reada.c

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/*
 * 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"

#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;
    u32         blocksize;
    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) {
                printk(KERN_DEBUG "generation mismatch for "
                        "(%llu,%d,%llu) %llu != %llu\n",
                       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_mapping_tree *map_tree = &fs_info->mapping_tree;
    struct btrfs_bio *bbio = NULL;
    struct btrfs_device *dev;
    struct btrfs_device *prev_dev;
    u32 blocksize;
    u64 length;
    int nzones = 0;
    int i;
    unsigned long index = logical >> PAGE_CACHE_SHIFT;

    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 = btrfs_level_size(root, level);
    re->logical = logical;
    re->blocksize = blocksize;
    re->top = *top;
    INIT_LIST_HEAD(&re->extctl);
    spin_lock_init(&re->lock);
    re->refcnt = 1;

    /*
     * map block
     */
    length = blocksize;
    ret = btrfs_map_block(map_tree, REQ_WRITE, logical, &length, &bbio, 0);
    if (ret || !bbio || length < blocksize)
        goto error;

    if (bbio->num_stripes > BTRFS_MAX_MIRRORS) {
        printk(KERN_ERR "btrfs readahead: more than %d copies not "
                "supported", BTRFS_MAX_MIRRORS);
        goto error;
    }

    for (nzones = 0; nzones < bbio->num_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 */
    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);
        goto error;
    }
    if (ret) {
        spin_unlock(&fs_info->reada_lock);
        goto error;
    }
    prev_dev = NULL;
    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;
        }
        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);
                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);
            goto error;
        }
    }
    spin_unlock(&fs_info->reada_lock);

    kfree(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);
    }
    kfree(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 -1;
    }

    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;
    u32 blocksize;
    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 + re->blocksize;
    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;
    blocksize = re->blocksize;

    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, blocksize,
             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();
    }
    rmw->work.func = reada_start_machine_worker;
    rmw->fs_info = fs_info;

    btrfs_queue_worker(&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\n", 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 "\n");
            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, re->blocksize,
                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 "\n");
            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, re->blocksize, 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 "\n");
        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;
    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);

    reada_add_block(rc, start, &max_key, level, generation);

    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, rc->elems < 10 ? 1 : 0);
    }

    dump_devs(rc->root->fs_info, 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);
}