RDS: RDMA support

Some transports may support RDMA features. This handles the
non-transport-specific parts, like pinning user pages and
tracking mapped regions.

Signed-off-by: Andy Grover <andy.grover@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 679 insertions, 0 deletions

diff --git a/net/rds/rdma.c b/net/rds/rdma.c
new file mode 100644
index 000000000000..eaeeb91e1119
--- /dev/null
+++ b/net/rds/rdma.c
@@ -0,0 +1,679 @@
+/*
+ * Copyright (c) 2007 Oracle.  All rights reserved.
+ *
+ * This software is available to you under a choice of one of two
+ * licenses.  You may choose to be licensed under the terms of the GNU
+ * General Public License (GPL) Version 2, available from the file
+ * COPYING in the main directory of this source tree, or the
+ * OpenIB.org BSD license below:
+ *
+ *     Redistribution and use in source and binary forms, with or
+ *     without modification, are permitted provided that the following
+ *     conditions are met:
+ *
+ *      - Redistributions of source code must retain the above
+ *        copyright notice, this list of conditions and the following
+ *        disclaimer.
+ *
+ *      - Redistributions in binary form must reproduce the above
+ *        copyright notice, this list of conditions and the following
+ *        disclaimer in the documentation and/or other materials
+ *        provided with the distribution.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+ * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ */
+#include <linux/pagemap.h>
+#include <linux/rbtree.h>
+#include <linux/dma-mapping.h> /* for DMA_*_DEVICE */
+
+#include "rdma.h"
+
+/*
+ * XXX
+ *  - build with sparse
+ *  - should we limit the size of a mr region?  let transport return failure?
+ *  - should we detect duplicate keys on a socket?  hmm.
+ *  - an rdma is an mlock, apply rlimit?
+ */
+
+/*
+ * get the number of pages by looking at the page indices that the start and
+ * end addresses fall in.
+ *
+ * Returns 0 if the vec is invalid.  It is invalid if the number of bytes
+ * causes the address to wrap or overflows an unsigned int.  This comes
+ * from being stored in the 'length' member of 'struct scatterlist'.
+ */
+static unsigned int rds_pages_in_vec(struct rds_iovec *vec)
+{
+	if ((vec->addr + vec->bytes <= vec->addr) ||
+	    (vec->bytes > (u64)UINT_MAX))
+		return 0;
+
+	return ((vec->addr + vec->bytes + PAGE_SIZE - 1) >> PAGE_SHIFT) -
+		(vec->addr >> PAGE_SHIFT);
+}
+
+static struct rds_mr *rds_mr_tree_walk(struct rb_root *root, u64 key,
+				       struct rds_mr *insert)
+{
+	struct rb_node **p = &root->rb_node;
+	struct rb_node *parent = NULL;
+	struct rds_mr *mr;
+
+	while (*p) {
+		parent = *p;
+		mr = rb_entry(parent, struct rds_mr, r_rb_node);
+
+		if (key < mr->r_key)
+			p = &(*p)->rb_left;
+		else if (key > mr->r_key)
+			p = &(*p)->rb_right;
+		else
+			return mr;
+	}
+
+	if (insert) {
+		rb_link_node(&insert->r_rb_node, parent, p);
+		rb_insert_color(&insert->r_rb_node, root);
+		atomic_inc(&insert->r_refcount);
+	}
+	return NULL;
+}
+
+/*
+ * Destroy the transport-specific part of a MR.
+ */
+static void rds_destroy_mr(struct rds_mr *mr)
+{
+	struct rds_sock *rs = mr->r_sock;
+	void *trans_private = NULL;
+	unsigned long flags;
+
+	rdsdebug("RDS: destroy mr key is %x refcnt %u\n",
+			mr->r_key, atomic_read(&mr->r_refcount));
+
+	if (test_and_set_bit(RDS_MR_DEAD, &mr->r_state))
+		return;
+
+	spin_lock_irqsave(&rs->rs_rdma_lock, flags);
+	if (!RB_EMPTY_NODE(&mr->r_rb_node))
+		rb_erase(&mr->r_rb_node, &rs->rs_rdma_keys);
+	trans_private = mr->r_trans_private;
+	mr->r_trans_private = NULL;
+	spin_unlock_irqrestore(&rs->rs_rdma_lock, flags);
+
+	if (trans_private)
+		mr->r_trans->free_mr(trans_private, mr->r_invalidate);
+}
+
+void __rds_put_mr_final(struct rds_mr *mr)
+{
+	rds_destroy_mr(mr);
+	kfree(mr);
+}
+
+/*
+ * By the time this is called we can't have any more ioctls called on
+ * the socket so we don't need to worry about racing with others.
