linux/mm/backing-dev.c

// SPDX-License-Identifier: GPL-2.0-only

#include <linux/blkdev.h>
#include <linux/wait.h>
#include <linux/rbtree.h>
#include <linux/kthread.h>
#include <linux/backing-dev.h>
#include <linux/blk-cgroup.h>
#include <linux/freezer.h>
#include <linux/fs.h>
#include <linux/pagemap.h>
#include <linux/mm.h>
#include <linux/sched/mm.h>
#include <linux/sched.h>
#include <linux/module.h>
#include <linux/writeback.h>
#include <linux/device.h>
#include <trace/events/writeback.h>

struct backing_dev_info noop_backing_dev_info;
EXPORT_SYMBOL_GPL(noop_backing_dev_info);

static struct class *bdi_class;
static const char *bdi_unknown_name = "(unknown)";

/*
 * bdi_lock protects bdi_tree and updates to bdi_list. bdi_list has RCU
 * reader side locking.
 */
DEFINE_SPINLOCK(bdi_lock);
static u64 bdi_id_cursor;
static struct rb_root bdi_tree = RB_ROOT;
LIST_HEAD(bdi_list);

/* bdi_wq serves all asynchronous writeback tasks */
struct workqueue_struct *bdi_wq;

#define K(x) ((x) << (PAGE_SHIFT - 10))

#ifdef CONFIG_DEBUG_FS
#include <linux/debugfs.h>
#include <linux/seq_file.h>

static struct dentry *bdi_debug_root;

static void bdi_debug_init(void)
{
	bdi_debug_root = debugfs_create_dir("bdi", NULL);
}

static int bdi_debug_stats_show(struct seq_file *m, void *v)
{
	struct backing_dev_info *bdi = m->private;
	struct bdi_writeback *wb = &bdi->wb;
	unsigned long background_thresh;
	unsigned long dirty_thresh;
	unsigned long wb_thresh;
	unsigned long nr_dirty, nr_io, nr_more_io, nr_dirty_time;
	struct inode *inode;

	nr_dirty = nr_io = nr_more_io = nr_dirty_time = 0;
	spin_lock(&wb->list_lock);
	list_for_each_entry(inode, &wb->b_dirty, i_io_list)
		nr_dirty++;
	list_for_each_entry(inode, &wb->b_io, i_io_list)
		nr_io++;
	list_for_each_entry(inode, &wb->b_more_io, i_io_list)
		nr_more_io++;
	list_for_each_entry(inode, &wb->b_dirty_time, i_io_list)
		if (inode->i_state & I_DIRTY_TIME)
			nr_dirty_time++;
	spin_unlock(&wb->list_lock);

	global_dirty_limits(&background_thresh, &dirty_thresh);
	wb_thresh = wb_calc_thresh(wb, dirty_thresh);

	seq_printf(m,
		   "BdiWriteback:       %10lu kB\n"
		   "BdiReclaimable:     %10lu kB\n"
		   "BdiDirtyThresh:     %10lu kB\n"
		   "DirtyThresh:        %10lu kB\n"
		   "BackgroundThresh:   %10lu kB\n"
		   "BdiDirtied:         %10lu kB\n"
		   "BdiWritten:         %10lu kB\n"
		   "BdiWriteBandwidth:  %10lu kBps\n"
		   "b_dirty:            %10lu\n"
		   "b_io:               %10lu\n"
		   "b_more_io:          %10lu\n"
		   "b_dirty_time:       %10lu\n"
		   "bdi_list:           %10u\n"
		   "state:              %10lx\n",
		   (unsigned long) K(wb_stat(wb, WB_WRITEBACK)),
		   (unsigned long) K(wb_stat(wb, WB_RECLAIMABLE)),
		   K(wb_thresh),
		   K(dirty_thresh),
		   K(background_thresh),
		   (unsigned long) K(wb_stat(wb, WB_DIRTIED)),
		   (unsigned long) K(wb_stat(wb, WB_WRITTEN)),
		   (unsigned long) K(wb->write_bandwidth),
		   nr_dirty,
		   nr_io,
		   nr_more_io,
		   nr_dirty_time,
		   !list_empty(&bdi->bdi_list), bdi->wb.state);

	return 0;
}
DEFINE_SHOW_ATTRIBUTE(bdi_debug_stats);

