/*
   * Copyright (C) 2011 STRATO.  All rights reserved.
   *
   * This program is free software; you can redistribute it and/or
   * modify it under the terms of the GNU General Public
   * License v2 as published by the Free Software Foundation.
   *
   * This program is distributed in the hope that it will be useful,
   * but WITHOUT ANY WARRANTY; without even the implied warranty of
   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   * General Public License for more details.
   *
   * You should have received a copy of the GNU General Public
   * License along with this program; if not, write to the
   * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
   * Boston, MA 021110-1307, USA.
   */
  
  #include <linux/sched.h>
  #include <linux/pagemap.h>
  #include <linux/writeback.h>
  #include <linux/blkdev.h>
  #include <linux/rbtree.h>
  #include <linux/slab.h>
  #include <linux/workqueue.h>
  #include "ctree.h"
  #include "volumes.h"
  #include "disk-io.h"
  #include "transaction.h"

  #include "dev-replace.h"

  
  #undef DEBUG
  
  /*
   * This is the implementation for the generic read ahead framework.
   *
   * To trigger a readahead, btrfs_reada_add must be called. It will start
   * a read ahead for the given range [start, end) on tree root. The returned
   * handle can either be used to wait on the readahead to finish
   * (btrfs_reada_wait), or to send it to the background (btrfs_reada_detach).
   *
   * The read ahead works as follows:
   * On btrfs_reada_add, the root of the tree is inserted into a radix_tree.
   * reada_start_machine will then search for extents to prefetch and trigger
   * some reads. When a read finishes for a node, all contained node/leaf
   * pointers that lie in the given range will also be enqueued. The reads will
   * be triggered in sequential order, thus giving a big win over a naive
   * enumeration. It will also make use of multi-device layouts. Each disk
   * will have its on read pointer and all disks will by utilized in parallel.
   * Also will no two disks read both sides of a mirror simultaneously, as this
   * would waste seeking capacity. Instead both disks will read different parts
   * of the filesystem.
   * Any number of readaheads can be started in parallel. The read order will be
   * determined globally, i.e. 2 parallel readaheads will normally finish faster
   * than the 2 started one after another.
   */

  #define MAX_IN_FLIGHT 6
  
  struct reada_extctl {
  	struct list_head	list;
  	struct reada_control	*rc;
  	u64			generation;
  };
  
  struct reada_extent {
  	u64			logical;
  	struct btrfs_key	top;

  	int			err;
  	struct list_head	extctl;

  	int 			refcnt;

  	spinlock_t		lock;

  	struct reada_zone	*zones[BTRFS_MAX_MIRRORS];

  	int			nzones;
  	struct btrfs_device	*scheduled_for;
  };
  
  struct reada_zone {
  	u64			start;
  	u64			end;
  	u64			elems;
  	struct list_head	list;
  	spinlock_t		lock;
  	int			locked;
  	struct btrfs_device	*device;

  	struct btrfs_device	*devs[BTRFS_MAX_MIRRORS]; /* full list, incl
  							   * self */

  	int			ndevs;
  	struct kref		refcnt;
  };
  
  struct reada_machine_work {

  	struct btrfs_work	work;

  	struct btrfs_fs_info	*fs_info;
  };
  
  static void reada_extent_put(struct btrfs_fs_info *, struct reada_extent *);
  static void reada_control_release(struct kref *kref);
  static void reada_zone_release(struct kref *kref);
  static void reada_start_machine(struct btrfs_fs_info *fs_info);
  static void __reada_start_machine(struct btrfs_fs_info *fs_info);
  
  static int reada_add_block(struct reada_control *rc, u64 logical,
  			   struct btrfs_key *top, int level, u64 generation);
  
  /* recurses */
  /* in case of err, eb might be NULL */
  static int __readahead_hook(struct btrfs_root *root, struct extent_buffer *eb,
  			    u64 start, int err)
  {
  	int level = 0;
  	int nritems;
  	int i;
  	u64 bytenr;
  	u64 generation;
  	struct reada_extent *re;
  	struct btrfs_fs_info *fs_info = root->fs_info;
  	struct list_head list;
  	unsigned long index = start >> PAGE_CACHE_SHIFT;
  	struct btrfs_device *for_dev;
  
  	if (eb)
  		level = btrfs_header_level(eb);
  
