Btrfs: do extent allocation and reference count updates in the background

The extent allocation tree maintains a reference count and full
back reference information for every extent allocated in the
filesystem.  For subvolume and snapshot trees, every time
a block goes through COW, the new copy of the block adds a reference
on every block it points to.

If a btree node points to 150 leaves, then the COW code needs to go
and add backrefs on 150 different extents, which might be spread all
over the extent allocation tree.

These updates currently happen during btrfs_cow_block, and most COWs
happen during btrfs_search_slot.  btrfs_search_slot has locks held
on both the parent and the node we are COWing, and so we really want
to avoid IO during the COW if we can.

This commit adds an rbtree of pending reference count updates and extent
allocations.  The tree is ordered by byte number of the extent and byte number
of the parent for the back reference.  The tree allows us to:

1) Modify back references in something close to disk order, reducing seeks
2) Significantly reduce the number of modifications made as block pointers
are balanced around
3) Do all of the extent insertion and back reference modifications outside
of the performance critical btrfs_search_slot code.

#3 has the added benefit of greatly reducing the btrfs stack footprint.
The extent allocation tree modifications are done without the deep
(and somewhat recursive) call chains used in the past.

These delayed back reference updates must be done before the transaction
commits, and so the rbtree is tied to the transaction.  Throttling is
implemented to help keep the queue of backrefs at a reasonable size.

Since there was a similar mechanism in place for the extent tree
extents, that is removed and replaced by the delayed reference tree.

Yan Zheng <yan.zheng@oracle.com> helped review and fixup this code.