static void bdi_debug_register(struct backing_dev_info *bdi, const char *name)
{
	bdi->debug_dir = debugfs_create_dir(name, bdi_debug_root);

	debugfs_create_file("stats", 0444, bdi->debug_dir, bdi,
			    &bdi_debug_stats_fops);
}

static void bdi_debug_unregister(struct backing_dev_info *bdi)
{
	debugfs_remove_recursive(bdi->debug_dir);
}
#else
static inline void bdi_debug_init(void)
{
}
static inline void bdi_debug_register(struct backing_dev_info *bdi,
				      const char *name)
{
}
static inline void bdi_debug_unregister(struct backing_dev_info *bdi)
{
}
#endif

static ssize_t read_ahead_kb_store(struct device *dev,
				  struct device_attribute *attr,
				  const char *buf, size_t count)
{
	struct backing_dev_info *bdi = dev_get_drvdata(dev);
	unsigned long read_ahead_kb;
	ssize_t ret;

	ret = kstrtoul(buf, 10, &read_ahead_kb);
	if (ret < 0)
		return ret;

	bdi->ra_pages = read_ahead_kb >> (PAGE_SHIFT - 10);

	return count;
}

#define BDI_SHOW(name, expr)						\
static ssize_t name##_show(struct device *dev,				\
			   struct device_attribute *attr, char *buf)	\
{									\
	struct backing_dev_info *bdi = dev_get_drvdata(dev);		\
									\
	return sysfs_emit(buf, "%lld\n", (long long)expr);		\
}									\
static DEVICE_ATTR_RW(name);

BDI_SHOW(read_ahead_kb, K(bdi->ra_pages))

static ssize_t min_ratio_store(struct device *dev,
		struct device_attribute *attr, const char *buf, size_t count)
{
	struct backing_dev_info *bdi = dev_get_drvdata(dev);
	unsigned int ratio;
	ssize_t ret;

	ret = kstrtouint(buf, 10, &ratio);
	if (ret < 0)
		return ret;

	ret = bdi_set_min_ratio(bdi, ratio);
	if (!ret)
		ret = count;

	return ret;
}
BDI_SHOW(min_ratio, bdi->min_ratio)

static ssize_t max_ratio_store(struct device *dev,
		struct device_attribute *attr, const char *buf, size_t count)
{
	struct backing_dev_info *bdi = dev_get_drvdata(dev);
	unsigned int ratio;
	ssize_t ret;

	ret = kstrtouint(buf, 10, &ratio);
	if (ret < 0)
		return ret;

	ret = bdi_set_max_ratio(bdi, ratio);
	if (!ret)
		ret = count;

	return ret;
}
BDI_SHOW(max_ratio, bdi->max_ratio)

static ssize_t stable_pages_required_show(struct device *dev,
					  struct device_attribute *attr,
					  char *buf)
{
	dev_warn_once(dev,
		"the stable_pages_required attribute has been removed. Use the stable_writes queue attribute instead.\n");
	return sysfs_emit(buf, "%d\n", 0);
}
static DEVICE_ATTR_RO(stable_pages_required);

static struct attribute *bdi_dev_attrs[] = {
	&dev_attr_read_ahead_kb.attr,
	&dev_attr_min_ratio.attr,
	&dev_attr_max_ratio.attr,
	&dev_attr_stable_pages_required.attr,
	NULL,
};
ATTRIBUTE_GROUPS(bdi_dev);

static __init int bdi_class_init(void)
{
	bdi_class = class_create(THIS_MODULE, "bdi");
	if (IS_ERR(bdi_class))
		return PTR_ERR(bdi_class);

	bdi_class->dev_groups = bdi_dev_groups;
	bdi_debug_init();

	return 0;
}
postcore_initcall(bdi_class_init);

static int __init default_bdi_init(void)
{
	bdi_wq = alloc_workqueue("writeback", WQ_MEM_RECLAIM | WQ_UNBOUND |
				 WQ_SYSFS, 0);
	if (!bdi_wq)
		return -ENOMEM;
	return 0;
}
subsys_initcall(default_bdi_init);