  	/* find extent */
  	spin_lock(&fs_info->reada_lock);
  	re = radix_tree_lookup(&fs_info->reada_tree, index);
  	if (re)

  		re->refcnt++;

  	spin_unlock(&fs_info->reada_lock);
  
  	if (!re)
  		return -1;
  
  	spin_lock(&re->lock);
  	/*
  	 * just take the full list from the extent. afterwards we
  	 * don't need the lock anymore
  	 */
  	list_replace_init(&re->extctl, &list);
  	for_dev = re->scheduled_for;
  	re->scheduled_for = NULL;
  	spin_unlock(&re->lock);
  
  	if (err == 0) {
  		nritems = level ? btrfs_header_nritems(eb) : 0;
  		generation = btrfs_header_generation(eb);
  		/*
  		 * FIXME: currently we just set nritems to 0 if this is a leaf,
  		 * effectively ignoring the content. In a next step we could
  		 * trigger more readahead depending from the content, e.g.
  		 * fetch the checksums for the extents in the leaf.
  		 */
  	} else {
  		/*
  		 * this is the error case, the extent buffer has not been
  		 * read correctly. We won't access anything from it and
  		 * just cleanup our data structures. Effectively this will
  		 * cut the branch below this node from read ahead.
  		 */
  		nritems = 0;
  		generation = 0;
  	}
  
  	for (i = 0; i < nritems; i++) {
  		struct reada_extctl *rec;
  		u64 n_gen;
  		struct btrfs_key key;
  		struct btrfs_key next_key;
  
  		btrfs_node_key_to_cpu(eb, &key, i);
  		if (i + 1 < nritems)
  			btrfs_node_key_to_cpu(eb, &next_key, i + 1);
  		else
  			next_key = re->top;
  		bytenr = btrfs_node_blockptr(eb, i);
  		n_gen = btrfs_node_ptr_generation(eb, i);
  
  		list_for_each_entry(rec, &list, list) {
  			struct reada_control *rc = rec->rc;
  
  			/*
  			 * if the generation doesn't match, just ignore this
  			 * extctl. This will probably cut off a branch from
  			 * prefetch. Alternatively one could start a new (sub-)
  			 * prefetch for this branch, starting again from root.
  			 * FIXME: move the generation check out of this loop
  			 */
  #ifdef DEBUG
  			if (rec->generation != generation) {

  				btrfs_debug(root->fs_info,
  					   "generation mismatch for (%llu,%d,%llu) %llu != %llu",

  				       key.objectid, key.type, key.offset,
  				       rec->generation, generation);
  			}
  #endif
  			if (rec->generation == generation &&
  			    btrfs_comp_cpu_keys(&key, &rc->key_end) < 0 &&
  			    btrfs_comp_cpu_keys(&next_key, &rc->key_start) > 0)
  				reada_add_block(rc, bytenr, &next_key,
  						level - 1, n_gen);
  		}
  	}
  	/*
  	 * free extctl records
  	 */
  	while (!list_empty(&list)) {
  		struct reada_control *rc;
  		struct reada_extctl *rec;
  
  		rec = list_first_entry(&list, struct reada_extctl, list);
  		list_del(&rec->list);
  		rc = rec->rc;
  		kfree(rec);
  
  		kref_get(&rc->refcnt);
  		if (atomic_dec_and_test(&rc->elems)) {
  			kref_put(&rc->refcnt, reada_control_release);
  			wake_up(&rc->wait);
  		}
  		kref_put(&rc->refcnt, reada_control_release);
  
  		reada_extent_put(fs_info, re);	/* one ref for each entry */
  	}
  	reada_extent_put(fs_info, re);	/* our ref */
  	if (for_dev)
  		atomic_dec(&for_dev->reada_in_flight);
  
  	return 0;
  }
  
  /*
   * start is passed separately in case eb in NULL, which may be the case with
   * failed I/O
   */
  int btree_readahead_hook(struct btrfs_root *root, struct extent_buffer *eb,
  			 u64 start, int err)
  {
  	int ret;
  
  	ret = __readahead_hook(root, eb, start, err);
  
  	reada_start_machine(root->fs_info);
  
  	return ret;
  }
  
  static struct reada_zone *reada_find_zone(struct btrfs_fs_info *fs_info,
  					  struct btrfs_device *dev, u64 logical,

  					  struct btrfs_bio *bbio)

