diff --git a/fs/xfs/Makefile b/fs/xfs/Makefile
index 3542d94..01857b0 100644
--- a/fs/xfs/Makefile
+++ b/fs/xfs/Makefile
@@ -39,6 +39,7 @@ xfs-y				+= $(addprefix libxfs/, \
 				   xfs_btree.o \
 				   xfs_da_btree.o \
 				   xfs_da_format.o \
+				   xfs_defer.o \
 				   xfs_dir2.o \
 				   xfs_dir2_block.o \
 				   xfs_dir2_data.o \
diff --git a/fs/xfs/libxfs/xfs_defer.c b/fs/xfs/libxfs/xfs_defer.c
new file mode 100644
index 0000000..2809db7
--- /dev/null
+++ b/fs/xfs/libxfs/xfs_defer.c
@@ -0,0 +1,444 @@
+/*
+ * Copyright (C) 2016 Oracle.  All Rights Reserved.
+ *
+ * Author: Darrick J. Wong <darrick.wong@oracle.com>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it would 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 the Free Software Foundation,
+ * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301, USA.
+ */
+#include "xfs.h"
+#include "xfs_fs.h"
+#include "xfs_shared.h"
+#include "xfs_format.h"
+#include "xfs_log_format.h"
+#include "xfs_trans_resv.h"
+#include "xfs_bit.h"
+#include "xfs_sb.h"
+#include "xfs_mount.h"
+#include "xfs_defer.h"
+#include "xfs_trans.h"
+#include "xfs_trace.h"
+
+/*
+ * Deferred Operations in XFS
+ *
+ * Due to the way locking rules work in XFS, certain transactions (block
+ * mapping and unmapping, typically) have permanent reservations so that
+ * we can roll the transaction to adhere to AG locking order rules and
+ * to unlock buffers between metadata updates.  Prior to rmap/reflink,
+ * the mapping code had a mechanism to perform these deferrals for
+ * extents that were going to be freed; this code makes that facility
+ * more generic.
+ *
+ * When adding the reverse mapping and reflink features, it became
+ * necessary to perform complex remapping multi-transactions to comply
+ * with AG locking order rules, and to be able to spread a single
+ * refcount update operation (an operation on an n-block extent can
+ * update as many as n records!) among multiple transactions.  XFS can
+ * roll a transaction to facilitate this, but using this facility
+ * requires us to log "intent" items in case log recovery needs to
+ * redo the operation, and to log "done" items to indicate that redo
+ * is not necessary.
+ *
+ * Deferred work is tracked in xfs_defer_pending items.  Each pending
+ * item tracks one type of deferred work.  Incoming work items (which
+ * have not yet had an intent logged) are attached to a pending item
+ * on the dop_intake list, where they wait for the caller to finish
+ * the deferred operations.
+ *
+ * Finishing a set of deferred operations is an involved process.  To
+ * start, we define "rolling a deferred-op transaction" as follows:
+ *
+ * > For each xfs_defer_pending item on the dop_intake list,
+ *   - Sort the work items in AG order.  XFS locking
+ *     order rules require us to lock buffers in AG order.
+ *   - Create a log intent item for that type.
+ *   - Attach it to the pending item.
+ *   - Move the pending item from the dop_intake list to the
+ *     dop_pending list.
+ * > Roll the transaction.
+ *
+ * NOTE: To avoid exceeding the transaction reservation, we limit the
+ * number of items that we attach to a given xfs_defer_pending.
+ *
+ * The actual finishing process looks like this:
+ *
+ * > For each xfs_defer_pending in the dop_pending list,
+ *   - Roll the deferred-op transaction as above.
+ *   - Create a log done item for that type, and attach it to the
+ *     log intent item.
+ *   - For each work item attached to the log intent item,
+ *     * Perform the described action.
+ *     * Attach the work item to the log done item.
+ *
+ * The key here is that we must log an intent item for all pending
+ * work items every time we roll the transaction, and that we must log
+ * a done item as soon as the work is completed.  With this mechanism
+ * we can perform complex remapping operations, chaining intent items
+ * as needed.