/*
 * This function is used when the first inode for this wb is marked dirty. It
 * wakes-up the corresponding bdi thread which should then take care of the
 * periodic background write-out of dirty inodes. Since the write-out would
 * starts only 'dirty_writeback_interval' centisecs from now anyway, we just
 * set up a timer which wakes the bdi thread up later.
 *
 * Note, we wouldn't bother setting up the timer, but this function is on the
 * fast-path (used by '__mark_inode_dirty()'), so we save few context switches
 * by delaying the wake-up.
 *
 * We have to be careful not to postpone flush work if it is scheduled for
 * earlier. Thus we use queue_delayed_work().
 */
void wb_wakeup_delayed(struct bdi_writeback *wb)
{
	unsigned long timeout;

	timeout = msecs_to_jiffies(dirty_writeback_interval * 10);
	spin_lock_irq(&wb->work_lock);
	if (test_bit(WB_registered, &wb->state))
		queue_delayed_work(bdi_wq, &wb->dwork, timeout);
	spin_unlock_irq(&wb->work_lock);
}

static void wb_update_bandwidth_workfn(struct work_struct *work)
{
	struct bdi_writeback *wb = container_of(to_delayed_work(work),
						struct bdi_writeback, bw_dwork);

	wb_update_bandwidth(wb);
}

/*
 * Initial write bandwidth: 100 MB/s
 */
#define INIT_BW		(100 << (20 - PAGE_SHIFT))

static int wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi,
		   gfp_t gfp)
{
	int i, err;

	memset(wb, 0, sizeof(*wb));

	wb->bdi = bdi;
	wb->last_old_flush = jiffies;
	INIT_LIST_HEAD(&wb->b_dirty);
	INIT_LIST_HEAD(&wb->b_io);
	INIT_LIST_HEAD(&wb->b_more_io);
	INIT_LIST_HEAD(&wb->b_dirty_time);
	spin_lock_init(&wb->list_lock);

	atomic_set(&wb->writeback_inodes, 0);
	wb->bw_time_stamp = jiffies;
	wb->balanced_dirty_ratelimit = INIT_BW;
	wb->dirty_ratelimit = INIT_BW;
	wb->write_bandwidth = INIT_BW;
	wb->avg_write_bandwidth = INIT_BW;

	spin_lock_init(&wb->work_lock);
	INIT_LIST_HEAD(&wb->work_list);
	INIT_DELAYED_WORK(&wb->dwork, wb_workfn);
	INIT_DELAYED_WORK(&wb->bw_dwork, wb_update_bandwidth_workfn);
	wb->dirty_sleep = jiffies;

	err = fprop_local_init_percpu(&wb->completions, gfp);
	if (err)
		return err;

	for (i = 0; i < NR_WB_STAT_ITEMS; i++) {
		err = percpu_counter_init(&wb->stat[i], 0, gfp);
		if (err)
			goto out_destroy_stat;
	}

	return 0;

out_destroy_stat:
	while (i--)
		percpu_counter_destroy(&wb->stat[i]);
	fprop_local_destroy_percpu(&wb->completions);
	return err;
}

static void cgwb_remove_from_bdi_list(struct bdi_writeback *wb);

/*
 * Remove bdi from the global list and shutdown any threads we have running
 */
static void wb_shutdown(struct bdi_writeback *wb)
{
	/* Make sure nobody queues further work */
	spin_lock_irq(&wb->work_lock);
	if (!test_and_clear_bit(WB_registered, &wb->state)) {
		spin_unlock_irq(&wb->work_lock);
		return;
	}
	spin_unlock_irq(&wb->work_lock);

	cgwb_remove_from_bdi_list(wb);
	/*
	 * Drain work list and shutdown the delayed_work.  !WB_registered
	 * tells wb_workfn() that @wb is dying and its work_list needs to
	 * be drained no matter what.
	 */
	mod_delayed_work(bdi_wq, &wb->dwork, 0);
	flush_delayed_work(&wb->dwork);
	WARN_ON(!list_empty(&wb->work_list));
	flush_delayed_work(&wb->bw_dwork);
}

static void wb_exit(struct bdi_writeback *wb)
{
	int i;