  {
  	int ret;

  	struct reada_zone *zone;
  	struct btrfs_block_group_cache *cache = NULL;
  	u64 start;
  	u64 end;
  	int i;

  	zone = NULL;
  	spin_lock(&fs_info->reada_lock);
  	ret = radix_tree_gang_lookup(&dev->reada_zones, (void **)&zone,
  				     logical >> PAGE_CACHE_SHIFT, 1);
  	if (ret == 1)
  		kref_get(&zone->refcnt);
  	spin_unlock(&fs_info->reada_lock);
  
  	if (ret == 1) {
  		if (logical >= zone->start && logical < zone->end)
  			return zone;
  		spin_lock(&fs_info->reada_lock);
  		kref_put(&zone->refcnt, reada_zone_release);
  		spin_unlock(&fs_info->reada_lock);
  	}

  	cache = btrfs_lookup_block_group(fs_info, logical);
  	if (!cache)
  		return NULL;
  
  	start = cache->key.objectid;
  	end = start + cache->key.offset - 1;
  	btrfs_put_block_group(cache);
  
  	zone = kzalloc(sizeof(*zone), GFP_NOFS);
  	if (!zone)
  		return NULL;
  
  	zone->start = start;
  	zone->end = end;
  	INIT_LIST_HEAD(&zone->list);
  	spin_lock_init(&zone->lock);
  	zone->locked = 0;
  	kref_init(&zone->refcnt);
  	zone->elems = 0;
  	zone->device = dev; /* our device always sits at index 0 */

  	for (i = 0; i < bbio->num_stripes; ++i) {

  		/* bounds have already been checked */

  		zone->devs[i] = bbio->stripes[i].dev;

  	}

  	zone->ndevs = bbio->num_stripes;

  
  	spin_lock(&fs_info->reada_lock);
  	ret = radix_tree_insert(&dev->reada_zones,

  				(unsigned long)(zone->end >> PAGE_CACHE_SHIFT),

  				zone);

  	if (ret == -EEXIST) {

  		kfree(zone);

  		ret = radix_tree_gang_lookup(&dev->reada_zones, (void **)&zone,
  					     logical >> PAGE_CACHE_SHIFT, 1);
  		if (ret == 1)
  			kref_get(&zone->refcnt);

  	}

  	spin_unlock(&fs_info->reada_lock);

  
  	return zone;
  }
  
  static struct reada_extent *reada_find_extent(struct btrfs_root *root,
  					      u64 logical,
  					      struct btrfs_key *top, int level)
  {
  	int ret;

  	struct reada_extent *re = NULL;

  	struct reada_extent *re_exist = NULL;

  	struct btrfs_fs_info *fs_info = root->fs_info;

  	struct btrfs_bio *bbio = NULL;

  	struct btrfs_device *dev;

  	struct btrfs_device *prev_dev;

  	u32 blocksize;
  	u64 length;

  	int real_stripes;

  	int nzones = 0;
  	int i;
  	unsigned long index = logical >> PAGE_CACHE_SHIFT;

  	int dev_replace_is_ongoing;

  	spin_lock(&fs_info->reada_lock);
  	re = radix_tree_lookup(&fs_info->reada_tree, index);
  	if (re)

  		re->refcnt++;

  	spin_unlock(&fs_info->reada_lock);

  	if (re)

  		return re;
  
  	re = kzalloc(sizeof(*re), GFP_NOFS);
  	if (!re)
  		return NULL;

  	blocksize = root->nodesize;

  	re->logical = logical;

  	re->top = *top;
  	INIT_LIST_HEAD(&re->extctl);
  	spin_lock_init(&re->lock);

  	re->refcnt = 1;