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

1 files changed, 585 insertions, 0 deletions

diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c
new file mode 100644
index 000000000000..874565a1f634
--- /dev/null
+++ b/fs/btrfs/delayed-ref.c
@@ -0,0 +1,585 @@
+/*
+ * Copyright (C) 2009 Oracle.  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/sort.h>
+#include <linux/ftrace.h>
+#include "ctree.h"
+#include "delayed-ref.h"
+#include "transaction.h"
+
+/*
+ * delayed back reference update tracking.  For subvolume trees
+ * we queue up extent allocations and backref maintenance for
+ * delayed processing.   This avoids deep call chains where we
+ * add extents in the middle of btrfs_search_slot, and it allows
+ * us to buffer up frequently modified backrefs in an rb tree instead
+ * of hammering updates on the extent allocation tree.
+ *
+ * Right now this code is only used for reference counted trees, but
+ * the long term goal is to get rid of the similar code for delayed
+ * extent tree modifications.
+ */
+
+/*
+ * entries in the rb tree are ordered by the byte number of the extent
+ * and by the byte number of the parent block.
+ */
+static int comp_entry(struct btrfs_delayed_ref_node *ref,
+                      u64 bytenr, u64 parent)
+{
+        if (bytenr < ref->bytenr)
+                return -1;
+        if (bytenr > ref->bytenr)
+                return 1;
+        if (parent < ref->parent)
+                return -1;
+        if (parent > ref->parent)
+                return 1;
+        return 0;
+}
+
+/*
+ * insert a new ref into the rbtree.  This returns any existing refs
+ * for the same (bytenr,parent) tuple, or NULL if the new node was properly
+ * inserted.
+ */
+static struct btrfs_delayed_ref_node *tree_insert(struct rb_root *root,
+                                                  u64 bytenr, u64 parent,
+                                                  struct rb_node *node)
+{
+        struct rb_node **p = &root->rb_node;
+        struct rb_node *parent_node = NULL;
+        struct btrfs_delayed_ref_node *entry;
+        int cmp;
+
+        while (*p) {
+                parent_node = *p;
+                entry = rb_entry(parent_node, struct btrfs_delayed_ref_node,
+                                 rb_node);
+
+                cmp = comp_entry(entry, bytenr, parent);
+                if (cmp < 0)
+                        p = &(*p)->rb_left;
+                else if (cmp > 0)
+                        p = &(*p)->rb_right;
+                else
+                        return entry;
+        }
+
+        entry = rb_entry(node, struct btrfs_delayed_ref_node, rb_node);
+        rb_link_node(node, parent_node, p);
+        rb_insert_color(node, root);
+        return NULL;
+}
+
+/*
+ * find an entry based on (bytenr,parent).  This returns the delayed
+ * ref if it was able to find one, or NULL if nothing was in that spot
+ */
+static struct btrfs_delayed_ref_node *tree_search(struct rb_root *root,
+                                                  u64 bytenr, u64 parent)
+{
+        struct rb_node *n = root->rb_node;
+        struct btrfs_delayed_ref_node *entry;
+        int cmp;
+
+        while (n) {
+                entry = rb_entry(n, struct btrfs_delayed_ref_node, rb_node);
+                WARN_ON(!entry->in_tree);
+
+                cmp = comp_entry(entry, bytenr, parent);
+                if (cmp < 0)
+                        n = n->rb_left;
+                else if (cmp > 0)
+                        n = n->rb_right;
+                else
+                        return entry;
+        }
+        return NULL;
+}
+
+/*
+ * Locking on delayed refs is done by taking a lock on the head node,
+ * which has the (impossible) parent id of (u64)-1.  Once a lock is held
+ * on the head node, you're allowed (and required) to process all the
+ * delayed refs for a given byte number in the tree.
+ *
+ * This will walk forward in the rbtree until it finds a head node it
+ * is able to lock.  It might not lock the delayed ref you asked for,
+ * and so it will return the one it did lock in next_ret and return 0.
+ *
+ * If no locks are taken, next_ret is set to null and 1 is returned.  This
+ * means there are no more unlocked head nodes in the rbtree.
+ */
+int btrfs_lock_delayed_ref(struct btrfs_trans_handle *trans,
+                           struct btrfs_delayed_ref_node *ref,
+                           struct btrfs_delayed_ref_head **next_ret)
+{
+        struct rb_node *node;
+        struct btrfs_delayed_ref_head *head;
+        int ret = 0;
+
+        while (1) {
+                if (btrfs_delayed_ref_is_head(ref)) {
+                        head = btrfs_delayed_node_to_head(ref);
+                        if (mutex_trylock(&head->mutex)) {
+                                *next_ret = head;
+                                ret = 0;
+                                break;
+                        }
+                }
+                node = rb_next(&ref->rb_node);
+                if (!node) {
+                        ret = 1;
+                        *next_ret = NULL;
+                        break;
+                }
+                ref = rb_entry(node, struct btrfs_delayed_ref_node, rb_node);
+        }
+        return ret;
+}
+
+/*
+ * This checks to see if there are any delayed refs in the