	WARN_ON(delayed_work_pending(&wb->dwork));

	for (i = 0; i < NR_WB_STAT_ITEMS; i++)
		percpu_counter_destroy(&wb->stat[i]);

	fprop_local_destroy_percpu(&wb->completions);
}

#ifdef CONFIG_CGROUP_WRITEBACK

#include <linux/memcontrol.h>

/*
 * cgwb_lock protects bdi->cgwb_tree, blkcg->cgwb_list, offline_cgwbs and
 * memcg->cgwb_list.  bdi->cgwb_tree is also RCU protected.
 */
static DEFINE_SPINLOCK(cgwb_lock);
static struct workqueue_struct *cgwb_release_wq;

static LIST_HEAD(offline_cgwbs);
static void cleanup_offline_cgwbs_workfn(struct work_struct *work);
static DECLARE_WORK(cleanup_offline_cgwbs_work, cleanup_offline_cgwbs_workfn);

static void cgwb_release_workfn(struct work_struct *work)
{
	struct bdi_writeback *wb = container_of(work, struct bdi_writeback,
						release_work);
	struct backing_dev_info *bdi = wb->bdi;

	mutex_lock(&wb->bdi->cgwb_release_mutex);
	wb_shutdown(wb);

	css_put(wb->memcg_css);
	css_put(wb->blkcg_css);
	mutex_unlock(&wb->bdi->cgwb_release_mutex);

	/* triggers blkg destruction if no online users left */
	blkcg_unpin_online(wb->blkcg_css);

	fprop_local_destroy_percpu(&wb->memcg_completions);

	spin_lock_irq(&cgwb_lock);
	list_del(&wb->offline_node);
	spin_unlock_irq(&cgwb_lock);

	percpu_ref_exit(&wb->refcnt);
	wb_exit(wb);
	bdi_put(bdi);
	WARN_ON_ONCE(!list_empty(&wb->b_attached));
	kfree_rcu(wb, rcu);
}

static void cgwb_release(struct percpu_ref *refcnt)
{
	struct bdi_writeback *wb = container_of(refcnt, struct bdi_writeback,
						refcnt);
	queue_work(cgwb_release_wq, &wb->release_work);
}

static void cgwb_kill(struct bdi_writeback *wb)
{
	lockdep_assert_held(&cgwb_lock);

	WARN_ON(!radix_tree_delete(&wb->bdi->cgwb_tree, wb->memcg_css->id));
	list_del(&wb->memcg_node);
	list_del(&wb->blkcg_node);
	list_add(&wb->offline_node, &offline_cgwbs);
	percpu_ref_kill(&wb->refcnt);
}

static void cgwb_remove_from_bdi_list(struct bdi_writeback *wb)
{
	spin_lock_irq(&cgwb_lock);
	list_del_rcu(&wb->bdi_node);
	spin_unlock_irq(&cgwb_lock);
}

static int cgwb_create(struct backing_dev_info *bdi,
		       struct cgroup_subsys_state *memcg_css, gfp_t gfp)
{
	struct mem_cgroup *memcg;
	struct cgroup_subsys_state *blkcg_css;
	struct list_head *memcg_cgwb_list, *blkcg_cgwb_list;
	struct bdi_writeback *wb;
	unsigned long flags;
	int ret = 0;

	memcg = mem_cgroup_from_css(memcg_css);
	blkcg_css = cgroup_get_e_css(memcg_css->cgroup, &io_cgrp_subsys);
	memcg_cgwb_list = &memcg->cgwb_list;
	blkcg_cgwb_list = blkcg_get_cgwb_list(blkcg_css);

	/* look up again under lock and discard on blkcg mismatch */
	spin_lock_irqsave(&cgwb_lock, flags);
	wb = radix_tree_lookup(&bdi->cgwb_tree, memcg_css->id);
	if (wb && wb->blkcg_css != blkcg_css) {
		cgwb_kill(wb);
		wb = NULL;
	}
	spin_unlock_irqrestore(&cgwb_lock, flags);
	if (wb)
		goto out_put;

	/* need to create a new one */
	wb = kmalloc(sizeof(*wb), gfp);
	if (!wb) {
		ret = -ENOMEM;
		goto out_put;
	}

	ret = wb_init(wb, bdi, gfp);
	if (ret)
		goto err_free;

	ret = percpu_ref_init(&wb->refcnt, cgwb_release, 0, gfp);
	if (ret)
		goto err_wb_exit;

	ret = fprop_local_init_percpu(&wb->memcg_completions, gfp);
	if (ret)
		goto err_ref_exit;

	wb->memcg_css = memcg_css;
	wb->blkcg_css = blkcg_css;
	INIT_LIST_HEAD(&wb->b_attached);
	INIT_WORK(&wb->release_work, cgwb_release_workfn);
	set_bit(WB_registered, &wb->state);
	bdi_get(bdi);

