
 
 /*
  * 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_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().
  */
 static void blkcg_destroy_blkgs(struct blkcg *blkcg)
 {
         might_sleep();
 
         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 (need_resched() || !spin_trylock(&q->queue_lock)) {
                         /*
                          * Given that the system can accumulate a huge number
                          * of blkgs in pathological cases, check to see if we
                          * need to rescheduling to avoid softlockup.
                          */
                         spin_unlock_irq(&blkcg->lock);
                         cond_resched();
                         spin_lock_irq(&blkcg->lock);
                         continue;
                 }
 
                 blkg_destroy(blkg);
                 spin_unlock(&q->queue_lock);
         }
 
         spin_unlock_irq(&blkcg->lock);
 }
 
 /**
  * blkcg_pin_online - pin online state
  * @blkcg_css: blkcg of interest
  *
  * While pinned, a blkcg is kept online.  This is primarily used to
  * impedance-match blkg and cgwb lifetimes so that blkg doesn't go offline
  * while an associated cgwb is still active.
  */
 void blkcg_pin_online(struct cgroup_subsys_state *blkcg_css)
 {
         refcount_inc(&css_to_blkcg(blkcg_css)->online_pin);
 }
 
 /**
  * blkcg_unpin_online - unpin online state
  * @blkcg_css: blkcg of interest
  *
  * This is primarily used to impedance-match blkg and cgwb lifetimes so
  * that blkg doesn't go offline while an associated cgwb is still active.
  * When this count goes to zero, all active cgwbs have finished so the
  * blkcg can continue destruction by calling blkcg_destroy_blkgs().
  */
 void blkcg_unpin_online(struct cgroup_subsys_state *blkcg_css)
 {
         struct blkcg *blkcg = css_to_blkcg(blkcg_css);
 
         do {
                 if (!refcount_dec_and_test(&blkcg->online_pin))
                         break;
                 blkcg_destroy_blkgs(blkcg);
                 blkcg = blkcg_parent(blkcg);
         } while (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)
 {
         /* this prevents anyone from attaching or migrating to this blkcg */
         wb_blkcg_offline(css);
 
         /* put the base online pin allowing step 2 to be triggered */
         blkcg_unpin_online(css);
 }
 
 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);
 
         free_percpu(blkcg->lhead);
         kfree(blkcg);
 }
 
 static struct cgroup_subsys_state *
 blkcg_css_alloc(struct cgroup_subsys_state *parent_css)
 {
         struct blkcg *blkcg;
         int i;
 
         mutex_lock(&blkcg_pol_mutex);
 
         if (!parent_css) {
                 blkcg = &blkcg_root;
         } else {
                 blkcg = kzalloc(sizeof(*blkcg), GFP_KERNEL);
                 if (!blkcg)
                         goto unlock;
         }
 
         if (init_blkcg_llists(blkcg))
                 goto free_blkcg;
 
         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)
                         goto free_pd_blkcg;
 
                 blkcg->cpd[i] = cpd;
                 cpd->blkcg = blkcg;
                 cpd->plid = i;
         }
 
         spin_lock_init(&blkcg->lock);
         refcount_set(&blkcg->online_pin, 1);
         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);
 #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]);
         free_percpu(blkcg->lhead);
 free_blkcg:
         if (blkcg != &blkcg_root)
                 kfree(blkcg);
 unlock:
         mutex_unlock(&blkcg_pol_mutex);
         return ERR_PTR(-ENOMEM);
 }
 
 static int blkcg_css_online(struct cgroup_subsys_state *css)
 {
         struct blkcg *parent = blkcg_parent(css_to_blkcg(css));
 
         /*
          * blkcg_pin_online() is used to delay blkcg offline so that blkgs
          * don't go offline while cgwbs are still active on them.  Pin the
          * parent so that offline always happens towards the root.
          */
         if (parent)
                 blkcg_pin_online(&parent->css);
         return 0;
 }
 
 void blkg_init_queue(struct request_queue *q)
 {
         INIT_LIST_HEAD(&q->blkg_list);
         mutex_init(&q->blkcg_mutex);
 }
 
 int blkcg_init_disk(struct gendisk *disk)
 {
         struct request_queue *q = disk->queue;
         struct blkcg_gq *new_blkg, *blkg;
         bool preloaded;
         int ret;
 
         new_blkg = blkg_alloc(&blkcg_root, disk, GFP_KERNEL);
         if (!new_blkg)
                 return -ENOMEM;
 
         preloaded = !radix_tree_preload(GFP_KERNEL);
 
