RDMA/mlx5: Use an xarray for the children of an implicit ODP

Currently the child leaves are stored in the shared interval tree and
every lookup for a child must be done under the interval tree rwsem.

This is further complicated by dropping the rwsem during iteration (ie the
odp_lookup(), odp_next() pattern), which requires a very tricky an
difficult to understand locking scheme with SRCU.

Instead reserve the interval tree for the exclusive use of the mmu
notifier related code in umem_odp.c and give each implicit MR a xarray
containing all the child MRs.

Since the size of each child is 1GB of VA, a 1 level xarray will index 64G
of VA, and a 2 level will index 2TB, making xarray a much better
data structure choice than an interval tree.

The locking properties of xarray will be used in the next patches to
rework the implicit ODP locking scheme into something simpler.

At this point, the xarray is locked by the implicit MR's umem_mutex, and
read can also be locked by the odp_srcu.

Link: https://lore.kernel.org/r/20191009160934.3143-10-jgg@ziepe.caReviewed-by: Artemy Kovalyov <artemyko@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/hw/mlx5/mlx5_ib.h

@@ -617,10 +617,13 @@ struct mlx5_ib_mr {
 	u64			data_iova;
 	u64			pi_iova;
 
+	/* For ODP and implicit */
 	atomic_t		num_leaf_free;
 	wait_queue_head_t       q_leaf_free;
-	struct mlx5_async_work  cb_work;
 	atomic_t		num_pending_prefetch;
+	struct xarray		implicit_children;
+
+	struct mlx5_async_work  cb_work;
 };
 
 static inline bool is_odp_mr(struct mlx5_ib_mr *mr)

include/rdma/ib_umem_odp.h

@@ -156,22 +156,6 @@ int rbt_ib_umem_for_each_in_range(struct rb_root_cached *root,
 				  umem_call_back cb,
 				  bool blockable, void *cookie);
 
-/*
- * Find first region intersecting with address range.
- * Return NULL if not found
- */
-static inline struct ib_umem_odp *
-rbt_ib_umem_lookup(struct rb_root_cached *root, u64 addr, u64 length)
-{
-	struct interval_tree_node *node;
-
-	node = interval_tree_iter_first(root, addr, addr + length - 1);
-	if (!node)
-		return NULL;
-	return container_of(node, struct ib_umem_odp, interval_tree);
-
-}
-
 static inline int ib_umem_mmu_notifier_retry(struct ib_umem_odp *umem_odp,
 					     unsigned long mmu_seq)
 {