+ * btree for a given bytenr.  It returns one if it finds any
+ * and zero otherwise.
+ *
+ * If it only finds a head node, it returns 0.
+ *
+ * The idea is to use this when deciding if you can safely delete an
+ * extent from the extent allocation tree.  There may be a pending
+ * ref in the rbtree that adds or removes references, so as long as this
+ * returns one you need to leave the BTRFS_EXTENT_ITEM in the extent
+ * allocation tree.
+ */
+int btrfs_delayed_ref_pending(struct btrfs_trans_handle *trans, u64 bytenr)
+{
+        struct btrfs_delayed_ref_node *ref;
+        struct btrfs_delayed_ref_root *delayed_refs;
+        struct rb_node *prev_node;
+        int ret = 0;
+
+        delayed_refs = &trans->transaction->delayed_refs;
+        spin_lock(&delayed_refs->lock);
+
+        ref = tree_search(&delayed_refs->root, bytenr, (u64)-1);
+        if (ref) {
+                prev_node = rb_prev(&ref->rb_node);
+                if (!prev_node)
+                        goto out;
+                ref = rb_entry(prev_node, struct btrfs_delayed_ref_node,
+                               rb_node);
+                if (ref->bytenr == bytenr)
+                        ret = 1;
+        }
+out:
+        spin_unlock(&delayed_refs->lock);
+        return ret;
+}
+
+/*
+ * helper function to lookup reference count
+ *
+ * the head node for delayed ref is used to store the sum of all the
+ * reference count modifications queued up in the rbtree.  This way you
+ * can check to see what the reference count would be if all of the
+ * delayed refs are processed.
+ */
+int btrfs_lookup_extent_ref(struct btrfs_trans_handle *trans,
+                            struct btrfs_root *root, u64 bytenr,
+                            u64 num_bytes, u32 *refs)
+{
+        struct btrfs_delayed_ref_node *ref;
+        struct btrfs_delayed_ref_head *head;
+        struct btrfs_delayed_ref_root *delayed_refs;
+        struct btrfs_path *path;
+        struct extent_buffer *leaf;
+        struct btrfs_extent_item *ei;
+        struct btrfs_key key;
+        u32 num_refs;
+        int ret;
+
+        path = btrfs_alloc_path();
+        if (!path)
+                return -ENOMEM;
+
+        key.objectid = bytenr;
+        key.type = BTRFS_EXTENT_ITEM_KEY;
+        key.offset = num_bytes;
+        delayed_refs = &trans->transaction->delayed_refs;
+again:
+        ret = btrfs_search_slot(trans, root->fs_info->extent_root,
+                                &key, path, 0, 0);
+        if (ret < 0)
+                goto out;
+
+        if (ret == 0) {
+                leaf = path->nodes[0];
+                ei = btrfs_item_ptr(leaf, path->slots[0],
+                                    struct btrfs_extent_item);
+                num_refs = btrfs_extent_refs(leaf, ei);
+        } else {
+                num_refs = 0;
+                ret = 0;
+        }
+
+        spin_lock(&delayed_refs->lock);
+        ref = tree_search(&delayed_refs->root, bytenr, (u64)-1);
+        if (ref) {
+                head = btrfs_delayed_node_to_head(ref);
+                if (mutex_trylock(&head->mutex)) {
+                        num_refs += ref->ref_mod;
+                        mutex_unlock(&head->mutex);
+                        *refs = num_refs;
+                        goto out;
+                }
+
+                atomic_inc(&ref->refs);
+                spin_unlock(&delayed_refs->lock);
+
+                btrfs_release_path(root->fs_info->extent_root, path);
+
+                mutex_lock(&head->mutex);
+                mutex_unlock(&head->mutex);
+                btrfs_put_delayed_ref(ref);
+                goto again;
+        } else {
+                *refs = num_refs;
+        }
+out:
+        spin_unlock(&delayed_refs->lock);
+        btrfs_free_path(path);
+        return ret;
+}
+
+/*
+ * helper function to update an extent delayed ref in the
+ * rbtree.  existing and update must both have the same
+ * bytenr and parent
+ *
+ * This may free existing if the update cancels out whatever
+ * operation it was doing.
+ */
+static noinline void
+update_existing_ref(struct btrfs_trans_handle *trans,
+                    struct btrfs_delayed_ref_root *delayed_refs,
+                    struct btrfs_delayed_ref_node *existing,
+                    struct btrfs_delayed_ref_node *update)
+{
+        struct btrfs_delayed_ref *existing_ref;
+        struct btrfs_delayed_ref *ref;
+
+        existing_ref = btrfs_delayed_node_to_ref(existing);
+        ref = btrfs_delayed_node_to_ref(update);
+
+        if (ref->pin)
+                existing_ref->pin = 1;
+
+        if (ref->action != existing_ref->action) {
+                /*
+                 * this is effectively undoing either an add or a
+                 * drop.  We decrement the ref_mod, and if it goes
+                 * down to zero we just delete the entry without
+                 * every changing the extent allocation tree.
+                 */
+                existing->ref_mod--;
+                if (existing->ref_mod == 0) {
+                        rb_erase(&existing->rb_node,
+                                 &delayed_refs->root);
+                        existing->in_tree = 0;
+                        btrfs_put_delayed_ref(existing);
+                        delayed_refs->num_entries--;
+                        if (trans->delayed_ref_updates)
