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, 182 insertions, 0 deletions

diff --git a/fs/btrfs/delayed-ref.h b/fs/btrfs/delayed-ref.h
new file mode 100644
index 000000000000..37919e5c007f
--- /dev/null
+++ b/fs/btrfs/delayed-ref.h
@@ -0,0 +1,182 @@
+/*
+ * Copyright (C) 2008 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.
+ */
+#ifndef __DELAYED_REF__
+#define __DELAYED_REF__
+
+/* these are the possible values of struct btrfs_delayed_ref->action */
+#define BTRFS_ADD_DELAYED_REF    1 /* add one backref to the tree */
+#define BTRFS_DROP_DELAYED_REF   2 /* delete one backref from the tree */
+#define BTRFS_ADD_DELAYED_EXTENT 3 /* record a full extent allocation */
+
+struct btrfs_delayed_ref_node {
+        struct rb_node rb_node;
+
+        /* the starting bytenr of the extent */
+        u64 bytenr;
+
+        /* the parent our backref will point to */
+        u64 parent;
+
+        /* the size of the extent */
+        u64 num_bytes;
+
+        /* ref count on this data structure */
+        atomic_t refs;
+
+        /*
+         * how many refs is this entry adding or deleting.  For
+         * head refs, this may be a negative number because it is keeping
+         * track of the total mods done to the reference count.
+         * For individual refs, this will always be a positive number
+         *
+         * It may be more than one, since it is possible for a single
+         * parent to have more than one ref on an extent
+         */
+        int ref_mod;
+
+        /* is this node still in the rbtree? */
+        unsigned int in_tree:1;
+};
+
+/*
+ * the head refs are used to hold a lock on a given extent, which allows us
+ * to make sure that only one process is running the delayed refs
+ * at a time for a single extent.  They also store the sum of all the
+ * reference count modifications we've queued up.
+ */
+struct btrfs_delayed_ref_head {
+        struct btrfs_delayed_ref_node node;
+
+        /*
+         * the mutex is held while running the refs, and it is also
+         * held when checking the sum of reference modifications.
+         */
+        struct mutex mutex;
+
+        /*
+         * when a new extent is allocated, it is just reserved in memory
+         * The actual extent isn't inserted into the extent allocation tree
+         * until the delayed ref is processed.  must_insert_reserved is
+         * used to flag a delayed ref so the accounting can be updated
+         * when a full insert is done.
+         *
+         * It is possible the extent will be freed before it is ever
+         * inserted into the extent allocation tree.  In this case
+         * we need to update the in ram accounting to properly reflect
+         * the free has happened.
+         */
+        unsigned int must_insert_reserved:1;
+};
+
+struct btrfs_delayed_ref {
+        struct btrfs_delayed_ref_node node;
+
+        /* the root objectid our ref will point to */
+        u64 root;
+
+        /* the generation for the backref */
+        u64 generation;
+
+        /* owner_objectid of the backref  */
+        u64 owner_objectid;
+
+        /* operation done by this entry in the rbtree */
+        u8 action;
+
+        /* if pin == 1, when the extent is freed it will be pinned until
+         * transaction commit
+         */
+        unsigned int pin:1;
+};
+
+struct btrfs_delayed_ref_root {
+        struct rb_root root;
+
+        /* this spin lock protects the rbtree and the entries inside */
+        spinlock_t lock;
+
+        /* how many delayed ref updates we've queued, used by the
+         * throttling code
+         */
+        unsigned long num_entries;
+
+        /*
+         * set when the tree is flushing before a transaction commit,
+         * used by the throttling code to decide if new updates need
+         * to be run right away
+         */
+        int flushing;
+};
+
+static inline void btrfs_put_delayed_ref(struct btrfs_delayed_ref_node *ref)
+{
+        WARN_ON(atomic_read(&ref->refs) == 0);
+        if (atomic_dec_and_test(&ref->refs)) {
+                WARN_ON(ref->in_tree);
+                kfree(ref);
+        }
+}
+
+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 *
+btrfs_find_delayed_ref(struct btrfs_trans_handle *trans, u64 bytenr,
+                       u64 parent);
+int btrfs_delayed_ref_pending(struct btrfs_trans_handle *trans, u64 bytenr);
+int btrfs_lock_delayed_ref(struct btrfs_trans_handle *trans,
+                           struct btrfs_delayed_ref_node *ref,
+                           struct btrfs_delayed_ref_head **next_ret);
+int btrfs_lookup_extent_ref(struct btrfs_trans_handle *trans,
+                            struct btrfs_root *root, u64 bytenr,
+                            u64 num_bytes, u32 *refs);
+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);
+/*
+ * a node might live in a head or a regular ref, this lets you
+ * test for the proper type to use.
+ */
+static int btrfs_delayed_ref_is_head(struct btrfs_delayed_ref_node *node)
+{
+        return node->parent == (u64)-1;
+}
+
+/*
+ * helper functions to cast a node into its container
+ */
+static inline struct btrfs_delayed_ref *
+btrfs_delayed_node_to_ref(struct btrfs_delayed_ref_node *node)
+{
+        WARN_ON(btrfs_delayed_ref_is_head(node));
+        return container_of(node, struct btrfs_delayed_ref, node);
+
+}
+
+static inline struct btrfs_delayed_ref_head *
+btrfs_delayed_node_to_head(struct btrfs_delayed_ref_node *node)
+{
+        WARN_ON(!btrfs_delayed_ref_is_head(node));
+        return container_of(node, struct btrfs_delayed_ref_head, node);
+
+}
+#endif