         /* Make sure the root blkg exists. */
         /* spin_lock_irq can serve as RCU read-side critical section. */
         spin_lock_irq(&q->queue_lock);
         blkg = blkg_create(&blkcg_root, disk, new_blkg);
         if (IS_ERR(blkg))
                 goto err_unlock;
         q->root_blkg = blkg;
         spin_unlock_irq(&q->queue_lock);
 
         if (preloaded)
                 radix_tree_preload_end();
 
         ret = blk_ioprio_init(disk);
         if (ret)
                 goto err_destroy_all;
 
         return 0;
 
 err_destroy_all:
         blkg_destroy_all(disk);
         return ret;
 err_unlock:
         spin_unlock_irq(&q->queue_lock);
         if (preloaded)
                 radix_tree_preload_end();
         return PTR_ERR(blkg);
 }
 
 void blkcg_exit_disk(struct gendisk *disk)
 {
         blkg_destroy_all(disk);
         blk_throtl_exit(disk);
 }
 
 static void blkcg_exit(struct task_struct *tsk)
 {
         if (tsk->throttle_disk)
                 put_disk(tsk->throttle_disk);
         tsk->throttle_disk = NULL;
 }
 
 struct cgroup_subsys io_cgrp_subsys = {
         .css_alloc = blkcg_css_alloc,
         .css_online = blkcg_css_online,
         .css_offline = blkcg_css_offline,
         .css_free = blkcg_css_free,
         .css_rstat_flush = blkcg_rstat_flush,
         .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 gendisk
  * @disk: gendisk of interest
  * @pol: blkcg policy to activate
  *
  * Activate @pol on @disk.  Requires %GFP_KERNEL context.  @disk goes through
  * bypass mode to populate its blkgs with policy_data for @pol.
  *
  * Activation happens with @disk 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 gendisk *disk, const struct blkcg_policy *pol)
 {
         struct request_queue *q = disk->queue;
         struct blkg_policy_data *pd_prealloc = NULL;
         struct blkcg_gq *blkg, *pinned_blkg = NULL;
         int ret;
 
         if (blkcg_policy_enabled(q, pol))
                 return 0;
 
         if (queue_is_mq(q))
                 blk_mq_freeze_queue(q);
 retry:
         spin_lock_irq(&q->queue_lock);
 
         /* blkg_list is pushed at the head, reverse walk to initialize parents first */
         list_for_each_entry_reverse(blkg, &q->blkg_list, q_node) {
                 struct blkg_policy_data *pd;
 
                 if (blkg->pd[pol->plid])
                         continue;
 
                 /* If prealloc matches, use it; otherwise try GFP_NOWAIT */
                 if (blkg == pinned_blkg) {
                         pd = pd_prealloc;
                         pd_prealloc = NULL;
                 } else {
                         pd = pol->pd_alloc_fn(disk, blkg->blkcg,
                                               GFP_NOWAIT | __GFP_NOWARN);
                 }
 
                 if (!pd) {
                         /*
                          * GFP_NOWAIT failed.  Free the existing one and
                          * prealloc for @blkg w/ GFP_KERNEL.
                          */
                         if (pinned_blkg)
                                 blkg_put(pinned_blkg);
                         blkg_get(blkg);
                         pinned_blkg = blkg;
 
                         spin_unlock_irq(&q->queue_lock);
 