+ *
+ * This is an example of remapping the extent (E, E+B) into file X at
+ * offset A and dealing with the extent (C, C+B) already being mapped
+ * there:
+ * +-------------------------------------------------+
+ * | Unmap file X startblock C offset A length B     | t0
+ * | Intent to reduce refcount for extent (C, B)     |
+ * | Intent to remove rmap (X, C, A, B)              |
+ * | Intent to free extent (D, 1) (bmbt block)       |
+ * | Intent to map (X, A, B) at startblock E         |
+ * +-------------------------------------------------+
+ * | Map file X startblock E offset A length B       | t1
+ * | Done mapping (X, E, A, B)                       |
+ * | Intent to increase refcount for extent (E, B)   |
+ * | Intent to add rmap (X, E, A, B)                 |
+ * +-------------------------------------------------+
+ * | Reduce refcount for extent (C, B)               | t2
+ * | Done reducing refcount for extent (C, B)        |
+ * | Increase refcount for extent (E, B)             |
+ * | Done increasing refcount for extent (E, B)      |
+ * | Intent to free extent (C, B)                    |
+ * | Intent to free extent (F, 1) (refcountbt block) |
+ * | Intent to remove rmap (F, 1, REFC)              |
+ * +-------------------------------------------------+
+ * | Remove rmap (X, C, A, B)                        | t3
+ * | Done removing rmap (X, C, A, B)                 |
+ * | Add rmap (X, E, A, B)                           |
+ * | Done adding rmap (X, E, A, B)                   |
+ * | Remove rmap (F, 1, REFC)                        |
+ * | Done removing rmap (F, 1, REFC)                 |
+ * +-------------------------------------------------+
+ * | Free extent (C, B)                              | t4
+ * | Done freeing extent (C, B)                      |
+ * | Free extent (D, 1)                              |
+ * | Done freeing extent (D, 1)                      |
+ * | Free extent (F, 1)                              |
+ * | Done freeing extent (F, 1)                      |
+ * +-------------------------------------------------+
+ *
+ * If we should crash before t2 commits, log recovery replays
+ * the following intent items:
+ *
+ * - Intent to reduce refcount for extent (C, B)
+ * - Intent to remove rmap (X, C, A, B)
+ * - Intent to free extent (D, 1) (bmbt block)
+ * - Intent to increase refcount for extent (E, B)
+ * - Intent to add rmap (X, E, A, B)
+ *
+ * In the process of recovering, it should also generate and take care
+ * of these intent items:
+ *
+ * - Intent to free extent (C, B)
+ * - Intent to free extent (F, 1) (refcountbt block)
+ * - Intent to remove rmap (F, 1, REFC)
+ */
+
+static const struct xfs_defer_op_type *defer_op_types[XFS_DEFER_OPS_TYPE_MAX];
+
+/*
+ * For each pending item in the intake list, log its intent item and the
+ * associated extents, then add the entire intake list to the end of
+ * the pending list.
+ */
+STATIC void
+xfs_defer_intake_work(
+	struct xfs_trans		*tp,
+	struct xfs_defer_ops		*dop)
+{
+	struct list_head		*li;
+	struct xfs_defer_pending	*dfp;
+
+	list_for_each_entry(dfp, &dop->dop_intake, dfp_list) {
+		dfp->dfp_intent = dfp->dfp_type->create_intent(tp,
+				dfp->dfp_count);
+		list_sort(tp->t_mountp, &dfp->dfp_work,
+				dfp->dfp_type->diff_items);
+		list_for_each(li, &dfp->dfp_work)
+			dfp->dfp_type->log_item(tp, dfp->dfp_intent, li);
+	}
+
+	list_splice_tail_init(&dop->dop_intake, &dop->dop_pending);
+}
+
+/* Abort all the intents that were committed. */
+STATIC void
+xfs_defer_trans_abort(
+	struct xfs_trans		*tp,
+	struct xfs_defer_ops		*dop,
+	int				error)
+{
+	struct xfs_defer_pending	*dfp;
+
+	/*
+	 * If the transaction was committed, drop the intent reference
+	 * since we're bailing out of here. The other reference is
+	 * dropped when the intent hits the AIL.  If the transaction
+	 * was not committed, the intent is freed by the intent item
+	 * unlock handler on abort.
+	 */
+	if (!dop->dop_committed)
+		return;
+
+	/* Abort intent items. */
+	list_for_each_entry(dfp, &dop->dop_pending, dfp_list) {
+		if (dfp->dfp_committed)
+			dfp->dfp_type->abort_intent(dfp->dfp_intent);
+	}
+
+	/* Shut down FS. */
+	xfs_force_shutdown(tp->t_mountp, (error == -EFSCORRUPTED) ?