  
  	/*
  	 * map block
  	 */
  	length = blocksize;

  	ret = btrfs_map_block(fs_info, REQ_GET_READ_MIRRORS, logical, &length,
  			      &bbio, 0);

  	if (ret || !bbio || length < blocksize)

  		goto error;

  	if (bbio->num_stripes > BTRFS_MAX_MIRRORS) {

  		btrfs_err(root->fs_info,
  			   "readahead: more than %d copies not supported",
  			   BTRFS_MAX_MIRRORS);

  		goto error;
  	}

  	real_stripes = bbio->num_stripes - bbio->num_tgtdevs;
  	for (nzones = 0; nzones < real_stripes; ++nzones) {

  		struct reada_zone *zone;

  		dev = bbio->stripes[nzones].dev;
  		zone = reada_find_zone(fs_info, dev, logical, bbio);

  		if (!zone)
  			break;
  
  		re->zones[nzones] = zone;
  		spin_lock(&zone->lock);
  		if (!zone->elems)
  			kref_get(&zone->refcnt);
  		++zone->elems;
  		spin_unlock(&zone->lock);
  		spin_lock(&fs_info->reada_lock);
  		kref_put(&zone->refcnt, reada_zone_release);
  		spin_unlock(&fs_info->reada_lock);
  	}
  	re->nzones = nzones;
  	if (nzones == 0) {
  		/* not a single zone found, error and out */
  		goto error;
  	}
  
  	/* insert extent in reada_tree + all per-device trees, all or nothing */

  	btrfs_dev_replace_lock(&fs_info->dev_replace);

  	spin_lock(&fs_info->reada_lock);
  	ret = radix_tree_insert(&fs_info->reada_tree, index, re);

  	if (ret == -EEXIST) {
  		re_exist = radix_tree_lookup(&fs_info->reada_tree, index);
  		BUG_ON(!re_exist);

  		re_exist->refcnt++;

  		spin_unlock(&fs_info->reada_lock);

  		btrfs_dev_replace_unlock(&fs_info->dev_replace);

  		goto error;
  	}

  	if (ret) {
  		spin_unlock(&fs_info->reada_lock);

  		btrfs_dev_replace_unlock(&fs_info->dev_replace);

  		goto error;
  	}

  	prev_dev = NULL;

  	dev_replace_is_ongoing = btrfs_dev_replace_is_ongoing(
  			&fs_info->dev_replace);

  	for (i = 0; i < nzones; ++i) {

  		dev = bbio->stripes[i].dev;

  		if (dev == prev_dev) {
  			/*
  			 * in case of DUP, just add the first zone. As both
  			 * are on the same device, there's nothing to gain
  			 * from adding both.
  			 * Also, it wouldn't work, as the tree is per device
  			 * and adding would fail with EEXIST
  			 */
  			continue;
  		}

  		if (!dev->bdev) {

  			/*
  			 * cannot read ahead on missing device, but for RAID5/6,
  			 * REQ_GET_READ_MIRRORS return 1. So don't skip missing
  			 * device for such case.
  			 */
  			if (nzones > 1)
  				continue;

  		}

  		if (dev_replace_is_ongoing &&
  		    dev == fs_info->dev_replace.tgtdev) {
  			/*
  			 * as this device is selected for reading only as
  			 * a last resort, skip it for read ahead.
  			 */
  			continue;
  		}

  		prev_dev = dev;

  		ret = radix_tree_insert(&dev->reada_extents, index, re);
  		if (ret) {
  			while (--i >= 0) {

  				dev = bbio->stripes[i].dev;

  				BUG_ON(dev == NULL);

  				/* ignore whether the entry was inserted */

  				radix_tree_delete(&dev->reada_extents, index);
  			}
  			BUG_ON(fs_info == NULL);
  			radix_tree_delete(&fs_info->reada_tree, index);
  			spin_unlock(&fs_info->reada_lock);

  			btrfs_dev_replace_unlock(&fs_info->dev_replace);

  			goto error;
  		}
  	}
  	spin_unlock(&fs_info->reada_lock);

  	btrfs_dev_replace_unlock(&fs_info->dev_replace);

  	btrfs_put_bbio(bbio);

  	return re;
  
  error:
  	while (nzones) {
  		struct reada_zone *zone;
  
  		--nzones;
  		zone = re->zones[nzones];
  		kref_get(&zone->refcnt);
  		spin_lock(&zone->lock);
  		--zone->elems;
  		if (zone->elems == 0) {
  			/*
  			 * no fs_info->reada_lock needed, as this can't be
  			 * the last ref
  			 */
  			kref_put(&zone->refcnt, reada_zone_release);
  		}
  		spin_unlock(&zone->lock);
  
  		spin_lock(&fs_info->reada_lock);
  		kref_put(&zone->refcnt, reada_zone_release);
  		spin_unlock(&fs_info->reada_lock);
  	}

  	btrfs_put_bbio(bbio);

  	kfree(re);

  	return re_exist;