                         if (pd_prealloc)
                                 pol->pd_free_fn(pd_prealloc);
                         pd_prealloc = pol->pd_alloc_fn(disk, blkg->blkcg,
                                                        GFP_KERNEL);
                         if (pd_prealloc)
                                 goto retry;
                         else
                                 goto enomem;
                 }
 
                 spin_lock(&blkg->blkcg->lock);
 
                 pd->blkg = blkg;
                 pd->plid = pol->plid;
                 blkg->pd[pol->plid] = pd;
 
                 if (pol->pd_init_fn)
                         pol->pd_init_fn(pd);
 
                 if (pol->pd_online_fn)
                         pol->pd_online_fn(pd);
                 pd->online = true;
 
                 spin_unlock(&blkg->blkcg->lock);
         }
 
         __set_bit(pol->plid, q->blkcg_pols);
         ret = 0;
 
         spin_unlock_irq(&q->queue_lock);
 out:
         if (queue_is_mq(q))
                 blk_mq_unfreeze_queue(q);
         if (pinned_blkg)
                 blkg_put(pinned_blkg);
         if (pd_prealloc)
                 pol->pd_free_fn(pd_prealloc);
         return ret;
 
 enomem:
         /* alloc failed, take down everything */
         spin_lock_irq(&q->queue_lock);
         list_for_each_entry(blkg, &q->blkg_list, q_node) {
                 struct blkcg *blkcg = blkg->blkcg;
                 struct blkg_policy_data *pd;
 
                 spin_lock(&blkcg->lock);
                 pd = blkg->pd[pol->plid];
                 if (pd) {
                         if (pd->online && pol->pd_offline_fn)
                                 pol->pd_offline_fn(pd);
                         pd->online = false;
                         pol->pd_free_fn(pd);
                         blkg->pd[pol->plid] = NULL;
                 }
                 spin_unlock(&blkcg->lock);
         }
         spin_unlock_irq(&q->queue_lock);
         ret = -ENOMEM;
         goto out;
 }
 EXPORT_SYMBOL_GPL(blkcg_activate_policy);
 
 /**
  * blkcg_deactivate_policy - deactivate a blkcg policy on a gendisk
  * @disk: gendisk of interest
  * @pol: blkcg policy to deactivate
  *
  * Deactivate @pol on @disk.  Follows the same synchronization rules as
  * blkcg_activate_policy().
  */
 void blkcg_deactivate_policy(struct gendisk *disk,
                              const struct blkcg_policy *pol)
 {
         struct request_queue *q = disk->queue;
         struct blkcg_gq *blkg;
 
         if (!blkcg_policy_enabled(q, pol))
                 return;
 
         if (queue_is_mq(q))
                 blk_mq_freeze_queue(q);
 
         mutex_lock(&q->blkcg_mutex);
         spin_lock_irq(&q->queue_lock);
 
         __clear_bit(pol->plid, q->blkcg_pols);
 
         list_for_each_entry(blkg, &q->blkg_list, q_node) {
                 struct blkcg *blkcg = blkg->blkcg;
 
                 spin_lock(&blkcg->lock);
                 if (blkg->pd[pol->plid]) {
                         if (blkg->pd[pol->plid]->online && 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(&blkcg->lock);
         }
 
         spin_unlock_irq(&q->queue_lock);
         mutex_unlock(&q->blkcg_mutex);
 
         if (queue_is_mq(q))
                 blk_mq_unfreeze_queue(q);
 }
 EXPORT_SYMBOL_GPL(blkcg_deactivate_policy);
 
 static void blkcg_free_all_cpd(struct blkcg_policy *pol)
 {
         struct blkcg *blkcg;
 
         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_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;
                 }
         }
 
         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)
                 blkcg_free_all_cpd(pol);
 
         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)
 {
         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)
                 blkcg_free_all_cpd(pol);
 
         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);
 
         /* negative use_delay means no scaling, see blkcg_set_delay() */
         if (atomic_read(&blkg->use_delay) < 0)
                 return;
 
         /*
          * 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_try_cmpxchg(&blkg->delay_start, &old, now)) {
                 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)
 {
         unsigned long pflags;
         bool clamp;
         u64 now = blk_time_get_ns();
         u64 exp;
         u64 delay_nsec = 0;
         int tok;
 
         while (blkg->parent) {
                 int use_delay = atomic_read(&blkg->use_delay);
 
                 if (use_delay) {
                         u64 this_delay;
 