+			SHUTDOWN_CORRUPT_INCORE : SHUTDOWN_META_IO_ERROR);
+}
+
+/* Roll a transaction so we can do some deferred op processing. */
+STATIC int
+xfs_defer_trans_roll(
+	struct xfs_trans		**tp,
+	struct xfs_defer_ops		*dop,
+	struct xfs_inode		*ip)
+{
+	int				i;
+	int				error;
+
+	/* Log all the joined inodes except the one we passed in. */
+	for (i = 0; i < XFS_DEFER_OPS_NR_INODES && dop->dop_inodes[i]; i++) {
+		if (dop->dop_inodes[i] == ip)
+			continue;
+		xfs_trans_log_inode(*tp, dop->dop_inodes[i], XFS_ILOG_CORE);
+	}
+
+	/* Roll the transaction. */
+	error = xfs_trans_roll(tp, ip);
+	if (error) {
+		xfs_defer_trans_abort(*tp, dop, error);
+		return error;
+	}
+	dop->dop_committed = true;
+
+	/* Rejoin the joined inodes except the one we passed in. */
+	for (i = 0; i < XFS_DEFER_OPS_NR_INODES && dop->dop_inodes[i]; i++) {
+		if (dop->dop_inodes[i] == ip)
+			continue;
+		xfs_trans_ijoin(*tp, dop->dop_inodes[i], 0);
+	}
+
+	return error;
+}
+
+/* Do we have any work items to finish? */
+bool
+xfs_defer_has_unfinished_work(
+	struct xfs_defer_ops		*dop)
+{
+	return !list_empty(&dop->dop_pending) || !list_empty(&dop->dop_intake);
+}
+
+/*
+ * Add this inode to the deferred op.  Each joined inode is relogged
+ * each time we roll the transaction, in addition to any inode passed
+ * to xfs_defer_finish().
+ */
+int
+xfs_defer_join(
+	struct xfs_defer_ops		*dop,
+	struct xfs_inode		*ip)
+{
+	int				i;
+
+	for (i = 0; i < XFS_DEFER_OPS_NR_INODES; i++) {
+		if (dop->dop_inodes[i] == ip)
+			return 0;
+		else if (dop->dop_inodes[i] == NULL) {
+			dop->dop_inodes[i] = ip;
+			return 0;
+		}
+	}
+
+	return -EFSCORRUPTED;
+}
+
+/*
+ * Finish all the pending work.  This involves logging intent items for
+ * any work items that wandered in since the last transaction roll (if
+ * one has even happened), rolling the transaction, and finishing the
+ * work items in the first item on the logged-and-pending list.
+ *
+ * If an inode is provided, relog it to the new transaction.
+ */
+int
+xfs_defer_finish(
+	struct xfs_trans		**tp,
+	struct xfs_defer_ops		*dop,
+	struct xfs_inode		*ip)
+{
+	struct xfs_defer_pending	*dfp;
+	struct list_head		*li;
+	struct list_head		*n;
+	void				*done_item = NULL;
+	void				*state;
+	int				error = 0;
+	void				(*cleanup_fn)(struct xfs_trans *, void *, int);
+
+	ASSERT((*tp)->t_flags & XFS_TRANS_PERM_LOG_RES);
+
+	/* Until we run out of pending work to finish... */
+	while (xfs_defer_has_unfinished_work(dop)) {
+		/* Log intents for work items sitting in the intake. */
+		xfs_defer_intake_work(*tp, dop);
+
+		/* Roll the transaction. */
+		error = xfs_defer_trans_roll(tp, dop, ip);
+		if (error)
+			goto out;
+
+		/* Mark all pending intents as committed. */
+		list_for_each_entry_reverse(dfp, &dop->dop_pending, dfp_list) {
+			if (dfp->dfp_committed)
+				break;
+			dfp->dfp_committed = true;
+		}
+
+		/* Log an intent-done item for the first pending item. */
+		dfp = list_first_entry(&dop->dop_pending,
+				struct xfs_defer_pending, dfp_list);
+		done_item = dfp->dfp_type->create_done(*tp, dfp->dfp_intent,
+				dfp->dfp_count);
+		cleanup_fn = dfp->dfp_type->finish_cleanup;
+
+		/* Finish the work items. */
+		state = NULL;
+		list_for_each_safe(li, n, &dfp->dfp_work) {
+			list_del(li);
+			dfp->dfp_count--;
+			error = dfp->dfp_type->finish_item(*tp, dop, li,
+					done_item, &state);
+			if (error) {
+				/*
+				 * Clean up after ourselves and jump out.