  }

  static void reada_extent_put(struct btrfs_fs_info *fs_info,
  			     struct reada_extent *re)
  {
  	int i;
  	unsigned long index = re->logical >> PAGE_CACHE_SHIFT;
  
  	spin_lock(&fs_info->reada_lock);

  	if (--re->refcnt) {

  		spin_unlock(&fs_info->reada_lock);
  		return;
  	}
  
  	radix_tree_delete(&fs_info->reada_tree, index);
  	for (i = 0; i < re->nzones; ++i) {
  		struct reada_zone *zone = re->zones[i];
  
  		radix_tree_delete(&zone->device->reada_extents, index);
  	}
  
  	spin_unlock(&fs_info->reada_lock);
  
  	for (i = 0; i < re->nzones; ++i) {
  		struct reada_zone *zone = re->zones[i];
  
  		kref_get(&zone->refcnt);
  		spin_lock(&zone->lock);
  		--zone->elems;
  		if (zone->elems == 0) {
  			/* no fs_info->reada_lock needed, as this can't be
  			 * the last ref */
  			kref_put(&zone->refcnt, reada_zone_release);
  		}
  		spin_unlock(&zone->lock);
  
  		spin_lock(&fs_info->reada_lock);
  		kref_put(&zone->refcnt, reada_zone_release);
  		spin_unlock(&fs_info->reada_lock);
  	}
  	if (re->scheduled_for)
  		atomic_dec(&re->scheduled_for->reada_in_flight);
  
  	kfree(re);
  }
  
  static void reada_zone_release(struct kref *kref)
  {
  	struct reada_zone *zone = container_of(kref, struct reada_zone, refcnt);
  
  	radix_tree_delete(&zone->device->reada_zones,
  			  zone->end >> PAGE_CACHE_SHIFT);
  
  	kfree(zone);
  }
  
  static void reada_control_release(struct kref *kref)
  {
  	struct reada_control *rc = container_of(kref, struct reada_control,
  						refcnt);
  
  	kfree(rc);
  }
  
  static int reada_add_block(struct reada_control *rc, u64 logical,
  			   struct btrfs_key *top, int level, u64 generation)
  {
  	struct btrfs_root *root = rc->root;
  	struct reada_extent *re;
  	struct reada_extctl *rec;
  
  	re = reada_find_extent(root, logical, top, level); /* takes one ref */
  	if (!re)
  		return -1;
  
  	rec = kzalloc(sizeof(*rec), GFP_NOFS);
  	if (!rec) {
  		reada_extent_put(root->fs_info, re);

  		return -ENOMEM;

  	}
  
  	rec->rc = rc;
  	rec->generation = generation;
  	atomic_inc(&rc->elems);
  
  	spin_lock(&re->lock);
  	list_add_tail(&rec->list, &re->extctl);
  	spin_unlock(&re->lock);
  
  	/* leave the ref on the extent */
  
  	return 0;
  }
  
  /*
   * called with fs_info->reada_lock held
   */
  static void reada_peer_zones_set_lock(struct reada_zone *zone, int lock)
  {
  	int i;
  	unsigned long index = zone->end >> PAGE_CACHE_SHIFT;
  
  	for (i = 0; i < zone->ndevs; ++i) {
  		struct reada_zone *peer;
  		peer = radix_tree_lookup(&zone->devs[i]->reada_zones, index);
  		if (peer && peer->device != zone->device)
  			peer->locked = lock;
  	}
  }
  