	/*
	 * The root wb determines the registered state of the whole bdi and
	 * memcg_cgwb_list and blkcg_cgwb_list's next pointers indicate
	 * whether they're still online.  Don't link @wb if any is dead.
	 * See wb_memcg_offline() and wb_blkcg_offline().
	 */
	ret = -ENODEV;
	spin_lock_irqsave(&cgwb_lock, flags);
	if (test_bit(WB_registered, &bdi->wb.state) &&
	    blkcg_cgwb_list->next && memcg_cgwb_list->next) {
		/* we might have raced another instance of this function */
		ret = radix_tree_insert(&bdi->cgwb_tree, memcg_css->id, wb);
		if (!ret) {
			list_add_tail_rcu(&wb->bdi_node, &bdi->wb_list);
			list_add(&wb->memcg_node, memcg_cgwb_list);
			list_add(&wb->blkcg_node, blkcg_cgwb_list);
			blkcg_pin_online(blkcg_css);
			css_get(memcg_css);
			css_get(blkcg_css);
		}
	}
	spin_unlock_irqrestore(&cgwb_lock, flags);
	if (ret) {
		if (ret == -EEXIST)
			ret = 0;
		goto err_fprop_exit;
	}
	goto out_put;

err_fprop_exit:
	bdi_put(bdi);
	fprop_local_destroy_percpu(&wb->memcg_completions);
err_ref_exit:
	percpu_ref_exit(&wb->refcnt);
err_wb_exit:
	wb_exit(wb);
err_free:
	kfree(wb);
out_put:
	css_put(blkcg_css);
	return ret;
}

/**
 * wb_get_lookup - get wb for a given memcg
 * @bdi: target bdi
 * @memcg_css: cgroup_subsys_state of the target memcg (must have positive ref)
 *
 * Try to get the wb for @memcg_css on @bdi.  The returned wb has its
 * refcount incremented.
 *
 * This function uses css_get() on @memcg_css and thus expects its refcnt
 * to be positive on invocation.  IOW, rcu_read_lock() protection on
 * @memcg_css isn't enough.  try_get it before calling this function.
 *
 * A wb is keyed by its associated memcg.  As blkcg implicitly enables
 * memcg on the default hierarchy, memcg association is guaranteed to be
 * more specific (equal or descendant to the associated blkcg) and thus can
 * identify both the memcg and blkcg associations.
 *
 * Because the blkcg associated with a memcg may change as blkcg is enabled
 * and disabled closer to root in the hierarchy, each wb keeps track of
 * both the memcg and blkcg associated with it and verifies the blkcg on
 * each lookup.  On mismatch, the existing wb is discarded and a new one is
 * created.
 */
struct bdi_writeback *wb_get_lookup(struct backing_dev_info *bdi,
				    struct cgroup_subsys_state *memcg_css)
{
	struct bdi_writeback *wb;

	if (!memcg_css->parent)
		return &bdi->wb;

	rcu_read_lock();
	wb = radix_tree_lookup(&bdi->cgwb_tree, memcg_css->id);
	if (wb) {
		struct cgroup_subsys_state *blkcg_css;

		/* see whether the blkcg association has changed */
		blkcg_css = cgroup_get_e_css(memcg_css->cgroup, &io_cgrp_subsys);
		if (unlikely(wb->blkcg_css != blkcg_css || !wb_tryget(wb)))
			wb = NULL;
		css_put(blkcg_css);
	}
	rcu_read_unlock();

	return wb;
}

/**
 * wb_get_create - get wb for a given memcg, create if necessary
 * @bdi: target bdi
 * @memcg_css: cgroup_subsys_state of the target memcg (must have positive ref)
 * @gfp: allocation mask to use
 *
 * Try to get the wb for @memcg_css on @bdi.  If it doesn't exist, try to
 * create one.  See wb_get_lookup() for more details.
 */
struct bdi_writeback *wb_get_create(struct backing_dev_info *bdi,
				    struct cgroup_subsys_state *memcg_css,
				    gfp_t gfp)
{
	struct bdi_writeback *wb;

	might_alloc(gfp);

	if (!memcg_css->parent)
		return &bdi->wb;

	do {
		wb = wb_get_lookup(bdi, memcg_css);
	} while (!wb && !cgwb_create(bdi, memcg_css, gfp));

	return wb;
}

static int cgwb_bdi_init(struct backing_dev_info *bdi)
{
	int ret;