+                                trans->delayed_ref_updates--;
+                }
+        } else {
+                if (existing_ref->action == BTRFS_ADD_DELAYED_REF) {
+                        /* if we're adding refs, make sure all the
+                         * details match up.  The extent could
+                         * have been totally freed and reallocated
+                         * by a different owner before the delayed
+                         * ref entries were removed.
+                         */
+                        existing_ref->owner_objectid = ref->owner_objectid;
+                        existing_ref->generation = ref->generation;
+                        existing_ref->root = ref->root;
+                        existing->num_bytes = update->num_bytes;
+                }
+                /*
+                 * the action on the existing ref matches
+                 * the action on the ref we're trying to add.
+                 * Bump the ref_mod by one so the backref that
+                 * is eventually added/removed has the correct
+                 * reference count
+                 */
+                existing->ref_mod += update->ref_mod;
+        }
+}
+
+/*
+ * helper function to update the accounting in the head ref
+ * existing and update must have the same bytenr
+ */
+static noinline void
+update_existing_head_ref(struct btrfs_delayed_ref_node *existing,
+                         struct btrfs_delayed_ref_node *update)
+{
+        struct btrfs_delayed_ref_head *existing_ref;
+        struct btrfs_delayed_ref_head *ref;
+
+        existing_ref = btrfs_delayed_node_to_head(existing);
+        ref = btrfs_delayed_node_to_head(update);
+
+        if (ref->must_insert_reserved) {
+                /* if the extent was freed and then
+                 * reallocated before the delayed ref
+                 * entries were processed, we can end up
+                 * with an existing head ref without
+                 * the must_insert_reserved flag set.
+                 * Set it again here
+                 */
+                existing_ref->must_insert_reserved = ref->must_insert_reserved;
+
+                /*
+                 * update the num_bytes so we make sure the accounting
+                 * is done correctly
+                 */
+                existing->num_bytes = update->num_bytes;
+
+        }
+
+        /*
+         * update the reference mod on the head to reflect this new operation
+         */
+        existing->ref_mod += update->ref_mod;
+}
+
+/*
+ * helper function to actually insert a delayed ref into the rbtree.
+ * this does all the dirty work in terms of maintaining the correct
+ * overall modification count in the head node and properly dealing
+ * with updating existing nodes as new modifications are queued.
+ */
+static noinline int __btrfs_add_delayed_ref(struct btrfs_trans_handle *trans,
+                          struct btrfs_delayed_ref_node *ref,
+                          u64 bytenr, u64 num_bytes, u64 parent, u64 ref_root,
+                          u64 ref_generation, u64 owner_objectid, int action,
+                          int pin)
+{
+        struct btrfs_delayed_ref_node *existing;
+        struct btrfs_delayed_ref *full_ref;
+        struct btrfs_delayed_ref_head *head_ref;
+        struct btrfs_delayed_ref_root *delayed_refs;
+        int count_mod = 1;
+        int must_insert_reserved = 0;
+
+        /*
+         * the head node stores the sum of all the mods, so dropping a ref
+         * should drop the sum in the head node by one.
+         */
+        if (parent == (u64)-1 && action == BTRFS_DROP_DELAYED_REF)
+                count_mod = -1;
+
+        /*
+         * BTRFS_ADD_DELAYED_EXTENT means that we need to update
+         * the reserved accounting when the extent is finally added, or
+         * if a later modification deletes the delayed ref without ever
+         * inserting the extent into the extent allocation tree.
+         * ref->must_insert_reserved is the flag used to record
+         * that accounting mods are required.
+         *
+         * Once we record must_insert_reserved, switch the action to
+         * BTRFS_ADD_DELAYED_REF because other special casing is not required.
+         */
+        if (action == BTRFS_ADD_DELAYED_EXTENT) {
+                must_insert_reserved = 1;
+                action = BTRFS_ADD_DELAYED_REF;
+        } else {
+                must_insert_reserved = 0;
+        }
+
+
+        delayed_refs = &trans->transaction->delayed_refs;
+
+        /* first set the basic ref node struct up */
+        atomic_set(&ref->refs, 1);
+        ref->bytenr = bytenr;
+        ref->parent = parent;
+        ref->ref_mod = count_mod;
+        ref->in_tree = 1;
+        ref->num_bytes = num_bytes;
+
+        if (btrfs_delayed_ref_is_head(ref)) {
+                head_ref = btrfs_delayed_node_to_head(ref);
+                head_ref->must_insert_reserved = must_insert_reserved;
+                mutex_init(&head_ref->mutex);
+        } else {
+                full_ref = btrfs_delayed_node_to_ref(ref);
+                full_ref->root = ref_root;
+                full_ref->generation = ref_generation;
+                full_ref->owner_objectid = owner_objectid;
+                full_ref->pin = pin;
+                full_ref->action = action;