  /*
   * called with fs_info->reada_lock held
   */
  static int reada_pick_zone(struct btrfs_device *dev)
  {
  	struct reada_zone *top_zone = NULL;
  	struct reada_zone *top_locked_zone = NULL;
  	u64 top_elems = 0;
  	u64 top_locked_elems = 0;
  	unsigned long index = 0;
  	int ret;
  
  	if (dev->reada_curr_zone) {
  		reada_peer_zones_set_lock(dev->reada_curr_zone, 0);
  		kref_put(&dev->reada_curr_zone->refcnt, reada_zone_release);
  		dev->reada_curr_zone = NULL;
  	}
  	/* pick the zone with the most elements */
  	while (1) {
  		struct reada_zone *zone;
  
  		ret = radix_tree_gang_lookup(&dev->reada_zones,
  					     (void **)&zone, index, 1);
  		if (ret == 0)
  			break;
  		index = (zone->end >> PAGE_CACHE_SHIFT) + 1;
  		if (zone->locked) {
  			if (zone->elems > top_locked_elems) {
  				top_locked_elems = zone->elems;
  				top_locked_zone = zone;
  			}
  		} else {
  			if (zone->elems > top_elems) {
  				top_elems = zone->elems;
  				top_zone = zone;
  			}
  		}
  	}
  	if (top_zone)
  		dev->reada_curr_zone = top_zone;
  	else if (top_locked_zone)
  		dev->reada_curr_zone = top_locked_zone;
  	else
  		return 0;
  
  	dev->reada_next = dev->reada_curr_zone->start;
  	kref_get(&dev->reada_curr_zone->refcnt);
  	reada_peer_zones_set_lock(dev->reada_curr_zone, 1);
  
  	return 1;
  }
  
  static int reada_start_machine_dev(struct btrfs_fs_info *fs_info,
  				   struct btrfs_device *dev)
  {
  	struct reada_extent *re = NULL;
  	int mirror_num = 0;
  	struct extent_buffer *eb = NULL;
  	u64 logical;

  	int ret;
  	int i;
  	int need_kick = 0;
  
  	spin_lock(&fs_info->reada_lock);
  	if (dev->reada_curr_zone == NULL) {
  		ret = reada_pick_zone(dev);
  		if (!ret) {
  			spin_unlock(&fs_info->reada_lock);
  			return 0;
  		}
  	}
  	/*
  	 * FIXME currently we issue the reads one extent at a time. If we have
  	 * a contiguous block of extents, we could also coagulate them or use
  	 * plugging to speed things up
  	 */
  	ret = radix_tree_gang_lookup(&dev->reada_extents, (void **)&re,
  				     dev->reada_next >> PAGE_CACHE_SHIFT, 1);
  	if (ret == 0 || re->logical >= dev->reada_curr_zone->end) {
  		ret = reada_pick_zone(dev);
  		if (!ret) {
  			spin_unlock(&fs_info->reada_lock);
  			return 0;
  		}
  		re = NULL;
  		ret = radix_tree_gang_lookup(&dev->reada_extents, (void **)&re,
  					dev->reada_next >> PAGE_CACHE_SHIFT, 1);
  	}
  	if (ret == 0) {
  		spin_unlock(&fs_info->reada_lock);
  		return 0;
  	}

  	dev->reada_next = re->logical + fs_info->tree_root->nodesize;

  	re->refcnt++;

  
  	spin_unlock(&fs_info->reada_lock);
  
  	/*
  	 * find mirror num
  	 */
  	for (i = 0; i < re->nzones; ++i) {
  		if (re->zones[i]->device == dev) {
  			mirror_num = i + 1;
  			break;
  		}
  	}
  	logical = re->logical;

  
  	spin_lock(&re->lock);
  	if (re->scheduled_for == NULL) {
  		re->scheduled_for = dev;
  		need_kick = 1;
  	}
  	spin_unlock(&re->lock);
  
  	reada_extent_put(fs_info, re);
  
  	if (!need_kick)
  		return 0;
  
  	atomic_inc(&dev->reada_in_flight);

  	ret = reada_tree_block_flagged(fs_info->extent_root, logical,

  			mirror_num, &eb);

  	if (ret)
  		__readahead_hook(fs_info->extent_root, NULL, logical, ret);
  	else if (eb)
  		__readahead_hook(fs_info->extent_root, eb, eb->start, ret);
  
  	if (eb)
  		free_extent_buffer(eb);
  
  	return 1;
  
  }

  static void reada_start_machine_worker(struct btrfs_work *work)