                         blkcg_scale_delay(blkg, now);
                         this_delay = atomic64_read(&blkg->delay_nsec);
                         if (this_delay > delay_nsec) {
                                 delay_nsec = this_delay;
                                 clamp = use_delay > 0;
                         }
                 }
                 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 0.25s. If there's 10's of seconds worth of delay then the
          * tasks will be delayed for 0.25 second for every syscall. If
          * blkcg_set_delay() was used as indicated by negative use_delay, the
          * caller is responsible for regulating the range.
          */
         if (clamp)
                 delay_nsec = min_t(u64, delay_nsec, 250 * NSEC_PER_MSEC);
 
         if (use_memdelay)
                 psi_memstall_enter(&pflags);
 
         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);
 
         if (use_memdelay)
                 psi_memstall_leave(&pflags);
 }
 
 /**
  * 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_disk 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 gendisk *disk = current->throttle_disk;
         struct blkcg *blkcg;
         struct blkcg_gq *blkg;
         bool use_memdelay = current->use_memdelay;
 
         if (!disk)
                 return;
 
         current->throttle_disk = NULL;
         current->use_memdelay = false;
 
         rcu_read_lock();
         blkcg = css_to_blkcg(blkcg_css());
         if (!blkcg)
                 goto out;
         blkg = blkg_lookup(blkcg, disk->queue);
         if (!blkg)
                 goto out;
         if (!blkg_tryget(blkg))
                 goto out;
         rcu_read_unlock();
 
         blkcg_maybe_throttle_blkg(blkg, use_memdelay);
         blkg_put(blkg);
         put_disk(disk);
         return;
 out:
         rcu_read_unlock();
 }
 
 /**
  * blkcg_schedule_throttle - this task needs to check for throttling
  * @disk: disk to throttle
  * @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 block_device 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 gendisk *disk, bool use_memdelay)
 {
         if (unlikely(current->flags & PF_KTHREAD))
                 return;
 
         if (current->throttle_disk != disk) {
                 if (test_bit(GD_DEAD, &disk->state))
                         return;
                 get_device(disk_to_dev(disk));
 
                 if (current->throttle_disk)
                         put_disk(current->throttle_disk);
                 current->throttle_disk = disk;
         }
 
         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)
 {
         if (WARN_ON_ONCE(atomic_read(&blkg->use_delay) < 0))
                 return;
         blkcg_scale_delay(blkg, now);
         atomic64_add(delta, &blkg->delay_nsec);
 }
 
 /**
  * blkg_tryget_closest - try and get a blkg ref on the closet blkg
  * @bio: target bio
  * @css: target css
  *
  * As the failure mode here is to walk up the blkg tree, this ensure that the
  * blkg->parent pointers are always valid.  This returns the blkg that it ended
  * up taking a reference on or %NULL if no reference was taken.
  */
 static inline struct blkcg_gq *blkg_tryget_closest(struct bio *bio,
                 struct cgroup_subsys_state *css)
 {
         struct blkcg_gq *blkg, *ret_blkg = NULL;
 
         rcu_read_lock();
         blkg = blkg_lookup_create(css_to_blkcg(css), bio->bi_bdev->bd_disk);
         while (blkg) {
                 if (blkg_tryget(blkg)) {
                         ret_blkg = blkg;
                         break;
                 }
                 blkg = blkg->parent;
         }
         rcu_read_unlock();
 
         return ret_blkg;
 }
 
 /**
  * bio_associate_blkg_from_css - associate a bio with a specified css
  * @bio: target bio
  * @css: target css
  *
  * Associate @bio with the blkg found by combining the css's blkg and the
  * request_queue of the @bio.  An association failure is handled by walking up
  * the blkg tree.  Therefore, the blkg associated can be anything between @blkg
  * and q->root_blkg.  This situation only happens when a cgroup is dying and
  * then the remaining bios will spill to the closest alive blkg.
  *
  * A reference will be taken on the blkg and will be released when @bio is
  * freed.
  */
 void bio_associate_blkg_from_css(struct bio *bio,
                                  struct cgroup_subsys_state *css)
 {
         if (bio->bi_blkg)
                 blkg_put(bio->bi_blkg);
 
         if (css && css->parent) {
                 bio->bi_blkg = blkg_tryget_closest(bio, css);
         } else {
                 blkg_get(bdev_get_queue(bio->bi_bdev)->root_blkg);
                 bio->bi_blkg = bdev_get_queue(bio->bi_bdev)->root_blkg;
         }
 }
 EXPORT_SYMBOL_GPL(bio_associate_blkg_from_css);
 