+ */
+void rds_rdma_drop_keys(struct rds_sock *rs)
+{
+	struct rds_mr *mr;
+	struct rb_node *node;
+
+	/* Release any MRs associated with this socket */
+	while ((node = rb_first(&rs->rs_rdma_keys))) {
+		mr = container_of(node, struct rds_mr, r_rb_node);
+		if (mr->r_trans == rs->rs_transport)
+			mr->r_invalidate = 0;
+		rds_mr_put(mr);
+	}
+
+	if (rs->rs_transport && rs->rs_transport->flush_mrs)
+		rs->rs_transport->flush_mrs();
+}
+
+/*
+ * Helper function to pin user pages.
+ */
+static int rds_pin_pages(unsigned long user_addr, unsigned int nr_pages,
+			struct page **pages, int write)
+{
+	int ret;
+
+	down_read(&current->mm->mmap_sem);
+	ret = get_user_pages(current, current->mm, user_addr,
+			     nr_pages, write, 0, pages, NULL);
+	up_read(&current->mm->mmap_sem);
+
+	if (0 <= ret && (unsigned) ret < nr_pages) {
+		while (ret--)
+			put_page(pages[ret]);
+		ret = -EFAULT;
+	}
+
+	return ret;
+}
+
+static int __rds_rdma_map(struct rds_sock *rs, struct rds_get_mr_args *args,
+				u64 *cookie_ret, struct rds_mr **mr_ret)
+{
+	struct rds_mr *mr = NULL, *found;
+	unsigned int nr_pages;
+	struct page **pages = NULL;
+	struct scatterlist *sg;
+	void *trans_private;
+	unsigned long flags;
+	rds_rdma_cookie_t cookie;
+	unsigned int nents;
+	long i;
+	int ret;
+
+	if (rs->rs_bound_addr == 0) {
+		ret = -ENOTCONN; /* XXX not a great errno */
+		goto out;
+	}
+
+	if (rs->rs_transport->get_mr == NULL) {
+		ret = -EOPNOTSUPP;
+		goto out;
+	}
+
+	nr_pages = rds_pages_in_vec(&args->vec);
+	if (nr_pages == 0) {
+		ret = -EINVAL;
+		goto out;
+	}
+
+	rdsdebug("RDS: get_mr addr %llx len %llu nr_pages %u\n",
+		args->vec.addr, args->vec.bytes, nr_pages);
+
+	/* XXX clamp nr_pages to limit the size of this alloc? */
+	pages = kcalloc(nr_pages, sizeof(struct page *), GFP_KERNEL);
+	if (pages == NULL) {
+		ret = -ENOMEM;
+		goto out;
+	}
+
+	mr = kzalloc(sizeof(struct rds_mr), GFP_KERNEL);
+	if (mr == NULL) {
+		ret = -ENOMEM;
+		goto out;
+	}
+
+	atomic_set(&mr->r_refcount, 1);
+	RB_CLEAR_NODE(&mr->r_rb_node);
+	mr->r_trans = rs->rs_transport;
+	mr->r_sock = rs;
+
+	if (args->flags & RDS_RDMA_USE_ONCE)
+		mr->r_use_once = 1;
+	if (args->flags & RDS_RDMA_INVALIDATE)
+		mr->r_invalidate = 1;
+	if (args->flags & RDS_RDMA_READWRITE)
+		mr->r_write = 1;
+
+	/*
+	 * Pin the pages that make up the user buffer and transfer the page
+	 * pointers to the mr's sg array.  We check to see if we've mapped
+	 * the whole region after transferring the partial page references
+	 * to the sg array so that we can have one page ref cleanup path.