+				 * xfs_defer_cancel will take care of freeing
+				 * all these lists and stuff.
+				 */
+				if (cleanup_fn)
+					cleanup_fn(*tp, state, error);
+				xfs_defer_trans_abort(*tp, dop, error);
+				goto out;
+			}
+		}
+		/* Done with the dfp, free it. */
+		list_del(&dfp->dfp_list);
+		kmem_free(dfp);
+
+		if (cleanup_fn)
+			cleanup_fn(*tp, state, error);
+	}
+
+out:
+	return error;
+}
+
+/*
+ * Free up any items left in the list.
+ */
+void
+xfs_defer_cancel(
+	struct xfs_defer_ops		*dop)
+{
+	struct xfs_defer_pending	*dfp;
+	struct xfs_defer_pending	*pli;
+	struct list_head		*pwi;
+	struct list_head		*n;
+
+	/*
+	 * Free the pending items.  Caller should already have arranged
+	 * for the intent items to be released.
+	 */
+	list_for_each_entry_safe(dfp, pli, &dop->dop_intake, dfp_list) {
+		list_del(&dfp->dfp_list);
+		list_for_each_safe(pwi, n, &dfp->dfp_work) {
+			list_del(pwi);
+			dfp->dfp_count--;
+			dfp->dfp_type->cancel_item(pwi);
+		}
+		ASSERT(dfp->dfp_count == 0);
+		kmem_free(dfp);
+	}
+	list_for_each_entry_safe(dfp, pli, &dop->dop_pending, dfp_list) {
+		list_del(&dfp->dfp_list);
+		list_for_each_safe(pwi, n, &dfp->dfp_work) {
+			list_del(pwi);
+			dfp->dfp_count--;
+			dfp->dfp_type->cancel_item(pwi);
+		}
+		ASSERT(dfp->dfp_count == 0);
+		kmem_free(dfp);
+	}
+}
+
+/* Add an item for later deferred processing. */
+void
+xfs_defer_add(
+	struct xfs_defer_ops		*dop,
+	enum xfs_defer_ops_type		type,
+	struct list_head		*li)
+{
+	struct xfs_defer_pending	*dfp = NULL;
+
+	/*
+	 * Add the item to a pending item at the end of the intake list.
+	 * If the last pending item has the same type, reuse it.  Else,
+	 * create a new pending item at the end of the intake list.
+	 */
+	if (!list_empty(&dop->dop_intake)) {
+		dfp = list_last_entry(&dop->dop_intake,
+				struct xfs_defer_pending, dfp_list);
+		if (dfp->dfp_type->type != type ||
+		    (dfp->dfp_type->max_items &&
+		     dfp->dfp_count >= dfp->dfp_type->max_items))
+			dfp = NULL;
+	}
+	if (!dfp) {
+		dfp = kmem_alloc(sizeof(struct xfs_defer_pending),
+				KM_SLEEP | KM_NOFS);
+		dfp->dfp_type = defer_op_types[type];
+		dfp->dfp_committed = false;
+		dfp->dfp_intent = NULL;
+		dfp->dfp_count = 0;
+		INIT_LIST_HEAD(&dfp->dfp_work);
+		list_add_tail(&dfp->dfp_list, &dop->dop_intake);
+	}
+
+	list_add_tail(li, &dfp->dfp_work);
+	dfp->dfp_count++;
+}
+
+/* Initialize a deferred operation list. */
+void
+xfs_defer_init_op_type(
+	const struct xfs_defer_op_type	*type)
+{
+	defer_op_types[type->type] = type;
+}
+
+/* Initialize a deferred operation. */
+void
+xfs_defer_init(
+	struct xfs_defer_ops		*dop,
+	xfs_fsblock_t			*fbp)
+{
+	dop->dop_committed = false;
+	dop->dop_low = false;
+	memset(&dop->dop_inodes, 0, sizeof(dop->dop_inodes));
+	*fbp = NULLFSBLOCK;
+	INIT_LIST_HEAD(&dop->dop_intake);
+	INIT_LIST_HEAD(&dop->dop_pending);
+}
diff --git a/fs/xfs/libxfs/xfs_defer.h b/fs/xfs/libxfs/xfs_defer.h
new file mode 100644
index 0000000..a227bd2
--- /dev/null
+++ b/fs/xfs/libxfs/xfs_defer.h
@@ -0,0 +1,95 @@
+/*
+ * Copyright (C) 2016 Oracle.  All Rights Reserved.