	INIT_RADIX_TREE(&bdi->cgwb_tree, GFP_ATOMIC);
	mutex_init(&bdi->cgwb_release_mutex);
	init_rwsem(&bdi->wb_switch_rwsem);

	ret = wb_init(&bdi->wb, bdi, GFP_KERNEL);
	if (!ret) {
		bdi->wb.memcg_css = &root_mem_cgroup->css;
		bdi->wb.blkcg_css = blkcg_root_css;
	}
	return ret;
}

static void cgwb_bdi_unregister(struct backing_dev_info *bdi)
{
	struct radix_tree_iter iter;
	void **slot;
	struct bdi_writeback *wb;

	WARN_ON(test_bit(WB_registered, &bdi->wb.state));

	spin_lock_irq(&cgwb_lock);
	radix_tree_for_each_slot(slot, &bdi->cgwb_tree, &iter, 0)
		cgwb_kill(*slot);
	spin_unlock_irq(&cgwb_lock);

	mutex_lock(&bdi->cgwb_release_mutex);
	spin_lock_irq(&cgwb_lock);
	while (!list_empty(&bdi->wb_list)) {
		wb = list_first_entry(&bdi->wb_list, struct bdi_writeback,
				      bdi_node);
		spin_unlock_irq(&cgwb_lock);
		wb_shutdown(wb);
		spin_lock_irq(&cgwb_lock);
	}
	spin_unlock_irq(&cgwb_lock);
	mutex_unlock(&bdi->cgwb_release_mutex);
}

/*
 * cleanup_offline_cgwbs_workfn - try to release dying cgwbs
 *
 * Try to release dying cgwbs by switching attached inodes to the nearest
 * living ancestor's writeback. Processed wbs are placed at the end
 * of the list to guarantee the forward progress.
 */
static void cleanup_offline_cgwbs_workfn(struct work_struct *work)
{
	struct bdi_writeback *wb;
	LIST_HEAD(processed);

	spin_lock_irq(&cgwb_lock);

	while (!list_empty(&offline_cgwbs)) {
		wb = list_first_entry(&offline_cgwbs, struct bdi_writeback,
				      offline_node);
		list_move(&wb->offline_node, &processed);

		/*
		 * If wb is dirty, cleaning up the writeback by switching
		 * attached inodes will result in an effective removal of any
		 * bandwidth restrictions, which isn't the goal.  Instead,
		 * it can be postponed until the next time, when all io
		 * will be likely completed.  If in the meantime some inodes
		 * will get re-dirtied, they should be eventually switched to
		 * a new cgwb.
		 */
		if (wb_has_dirty_io(wb))
			continue;

		if (!wb_tryget(wb))
			continue;

		spin_unlock_irq(&cgwb_lock);
		while (cleanup_offline_cgwb(wb))
			cond_resched();
		spin_lock_irq(&cgwb_lock);

		wb_put(wb);
	}

	if (!list_empty(&processed))
		list_splice_tail(&processed, &offline_cgwbs);

	spin_unlock_irq(&cgwb_lock);
}

/**
 * wb_memcg_offline - kill all wb's associated with a memcg being offlined
 * @memcg: memcg being offlined
 *
 * Also prevents creation of any new wb's associated with @memcg.
 */
void wb_memcg_offline(struct mem_cgroup *memcg)
{
	struct list_head *memcg_cgwb_list = &memcg->cgwb_list;
	struct bdi_writeback *wb, *next;

	spin_lock_irq(&cgwb_lock);
	list_for_each_entry_safe(wb, next, memcg_cgwb_list, memcg_node)
		cgwb_kill(wb);
	memcg_cgwb_list->next = NULL;	/* prevent new wb's */
	spin_unlock_irq(&cgwb_lock);

	queue_work(system_unbound_wq, &cleanup_offline_cgwbs_work);
}

/**
 * wb_blkcg_offline - kill all wb's associated with a blkcg being offlined
 * @css: blkcg being offlined
 *
 * Also prevents creation of any new wb's associated with @blkcg.
 */
void wb_blkcg_offline(struct cgroup_subsys_state *css)
{
	struct bdi_writeback *wb, *next;
	struct list_head *list = blkcg_get_cgwb_list(css);