  {
  	struct reada_machine_work *rmw;
  	struct btrfs_fs_info *fs_info;

  	int old_ioprio;

  
  	rmw = container_of(work, struct reada_machine_work, work);
  	fs_info = rmw->fs_info;
  
  	kfree(rmw);

  	old_ioprio = IOPRIO_PRIO_VALUE(task_nice_ioclass(current),
  				       task_nice_ioprio(current));
  	set_task_ioprio(current, BTRFS_IOPRIO_READA);

  	__reada_start_machine(fs_info);

  	set_task_ioprio(current, old_ioprio);

  }
  
  static void __reada_start_machine(struct btrfs_fs_info *fs_info)
  {
  	struct btrfs_device *device;
  	struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
  	u64 enqueued;
  	u64 total = 0;
  	int i;
  
  	do {
  		enqueued = 0;
  		list_for_each_entry(device, &fs_devices->devices, dev_list) {
  			if (atomic_read(&device->reada_in_flight) <
  			    MAX_IN_FLIGHT)
  				enqueued += reada_start_machine_dev(fs_info,
  								    device);
  		}
  		total += enqueued;
  	} while (enqueued && total < 10000);
  
  	if (enqueued == 0)
  		return;
  
  	/*
  	 * If everything is already in the cache, this is effectively single
  	 * threaded. To a) not hold the caller for too long and b) to utilize
  	 * more cores, we broke the loop above after 10000 iterations and now
  	 * enqueue to workers to finish it. This will distribute the load to
  	 * the cores.
  	 */
  	for (i = 0; i < 2; ++i)
  		reada_start_machine(fs_info);
  }
  
  static void reada_start_machine(struct btrfs_fs_info *fs_info)
  {
  	struct reada_machine_work *rmw;
  
  	rmw = kzalloc(sizeof(*rmw), GFP_NOFS);
  	if (!rmw) {
  		/* FIXME we cannot handle this properly right now */
  		BUG();
  	}

  	btrfs_init_work(&rmw->work, btrfs_readahead_helper,
  			reada_start_machine_worker, NULL, NULL);

  	rmw->fs_info = fs_info;

  	btrfs_queue_work(fs_info->readahead_workers, &rmw->work);

  }
  
  #ifdef DEBUG
  static void dump_devs(struct btrfs_fs_info *fs_info, int all)
  {
  	struct btrfs_device *device;
  	struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
  	unsigned long index;
  	int ret;
  	int i;
  	int j;
  	int cnt;
  
  	spin_lock(&fs_info->reada_lock);
  	list_for_each_entry(device, &fs_devices->devices, dev_list) {
  		printk(KERN_DEBUG "dev %lld has %d in flight
  ", device->devid,
  			atomic_read(&device->reada_in_flight));
  		index = 0;
  		while (1) {
  			struct reada_zone *zone;
  			ret = radix_tree_gang_lookup(&device->reada_zones,
  						     (void **)&zone, index, 1);
  			if (ret == 0)
  				break;
  			printk(KERN_DEBUG "  zone %llu-%llu elems %llu locked "
  				"%d devs", zone->start, zone->end, zone->elems,
  				zone->locked);
  			for (j = 0; j < zone->ndevs; ++j) {
  				printk(KERN_CONT " %lld",
  					zone->devs[j]->devid);
  			}
  			if (device->reada_curr_zone == zone)
  				printk(KERN_CONT " curr off %llu",
  					device->reada_next - zone->start);
  			printk(KERN_CONT "
  ");
  			index = (zone->end >> PAGE_CACHE_SHIFT) + 1;
  		}
  		cnt = 0;
  		index = 0;
  		while (all) {
  			struct reada_extent *re = NULL;
  
  			ret = radix_tree_gang_lookup(&device->reada_extents,
  						     (void **)&re, index, 1);
  			if (ret == 0)
  				break;
  			printk(KERN_DEBUG
  				"  re: logical %llu size %u empty %d for %lld",

  				re->logical, fs_info->tree_root->nodesize,

  				list_empty(&re->extctl), re->scheduled_for ?
  				re->scheduled_for->devid : -1);
  
  			for (i = 0; i < re->nzones; ++i) {
  				printk(KERN_CONT " zone %llu-%llu devs",
  					re->zones[i]->start,
  					re->zones[i]->end);
  				for (j = 0; j < re->zones[i]->ndevs; ++j) {
  					printk(KERN_CONT " %lld",
  						re->zones[i]->devs[j]->devid);
  				}
  			}
  			printk(KERN_CONT "
  ");
  			index = (re->logical >> PAGE_CACHE_SHIFT) + 1;
  			if (++cnt > 15)
  				break;
  		}
  	}
  