+ *
+ * Author: Darrick J. Wong <darrick.wong@oracle.com>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it would 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 the Free Software Foundation,
+ * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301, USA.
+ */
+#ifndef __XFS_DEFER_H__
+#define	__XFS_DEFER_H__
+
+struct xfs_defer_op_type;
+
+/*
+ * Save a log intent item and a list of extents, so that we can replay
+ * whatever action had to happen to the extent list and file the log done
+ * item.
+ */
+struct xfs_defer_pending {
+	const struct xfs_defer_op_type	*dfp_type;	/* function pointers */
+	struct list_head		dfp_list;	/* pending items */
+	bool				dfp_committed;	/* committed trans? */
+	void				*dfp_intent;	/* log intent item */
+	struct list_head		dfp_work;	/* work items */
+	unsigned int			dfp_count;	/* # extent items */
+};
+
+/*
+ * Header for deferred operation list.
+ *
+ * dop_low is used by the allocator to activate the lowspace algorithm -
+ * when free space is running low the extent allocator may choose to
+ * allocate an extent from an AG without leaving sufficient space for
+ * a btree split when inserting the new extent.  In this case the allocator
+ * will enable the lowspace algorithm which is supposed to allow further
+ * allocations (such as btree splits and newroots) to allocate from
+ * sequential AGs.  In order to avoid locking AGs out of order the lowspace
+ * algorithm will start searching for free space from AG 0.  If the correct
+ * transaction reservations have been made then this algorithm will eventually
+ * find all the space it needs.
+ */
+enum xfs_defer_ops_type {
+	XFS_DEFER_OPS_TYPE_MAX,
+};
+
+#define XFS_DEFER_OPS_NR_INODES	2	/* join up to two inodes */
+
+struct xfs_defer_ops {
+	bool			dop_committed;	/* did any trans commit? */
+	bool			dop_low;	/* alloc in low mode */
+	struct list_head	dop_intake;	/* unlogged pending work */
+	struct list_head	dop_pending;	/* logged pending work */
+
+	/* relog these inodes with each roll */
+	struct xfs_inode	*dop_inodes[XFS_DEFER_OPS_NR_INODES];
+};
+
+void xfs_defer_add(struct xfs_defer_ops *dop, enum xfs_defer_ops_type type,
+		struct list_head *h);
+int xfs_defer_finish(struct xfs_trans **tp, struct xfs_defer_ops *dop,
+		struct xfs_inode *ip);
+void xfs_defer_cancel(struct xfs_defer_ops *dop);
+void xfs_defer_init(struct xfs_defer_ops *dop, xfs_fsblock_t *fbp);
+bool xfs_defer_has_unfinished_work(struct xfs_defer_ops *dop);
+int xfs_defer_join(struct xfs_defer_ops *dop, struct xfs_inode *ip);
+
+/* Description of a deferred type. */
+struct xfs_defer_op_type {
+	enum xfs_defer_ops_type	type;
+	unsigned int		max_items;
+	void (*abort_intent)(void *);
+	void *(*create_done)(struct xfs_trans *, void *, unsigned int);
+	int (*finish_item)(struct xfs_trans *, struct xfs_defer_ops *,
+			struct list_head *, void *, void **);
+	void (*finish_cleanup)(struct xfs_trans *, void *, int);
+	void (*cancel_item)(struct list_head *);
+	int (*diff_items)(void *, struct list_head *, struct list_head *);
+	void *(*create_intent)(struct xfs_trans *, uint);
+	void (*log_item)(struct xfs_trans *, void *, struct list_head *);
+};
+
+void xfs_defer_init_op_type(const struct xfs_defer_op_type *type);
+
+#endif /* __XFS_DEFER_H__ */