+	 *
+	 * For now we have no flag that tells us whether the mapping is
+	 * r/o or r/w. We need to assume r/w, or we'll do a lot of RDMA to
+	 * the zero page.
+	 */
+	ret = rds_pin_pages(args->vec.addr & PAGE_MASK, nr_pages, pages, 1);
+	if (ret < 0)
+		goto out;
+
+	nents = ret;
+	sg = kcalloc(nents, sizeof(*sg), GFP_KERNEL);
+	if (sg == NULL) {
+		ret = -ENOMEM;
+		goto out;
+	}
+	WARN_ON(!nents);
+	sg_init_table(sg, nents);
+
+	/* Stick all pages into the scatterlist */
+	for (i = 0 ; i < nents; i++)
+		sg_set_page(&sg[i], pages[i], PAGE_SIZE, 0);
+
+	rdsdebug("RDS: trans_private nents is %u\n", nents);
+
+	/* Obtain a transport specific MR. If this succeeds, the
+	 * s/g list is now owned by the MR.
+	 * Note that dma_map() implies that pending writes are
+	 * flushed to RAM, so no dma_sync is needed here. */
+	trans_private = rs->rs_transport->get_mr(sg, nents, rs,
+						 &mr->r_key);
+
+	if (IS_ERR(trans_private)) {
+		for (i = 0 ; i < nents; i++)
+			put_page(sg_page(&sg[i]));
+		kfree(sg);
+		ret = PTR_ERR(trans_private);
+		goto out;
+	}
+
+	mr->r_trans_private = trans_private;
+
+	rdsdebug("RDS: get_mr put_user key is %x cookie_addr %p\n",
+	       mr->r_key, (void *)(unsigned long) args->cookie_addr);
+
+	/* The user may pass us an unaligned address, but we can only
+	 * map page aligned regions. So we keep the offset, and build
+	 * a 64bit cookie containing <R_Key, offset> and pass that
+	 * around. */
+	cookie = rds_rdma_make_cookie(mr->r_key, args->vec.addr & ~PAGE_MASK);
+	if (cookie_ret)
+		*cookie_ret = cookie;
+
+	if (args->cookie_addr && put_user(cookie, (u64 __user *)(unsigned long) args->cookie_addr)) {
+		ret = -EFAULT;
+		goto out;
+	}
+
+	/* Inserting the new MR into the rbtree bumps its
+	 * reference count. */
+	spin_lock_irqsave(&rs->rs_rdma_lock, flags);
+	found = rds_mr_tree_walk(&rs->rs_rdma_keys, mr->r_key, mr);
+	spin_unlock_irqrestore(&rs->rs_rdma_lock, flags);
+
+	BUG_ON(found && found != mr);
+
+	rdsdebug("RDS: get_mr key is %x\n", mr->r_key);
+	if (mr_ret) {
+		atomic_inc(&mr->r_refcount);
+		*mr_ret = mr;
+	}
+
+	ret = 0;
+out:
+	kfree(pages);
+	if (mr)
+		rds_mr_put(mr);
+	return ret;
+}
+
+int rds_get_mr(struct rds_sock *rs, char __user *optval, int optlen)
+{
+	struct rds_get_mr_args args;
+
+	if (optlen != sizeof(struct rds_get_mr_args))
+		return -EINVAL;
+
+	if (copy_from_user(&args, (struct rds_get_mr_args __user *)optval,
+			   sizeof(struct rds_get_mr_args)))
+		return -EFAULT;
+
+	return __rds_rdma_map(rs, &args, NULL, NULL);
+}
+
+/*
+ * Free the MR indicated by the given R_Key
+ */
+int rds_free_mr(struct rds_sock *rs, char __user *optval, int optlen)
+{
+	struct rds_free_mr_args args;
+	struct rds_mr *mr;
+	unsigned long flags;
+
+	if (optlen != sizeof(struct rds_free_mr_args))
+		return -EINVAL;
+
+	if (copy_from_user(&args, (struct rds_free_mr_args __user *)optval,
+			   sizeof(struct rds_free_mr_args)))
+		return -EFAULT;
+
+	/* Special case - a null cookie means flush all unused MRs */
+	if (args.cookie == 0) {
+		if (!rs->rs_transport || !rs->rs_transport->flush_mrs)
+			return -EINVAL;
+		rs->rs_transport->flush_mrs();
+		return 0;
+	}
+
+	/* Look up the MR given its R_key and remove it from the rbtree
+	 * so nobody else finds it.