  	index = 0;
  	cnt = 0;
  	while (all) {
  		struct reada_extent *re = NULL;
  
  		ret = radix_tree_gang_lookup(&fs_info->reada_tree, (void **)&re,
  					     index, 1);
  		if (ret == 0)
  			break;
  		if (!re->scheduled_for) {
  			index = (re->logical >> PAGE_CACHE_SHIFT) + 1;
  			continue;
  		}
  		printk(KERN_DEBUG
  			"re: logical %llu size %u list empty %d for %lld",

  			re->logical, fs_info->tree_root->nodesize,
  			list_empty(&re->extctl),

  			re->scheduled_for ? re->scheduled_for->devid : -1);
  		for (i = 0; i < re->nzones; ++i) {
  			printk(KERN_CONT " zone %llu-%llu devs",
  				re->zones[i]->start,
  				re->zones[i]->end);
  			for (i = 0; i < re->nzones; ++i) {
  				printk(KERN_CONT " zone %llu-%llu devs",
  					re->zones[i]->start,
  					re->zones[i]->end);
  				for (j = 0; j < re->zones[i]->ndevs; ++j) {
  					printk(KERN_CONT " %lld",
  						re->zones[i]->devs[j]->devid);
  				}
  			}
  		}
  		printk(KERN_CONT "
  ");
  		index = (re->logical >> PAGE_CACHE_SHIFT) + 1;
  	}
  	spin_unlock(&fs_info->reada_lock);
  }
  #endif
  
  /*
   * interface
   */
  struct reada_control *btrfs_reada_add(struct btrfs_root *root,
  			struct btrfs_key *key_start, struct btrfs_key *key_end)
  {
  	struct reada_control *rc;
  	u64 start;
  	u64 generation;
  	int level;

  	int ret;

  	struct extent_buffer *node;
  	static struct btrfs_key max_key = {
  		.objectid = (u64)-1,
  		.type = (u8)-1,
  		.offset = (u64)-1
  	};
  
  	rc = kzalloc(sizeof(*rc), GFP_NOFS);
  	if (!rc)
  		return ERR_PTR(-ENOMEM);
  
  	rc->root = root;
  	rc->key_start = *key_start;
  	rc->key_end = *key_end;
  	atomic_set(&rc->elems, 0);
  	init_waitqueue_head(&rc->wait);
  	kref_init(&rc->refcnt);
  	kref_get(&rc->refcnt); /* one ref for having elements */
  
  	node = btrfs_root_node(root);
  	start = node->start;
  	level = btrfs_header_level(node);
  	generation = btrfs_header_generation(node);
  	free_extent_buffer(node);

  	ret = reada_add_block(rc, start, &max_key, level, generation);
  	if (ret) {

  		kfree(rc);

  		return ERR_PTR(ret);

  	}

  
  	reada_start_machine(root->fs_info);
  
  	return rc;
  }
  
  #ifdef DEBUG
  int btrfs_reada_wait(void *handle)
  {
  	struct reada_control *rc = handle;
  
  	while (atomic_read(&rc->elems)) {
  		wait_event_timeout(rc->wait, atomic_read(&rc->elems) == 0,
  				   5 * HZ);

  		dump_devs(rc->root->fs_info,
  			  atomic_read(&rc->elems) < 10 ? 1 : 0);

  	}

  	dump_devs(rc->root->fs_info, atomic_read(&rc->elems) < 10 ? 1 : 0);

  
  	kref_put(&rc->refcnt, reada_control_release);
  
  	return 0;
  }
  #else
  int btrfs_reada_wait(void *handle)
  {
  	struct reada_control *rc = handle;
  
  	while (atomic_read(&rc->elems)) {
  		wait_event(rc->wait, atomic_read(&rc->elems) == 0);
  	}
  
  	kref_put(&rc->refcnt, reada_control_release);
  
  	return 0;
  }
  #endif
  
  void btrfs_reada_detach(void *handle)
  {
  	struct reada_control *rc = handle;
  
  	kref_put(&rc->refcnt, reada_control_release);
  }