	spin_lock_irq(&cgwb_lock);
	list_for_each_entry_safe(wb, next, list, blkcg_node)
		cgwb_kill(wb);
	list->next = NULL;	/* prevent new wb's */
	spin_unlock_irq(&cgwb_lock);
}

static void cgwb_bdi_register(struct backing_dev_info *bdi)
{
	spin_lock_irq(&cgwb_lock);
	list_add_tail_rcu(&bdi->wb.bdi_node, &bdi->wb_list);
	spin_unlock_irq(&cgwb_lock);
}

static int __init cgwb_init(void)
{
	/*
	 * There can be many concurrent release work items overwhelming
	 * system_wq.  Put them in a separate wq and limit concurrency.
	 * There's no point in executing many of these in parallel.
	 */
	cgwb_release_wq = alloc_workqueue("cgwb_release", 0, 1);
	if (!cgwb_release_wq)
		return -ENOMEM;

	return 0;
}
subsys_initcall(cgwb_init);

#else	/* CONFIG_CGROUP_WRITEBACK */

static int cgwb_bdi_init(struct backing_dev_info *bdi)
{
	return wb_init(&bdi->wb, bdi, GFP_KERNEL);
}

static void cgwb_bdi_unregister(struct backing_dev_info *bdi) { }

static void cgwb_bdi_register(struct backing_dev_info *bdi)
{
	list_add_tail_rcu(&bdi->wb.bdi_node, &bdi->wb_list);
}

static void cgwb_remove_from_bdi_list(struct bdi_writeback *wb)
{
	list_del_rcu(&wb->bdi_node);
}

#endif	/* CONFIG_CGROUP_WRITEBACK */

int bdi_init(struct backing_dev_info *bdi)
{
	int ret;

	bdi->dev = NULL;

	kref_init(&bdi->refcnt);
	bdi->min_ratio = 0;
	bdi->max_ratio = 100;
	bdi->max_prop_frac = FPROP_FRAC_BASE;
	INIT_LIST_HEAD(&bdi->bdi_list);
	INIT_LIST_HEAD(&bdi->wb_list);
	init_waitqueue_head(&bdi->wb_waitq);

	ret = cgwb_bdi_init(bdi);

	return ret;
}

struct backing_dev_info *bdi_alloc(int node_id)
{
	struct backing_dev_info *bdi;

	bdi = kzalloc_node(sizeof(*bdi), GFP_KERNEL, node_id);
	if (!bdi)
		return NULL;

	if (bdi_init(bdi)) {
		kfree(bdi);
		return NULL;
	}
	bdi->capabilities = BDI_CAP_WRITEBACK | BDI_CAP_WRITEBACK_ACCT;
	bdi->ra_pages = VM_READAHEAD_PAGES;
	bdi->io_pages = VM_READAHEAD_PAGES;
	timer_setup(&bdi->laptop_mode_wb_timer, laptop_mode_timer_fn, 0);
	return bdi;
}
EXPORT_SYMBOL(bdi_alloc);

static struct rb_node **bdi_lookup_rb_node(u64 id, struct rb_node **parentp)
{
	struct rb_node **p = &bdi_tree.rb_node;
	struct rb_node *parent = NULL;
	struct backing_dev_info *bdi;

	lockdep_assert_held(&bdi_lock);

	while (*p) {
		parent = *p;
		bdi = rb_entry(parent, struct backing_dev_info, rb_node);

		if (bdi->id > id)
			p = &(*p)->rb_left;
		else if (bdi->id < id)
			p = &(*p)->rb_right;
		else
			break;
	}

	if (parentp)
		*parentp = parent;
	return p;
}

/**
 * bdi_get_by_id - lookup and get bdi from its id
 * @id: bdi id to lookup
 *
 * Find bdi matching @id and get it.  Returns NULL if the matching bdi
 * doesn't exist or is already unregistered.
 */
struct backing_dev_info *bdi_get_by_id(u64 id)
{
	struct backing_dev_info *bdi = NULL;
	struct rb_node **p;

	spin_lock_bh(&bdi_lock);
	p = bdi_lookup_rb_node(id, NULL);
	if (*p) {
		bdi = rb_entry(*p, struct backing_dev_info, rb_node);
		bdi_get(bdi);
	}
	spin_unlock_bh(&bdi_lock);