 /**
  * bio_associate_blkg - associate a bio with a blkg
  * @bio: target bio
  *
  * Associate @bio with the blkg found from the bio's css and request_queue.
  * If one is not found, bio_lookup_blkg() creates the blkg.  If a blkg is
  * already associated, the css is reused and association redone as the
  * request_queue may have changed.
  */
 void bio_associate_blkg(struct bio *bio)
 {
         struct cgroup_subsys_state *css;
 
         if (blk_op_is_passthrough(bio->bi_opf))
                 return;
 
         rcu_read_lock();
 
         if (bio->bi_blkg)
                 css = bio_blkcg_css(bio);
         else
                 css = blkcg_css();
 
         bio_associate_blkg_from_css(bio, css);
 
         rcu_read_unlock();
 }
 EXPORT_SYMBOL_GPL(bio_associate_blkg);
 
 /**
  * bio_clone_blkg_association - clone blkg association from src to dst bio
  * @dst: destination bio
  * @src: source bio
  */
 void bio_clone_blkg_association(struct bio *dst, struct bio *src)
 {
         if (src->bi_blkg)
                 bio_associate_blkg_from_css(dst, bio_blkcg_css(src));
 }
 EXPORT_SYMBOL_GPL(bio_clone_blkg_association);
 
 static int blk_cgroup_io_type(struct bio *bio)
 {
         if (op_is_discard(bio->bi_opf))
                 return BLKG_IOSTAT_DISCARD;
         if (op_is_write(bio->bi_opf))
                 return BLKG_IOSTAT_WRITE;
         return BLKG_IOSTAT_READ;
 }
 
 void blk_cgroup_bio_start(struct bio *bio)
 {
         struct blkcg *blkcg = bio->bi_blkg->blkcg;
         int rwd = blk_cgroup_io_type(bio), cpu;
         struct blkg_iostat_set *bis;
         unsigned long flags;
 
         if (!cgroup_subsys_on_dfl(io_cgrp_subsys))
                 return;
 
         /* Root-level stats are sourced from system-wide IO stats */
         if (!cgroup_parent(blkcg->css.cgroup))
                 return;
 
         cpu = get_cpu();
         bis = per_cpu_ptr(bio->bi_blkg->iostat_cpu, cpu);
         flags = u64_stats_update_begin_irqsave(&bis->sync);
 
         /*
          * If the bio is flagged with BIO_CGROUP_ACCT it means this is a split
          * bio and we would have already accounted for the size of the bio.
          */
         if (!bio_flagged(bio, BIO_CGROUP_ACCT)) {
                 bio_set_flag(bio, BIO_CGROUP_ACCT);
                 bis->cur.bytes[rwd] += bio->bi_iter.bi_size;
         }
         bis->cur.ios[rwd]++;
 
         /*
          * If the iostat_cpu isn't in a lockless list, put it into the
          * list to indicate that a stat update is pending.
          */
         if (!READ_ONCE(bis->lqueued)) {
                 struct llist_head *lhead = this_cpu_ptr(blkcg->lhead);
 
                 llist_add(&bis->lnode, lhead);
                 WRITE_ONCE(bis->lqueued, true);
         }
 
         u64_stats_update_end_irqrestore(&bis->sync, flags);
         cgroup_rstat_updated(blkcg->css.cgroup, cpu);
         put_cpu();
 }
 
 bool blk_cgroup_congested(void)
 {
         struct blkcg *blkcg;
         bool ret = false;
 
         rcu_read_lock();
         for (blkcg = css_to_blkcg(blkcg_css()); blkcg;
              blkcg = blkcg_parent(blkcg)) {
                 if (atomic_read(&blkcg->congestion_count)) {
                         ret = true;
                         break;
                 }
         }
         rcu_read_unlock();
         return ret;
 }
 
 module_param(blkcg_debug_stats, bool, 0644);
 MODULE_PARM_DESC(blkcg_debug_stats, "True if you want debug stats, false if not");
 

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