+	 * This should also prevent races with rds_rdma_unuse.
+	 */
+	spin_lock_irqsave(&rs->rs_rdma_lock, flags);
+	mr = rds_mr_tree_walk(&rs->rs_rdma_keys, rds_rdma_cookie_key(args.cookie), NULL);
+	if (mr) {
+		rb_erase(&mr->r_rb_node, &rs->rs_rdma_keys);
+		RB_CLEAR_NODE(&mr->r_rb_node);
+		if (args.flags & RDS_RDMA_INVALIDATE)
+			mr->r_invalidate = 1;
+	}
+	spin_unlock_irqrestore(&rs->rs_rdma_lock, flags);
+
+	if (!mr)
+		return -EINVAL;
+
+	/*
+	 * call rds_destroy_mr() ourselves so that we're sure it's done by the time
+	 * we return.  If we let rds_mr_put() do it it might not happen until
+	 * someone else drops their ref.
+	 */
+	rds_destroy_mr(mr);
+	rds_mr_put(mr);
+	return 0;
+}
+
+/*
+ * This is called when we receive an extension header that
+ * tells us this MR was used. It allows us to implement
+ * use_once semantics
+ */
+void rds_rdma_unuse(struct rds_sock *rs, u32 r_key, int force)
+{
+	struct rds_mr *mr;
+	unsigned long flags;
+	int zot_me = 0;
+
+	spin_lock_irqsave(&rs->rs_rdma_lock, flags);
+	mr = rds_mr_tree_walk(&rs->rs_rdma_keys, r_key, NULL);
+	if (mr && (mr->r_use_once || force)) {
+		rb_erase(&mr->r_rb_node, &rs->rs_rdma_keys);
+		RB_CLEAR_NODE(&mr->r_rb_node);
+		zot_me = 1;
+	} else if (mr)
+		atomic_inc(&mr->r_refcount);
+	spin_unlock_irqrestore(&rs->rs_rdma_lock, flags);
+
+	/* May have to issue a dma_sync on this memory region.
+	 * Note we could avoid this if the operation was a RDMA READ,
+	 * but at this point we can't tell. */
+	if (mr != NULL) {
+		if (mr->r_trans->sync_mr)
+			mr->r_trans->sync_mr(mr->r_trans_private, DMA_FROM_DEVICE);
+
+		/* If the MR was marked as invalidate, this will
+		 * trigger an async flush. */
+		if (zot_me)
+			rds_destroy_mr(mr);
+		rds_mr_put(mr);
+	}
+}
+
+void rds_rdma_free_op(struct rds_rdma_op *ro)
+{
+	unsigned int i;
+
+	for (i = 0; i < ro->r_nents; i++) {
+		struct page *page = sg_page(&ro->r_sg[i]);
+
+		/* Mark page dirty if it was possibly modified, which
+		 * is the case for a RDMA_READ which copies from remote
+		 * to local memory */
+		if (!ro->r_write)
+			set_page_dirty(page);
+		put_page(page);
+	}
+
+	kfree(ro->r_notifier);
+	kfree(ro);
+}
+
+/*
+ * args is a pointer to an in-kernel copy in the sendmsg cmsg.