+        }
+
+        existing = tree_insert(&delayed_refs->root, bytenr,
+                               parent, &ref->rb_node);
+
+        if (existing) {
+                if (btrfs_delayed_ref_is_head(ref))
+                        update_existing_head_ref(existing, ref);
+                else
+                        update_existing_ref(trans, delayed_refs, existing, ref);
+
+                /*
+                 * we've updated the existing ref, free the newly
+                 * allocated ref
+                 */
+                kfree(ref);
+        } else {
+                delayed_refs->num_entries++;
+                trans->delayed_ref_updates++;
+        }
+        return 0;
+}
+
+/*
+ * add a delayed ref to the tree.  This does all of the accounting required
+ * to make sure the delayed ref is eventually processed before this
+ * transaction commits.
+ */
+int btrfs_add_delayed_ref(struct btrfs_trans_handle *trans,
+                          u64 bytenr, u64 num_bytes, u64 parent, u64 ref_root,
+                          u64 ref_generation, u64 owner_objectid, int action,
+                          int pin)
+{
+        struct btrfs_delayed_ref *ref;
+        struct btrfs_delayed_ref_head *head_ref;
+        struct btrfs_delayed_ref_root *delayed_refs;
+        int ret;
+
+        ref = kmalloc(sizeof(*ref), GFP_NOFS);
+        if (!ref)
+                return -ENOMEM;
+
+        /*
+         * the parent = 0 case comes from cases where we don't actually
+         * know the parent yet.  It will get updated later via a add/drop
+         * pair.
+         */
+        if (parent == 0)
+                parent = bytenr;
+
+        head_ref = kmalloc(sizeof(*head_ref), GFP_NOFS);
+        if (!head_ref) {
+                kfree(ref);
+                return -ENOMEM;
+        }
+        delayed_refs = &trans->transaction->delayed_refs;
+        spin_lock(&delayed_refs->lock);
+
+        /*
+         * insert both the head node and the new ref without dropping
+         * the spin lock
+         */
+        ret = __btrfs_add_delayed_ref(trans, &head_ref->node, bytenr, num_bytes,
+                                      (u64)-1, 0, 0, 0, action, pin);
+        BUG_ON(ret);
+
+        ret = __btrfs_add_delayed_ref(trans, &ref->node, bytenr, num_bytes,
+                                      parent, ref_root, ref_generation,
+                                      owner_objectid, action, pin);
+        BUG_ON(ret);
+        spin_unlock(&delayed_refs->lock);
+        return 0;
+}
+
+/*
+ * add a delayed ref to the tree.  This does all of the accounting required
+ * to make sure the delayed ref is eventually processed before this
+ * transaction commits.
+ *
+ * The main point of this call is to add and remove a backreference in a single
+ * shot, taking the lock only once, and only searching for the head node once.
+ *
+ * It is the same as doing a ref add and delete in two separate calls.
+ */
+int btrfs_update_delayed_ref(struct btrfs_trans_handle *trans,
+                          u64 bytenr, u64 num_bytes, u64 orig_parent,
+                          u64 parent, u64 orig_ref_root, u64 ref_root,
+                          u64 orig_ref_generation, u64 ref_generation,
+                          u64 owner_objectid, int pin)
+{
+        struct btrfs_delayed_ref *ref;
+        struct btrfs_delayed_ref *old_ref;
+        struct btrfs_delayed_ref_head *head_ref;
+        struct btrfs_delayed_ref_root *delayed_refs;
+        int ret;
+
+        ref = kmalloc(sizeof(*ref), GFP_NOFS);
+        if (!ref)
+                return -ENOMEM;
+
+        old_ref = kmalloc(sizeof(*old_ref), GFP_NOFS);
+        if (!old_ref) {
+                kfree(ref);
+                return -ENOMEM;
+        }
+
+        /*
+         * the parent = 0 case comes from cases where we don't actually
+         * know the parent yet.  It will get updated later via a add/drop
+         * pair.
+         */
+        if (parent == 0)
+                parent = bytenr;
+        if (orig_parent == 0)
+                orig_parent = bytenr;
+
+        head_ref = kmalloc(sizeof(*head_ref), GFP_NOFS);
+        if (!head_ref) {
+                kfree(ref);
+                kfree(old_ref);
+                return -ENOMEM;
+        }
+        delayed_refs = &trans->transaction->delayed_refs;
+        spin_lock(&delayed_refs->lock);
+
+        /*
+         * insert both the head node and the new ref without dropping
+         * the spin lock
+         */
+        ret = __btrfs_add_delayed_ref(trans, &head_ref->node, bytenr, num_bytes,
+                                      (u64)-1, 0, 0, 0,
+                                      BTRFS_ADD_DELAYED_REF, 0);
+        BUG_ON(ret);
+
+        ret = __btrfs_add_delayed_ref(trans, &ref->node, bytenr, num_bytes,
+                                      parent, ref_root, ref_generation,
+                                      owner_objectid, BTRFS_ADD_DELAYED_REF, 0);
+        BUG_ON(ret);
+
+        ret = __btrfs_add_delayed_ref(trans, &old_ref->node, bytenr, num_bytes,
+                                      orig_parent, orig_ref_root,
+                                      orig_ref_generation, owner_objectid,
+                                      BTRFS_DROP_DELAYED_REF, pin);
+        BUG_ON(ret);
+        spin_unlock(&delayed_refs->lock);
+        return 0;
+}