	return bdi;
}

int bdi_register_va(struct backing_dev_info *bdi, const char *fmt, va_list args)
{
	struct device *dev;
	struct rb_node *parent, **p;

	if (bdi->dev)	/* The driver needs to use separate queues per device */
		return 0;

	vsnprintf(bdi->dev_name, sizeof(bdi->dev_name), fmt, args);
	dev = device_create(bdi_class, NULL, MKDEV(0, 0), bdi, bdi->dev_name);
	if (IS_ERR(dev))
		return PTR_ERR(dev);

	cgwb_bdi_register(bdi);
	bdi->dev = dev;

	bdi_debug_register(bdi, dev_name(dev));
	set_bit(WB_registered, &bdi->wb.state);

	spin_lock_bh(&bdi_lock);

	bdi->id = ++bdi_id_cursor;

	p = bdi_lookup_rb_node(bdi->id, &parent);
	rb_link_node(&bdi->rb_node, parent, p);
	rb_insert_color(&bdi->rb_node, &bdi_tree);

	list_add_tail_rcu(&bdi->bdi_list, &bdi_list);

	spin_unlock_bh(&bdi_lock);

	trace_writeback_bdi_register(bdi);
	return 0;
}

int bdi_register(struct backing_dev_info *bdi, const char *fmt, ...)
{
	va_list args;
	int ret;

	va_start(args, fmt);
	ret = bdi_register_va(bdi, fmt, args);
	va_end(args);
	return ret;
}
EXPORT_SYMBOL(bdi_register);

void bdi_set_owner(struct backing_dev_info *bdi, struct device *owner)
{
	WARN_ON_ONCE(bdi->owner);
	bdi->owner = owner;
	get_device(owner);
}

/*
 * Remove bdi from bdi_list, and ensure that it is no longer visible
 */
static void bdi_remove_from_list(struct backing_dev_info *bdi)
{
	spin_lock_bh(&bdi_lock);
	rb_erase(&bdi->rb_node, &bdi_tree);
	list_del_rcu(&bdi->bdi_list);
	spin_unlock_bh(&bdi_lock);

	synchronize_rcu_expedited();
}

void bdi_unregister(struct backing_dev_info *bdi)
{
	del_timer_sync(&bdi->laptop_mode_wb_timer);

	/* make sure nobody finds us on the bdi_list anymore */
	bdi_remove_from_list(bdi);
	wb_shutdown(&bdi->wb);
	cgwb_bdi_unregister(bdi);

	/*
	 * If this BDI's min ratio has been set, use bdi_set_min_ratio() to
	 * update the global bdi_min_ratio.
	 */
	if (bdi->min_ratio)
		bdi_set_min_ratio(bdi, 0);

	if (bdi->dev) {
		bdi_debug_unregister(bdi);
		device_unregister(bdi->dev);
		bdi->dev = NULL;
	}

	if (bdi->owner) {
		put_device(bdi->owner);
		bdi->owner = NULL;
	}
}
EXPORT_SYMBOL(bdi_unregister);

static void release_bdi(struct kref *ref)
{
	struct backing_dev_info *bdi =
			container_of(ref, struct backing_dev_info, refcnt);

	WARN_ON_ONCE(test_bit(WB_registered, &bdi->wb.state));
	WARN_ON_ONCE(bdi->dev);
	wb_exit(&bdi->wb);
	kfree(bdi);
}

void bdi_put(struct backing_dev_info *bdi)
{
	kref_put(&bdi->refcnt, release_bdi);
}
EXPORT_SYMBOL(bdi_put);

struct backing_dev_info *inode_to_bdi(struct inode *inode)
{
	struct super_block *sb;

	if (!inode)
		return &noop_backing_dev_info;

	sb = inode->i_sb;
#ifdef CONFIG_BLOCK
	if (sb_is_blkdev_sb(sb))
		return I_BDEV(inode)->bd_disk->bdi;
#endif
	return sb->s_bdi;
}
EXPORT_SYMBOL(inode_to_bdi);

const char *bdi_dev_name(struct backing_dev_info *bdi)
{
	if (!bdi || !bdi->dev)
		return bdi_unknown_name;
	return bdi->dev_name;
}
EXPORT_SYMBOL_GPL(bdi_dev_name);