+ */
+static struct rds_rdma_op *rds_rdma_prepare(struct rds_sock *rs,
+					    struct rds_rdma_args *args)
+{
+	struct rds_iovec vec;
+	struct rds_rdma_op *op = NULL;
+	unsigned int nr_pages;
+	unsigned int max_pages;
+	unsigned int nr_bytes;
+	struct page **pages = NULL;
+	struct rds_iovec __user *local_vec;
+	struct scatterlist *sg;
+	unsigned int nr;
+	unsigned int i, j;
+	int ret;
+
+
+	if (rs->rs_bound_addr == 0) {
+		ret = -ENOTCONN; /* XXX not a great errno */
+		goto out;
+	}
+
+	if (args->nr_local > (u64)UINT_MAX) {
+		ret = -EMSGSIZE;
+		goto out;
+	}
+
+	nr_pages = 0;
+	max_pages = 0;
+
+	local_vec = (struct rds_iovec __user *)(unsigned long) args->local_vec_addr;
+
+	/* figure out the number of pages in the vector */
+	for (i = 0; i < args->nr_local; i++) {
+		if (copy_from_user(&vec, &local_vec[i],
+				   sizeof(struct rds_iovec))) {
+			ret = -EFAULT;
+			goto out;
+		}
+
+		nr = rds_pages_in_vec(&vec);
+		if (nr == 0) {
+			ret = -EINVAL;
+			goto out;
+		}
+
+		max_pages = max(nr, max_pages);
+		nr_pages += nr;
+	}
+
+	pages = kcalloc(max_pages, sizeof(struct page *), GFP_KERNEL);
+	if (pages == NULL) {
+		ret = -ENOMEM;
+		goto out;
+	}
+
+	op = kzalloc(offsetof(struct rds_rdma_op, r_sg[nr_pages]), GFP_KERNEL);
+	if (op == NULL) {
+		ret = -ENOMEM;
+		goto out;
+	}
+
+	op->r_write = !!(args->flags & RDS_RDMA_READWRITE);
+	op->r_fence = !!(args->flags & RDS_RDMA_FENCE);
+	op->r_notify = !!(args->flags & RDS_RDMA_NOTIFY_ME);
+	op->r_recverr = rs->rs_recverr;
+	WARN_ON(!nr_pages);
+	sg_init_table(op->r_sg, nr_pages);
+
+	if (op->r_notify || op->r_recverr) {
+		/* We allocate an uninitialized notifier here, because
+		 * we don't want to do that in the completion handler. We
+		 * would have to use GFP_ATOMIC there, and don't want to deal
+		 * with failed allocations.
+		 */
+		op->r_notifier = kmalloc(sizeof(struct rds_notifier), GFP_KERNEL);
+		if (!op->r_notifier) {
+			ret = -ENOMEM;
+			goto out;
+		}
+		op->r_notifier->n_user_token = args->user_token;
+		op->r_notifier->n_status = RDS_RDMA_SUCCESS;
+	}
+
+	/* The cookie contains the R_Key of the remote memory region, and
+	 * optionally an offset into it. This is how we implement RDMA into
+	 * unaligned memory.
+	 * When setting up the RDMA, we need to add that offset to the
+	 * destination address (which is really an offset into the MR)
+	 * FIXME: We may want to move this into ib_rdma.c
+	 */
+	op->r_key = rds_rdma_cookie_key(args->cookie);
+	op->r_remote_addr = args->remote_vec.addr + rds_rdma_cookie_offset(args->cookie);
+
+	nr_bytes = 0;
+
+	rdsdebug("RDS: rdma prepare nr_local %llu rva %llx rkey %x\n",
+	       (unsigned long long)args->nr_local,
+	       (unsigned long long)args->remote_vec.addr,
+	       op->r_key);
+
+	for (i = 0; i < args->nr_local; i++) {
+		if (copy_from_user(&vec, &local_vec[i],
+				   sizeof(struct rds_iovec))) {
+			ret = -EFAULT;
+			goto out;
+		}
+
+		nr = rds_pages_in_vec(&vec);
+		if (nr == 0) {
+			ret = -EINVAL;
+			goto out;
+		}
+
+		rs->rs_user_addr = vec.addr;
+		rs->rs_user_bytes = vec.bytes;
+
+		/* did the user change the vec under us? */
+		if (nr > max_pages || op->r_nents + nr > nr_pages) {
+			ret = -EINVAL;
+			goto out;
+		}
+		/* If it's a WRITE operation, we want to pin the pages for reading.
