Merge branch 'for-chris' of git://github.com/sensille/linux into integration

Conflicts:
	fs/btrfs/ctree.h

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 949 insertions, 0 deletions

diff --git a/fs/btrfs/reada.c b/fs/btrfs/reada.c
new file mode 100644
index 000000000000..2b701d082227
--- /dev/null
+++ b/fs/btrfs/reada.c
@@ -0,0 +1,949 @@
+/*
+ * 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_MIRRORS 2
+#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;
+        struct kref             refcnt;
+        spinlock_t              lock;
+        struct reada_zone       *zones[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[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)
+                kref_get(&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_multi_bio *multi)
+{
+        int ret;
+        int looped = 0;
+        struct reada_zone *zone;
+        struct btrfs_block_group_cache *cache = NULL;
+        u64 start;
+        u64 end;
+        int i;
+
+again:
+        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);
+        }
+
+        if (looped)
+                return NULL;
+
+        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 < multi->num_stripes; ++i) {
+                /* bounds have already been checked */
+                zone->devs[i] = multi->stripes[i].dev;
+        }
+        zone->ndevs = multi->num_stripes;
+
+        spin_lock(&fs_info->reada_lock);
+        ret = radix_tree_insert(&dev->reada_zones,
+                                (unsigned long)zone->end >> PAGE_CACHE_SHIFT,
+                                zone);
+        spin_unlock(&fs_info->reada_lock);
+
+        if (ret) {
+                kfree(zone);
+                looped = 1;
+                goto again;
+        }
+
+        return zone;
+}
+
+static struct reada_extent *reada_find_extent(struct btrfs_root *root,
+                                              u64 logical,
+                                              struct btrfs_key *top, int level)
+{
+        int ret;
+        int looped = 0;
+        struct reada_extent *re = NULL;
+        struct btrfs_fs_info *fs_info = root->fs_info;
+        struct btrfs_mapping_tree *map_tree = &fs_info->mapping_tree;
+        struct btrfs_multi_bio *multi = NULL;
+        struct btrfs_device *dev;
+        u32 blocksize;
+        u64 length;
+        int nzones = 0;
+        int i;
+        unsigned long index = logical >> PAGE_CACHE_SHIFT;
+
+again:
+        spin_lock(&fs_info->reada_lock);
+        re = radix_tree_lookup(&fs_info->reada_tree, index);
+        if (re)
+                kref_get(&re->refcnt);
+        spin_unlock(&fs_info->reada_lock);
+
+        if (re || looped)
+                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);
+        kref_init(&re->refcnt);
+
+        /*
+         * map block
+         */
+        length = blocksize;
+        ret = btrfs_map_block(map_tree, REQ_WRITE, logical, &length, &multi, 0);
+        if (ret || !multi || length < blocksize)
+                goto error;
+
+        if (multi->num_stripes > MAX_MIRRORS) {
+                printk(KERN_ERR "btrfs readahead: more than %d copies not "
+                                "supported", MAX_MIRRORS);
+                goto error;
+        }
+
+        for (nzones = 0; nzones < multi->num_stripes; ++nzones) {
+                struct reada_zone *zone;
+
+                dev = multi->stripes[nzones].dev;
+                zone = reada_find_zone(fs_info, dev, logical, multi);
+                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) {
+                spin_unlock(&fs_info->reada_lock);
+                if (ret != -ENOMEM) {
+                        /* someone inserted the extent in the meantime */
+                        looped = 1;
+                }
+                goto error;
+        }
+        for (i = 0; i < nzones; ++i) {
+                dev = multi->stripes[i].dev;
+                ret = radix_tree_insert(&dev->reada_extents, index, re);
+                if (ret) {
+                        while (--i >= 0) {
+                                dev = multi->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);
+
+        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(re);
+        if (looped)
+                goto again;
+        return NULL;
+}
+
+static void reada_kref_dummy(struct kref *kr)
+{
+}
+
+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 (!kref_put(&re->refcnt, reada_kref_dummy)) {
+                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;
+        kref_get(&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;
+
+        rmw = container_of(work, struct reada_machine_work, work);
+        fs_info = rmw->fs_info;
+
+        kfree(rmw);
+
+        __reada_start_machine(fs_info);
+}
+
+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);
+}