+		 * If it's a READ operation, we need to pin the pages for writing.
+		 */
+		ret = rds_pin_pages(vec.addr & PAGE_MASK, nr, pages, !op->r_write);
+		if (ret < 0)
+			goto out;
+
+		rdsdebug("RDS: nr_bytes %u nr %u vec.bytes %llu vec.addr %llx\n",
+		       nr_bytes, nr, vec.bytes, vec.addr);
+
+		nr_bytes += vec.bytes;
+
+		for (j = 0; j < nr; j++) {
+			unsigned int offset = vec.addr & ~PAGE_MASK;
+
+			sg = &op->r_sg[op->r_nents + j];
+			sg_set_page(sg, pages[j],
+					min_t(unsigned int, vec.bytes, PAGE_SIZE - offset),
+					offset);
+
+			rdsdebug("RDS: sg->offset %x sg->len %x vec.addr %llx vec.bytes %llu\n",
+			       sg->offset, sg->length, vec.addr, vec.bytes);
+
+			vec.addr += sg->length;
+			vec.bytes -= sg->length;
+		}
+
+		op->r_nents += nr;
+	}
+
+
+	if (nr_bytes > args->remote_vec.bytes) {
+		rdsdebug("RDS nr_bytes %u remote_bytes %u do not match\n",
+				nr_bytes,
+				(unsigned int) args->remote_vec.bytes);
+		ret = -EINVAL;
+		goto out;
+	}
+	op->r_bytes = nr_bytes;
+
+	ret = 0;
+out:
+	kfree(pages);
+	if (ret) {
+		if (op)
+			rds_rdma_free_op(op);
+		op = ERR_PTR(ret);
+	}
+	return op;
+}
+
+/*
+ * The application asks for a RDMA transfer.
+ * Extract all arguments and set up the rdma_op
+ */
+int rds_cmsg_rdma_args(struct rds_sock *rs, struct rds_message *rm,
+			  struct cmsghdr *cmsg)
+{
+	struct rds_rdma_op *op;
+
+	if (cmsg->cmsg_len < CMSG_LEN(sizeof(struct rds_rdma_args))
+	 || rm->m_rdma_op != NULL)
+		return -EINVAL;
+
+	op = rds_rdma_prepare(rs, CMSG_DATA(cmsg));
+	if (IS_ERR(op))
+		return PTR_ERR(op);
+	rds_stats_inc(s_send_rdma);
+	rm->m_rdma_op = op;
+	return 0;
+}
+
+/*
+ * The application wants us to pass an RDMA destination (aka MR)
+ * to the remote
+ */
+int rds_cmsg_rdma_dest(struct rds_sock *rs, struct rds_message *rm,
+			  struct cmsghdr *cmsg)
+{
+	unsigned long flags;
+	struct rds_mr *mr;
+	u32 r_key;
+	int err = 0;
+
+	if (cmsg->cmsg_len < CMSG_LEN(sizeof(rds_rdma_cookie_t))
+	 || rm->m_rdma_cookie != 0)
+		return -EINVAL;
+
+	memcpy(&rm->m_rdma_cookie, CMSG_DATA(cmsg), sizeof(rm->m_rdma_cookie));
+
+	/* We are reusing a previously mapped MR here. Most likely, the
+	 * application has written to the buffer, so we need to explicitly
+	 * flush those writes to RAM. Otherwise the HCA may not see them
+	 * when doing a DMA from that buffer.
+	 */
+	r_key = rds_rdma_cookie_key(rm->m_rdma_cookie);
+
+	spin_lock_irqsave(&rs->rs_rdma_lock, flags);
+	mr = rds_mr_tree_walk(&rs->rs_rdma_keys, r_key, NULL);
+	if (mr == NULL)
+		err = -EINVAL;	/* invalid r_key */
+	else
+		atomic_inc(&mr->r_refcount);
+	spin_unlock_irqrestore(&rs->rs_rdma_lock, flags);
+
+	if (mr) {
+		mr->r_trans->sync_mr(mr->r_trans_private, DMA_TO_DEVICE);
+		rm->m_rdma_mr = mr;
+	}
+	return err;
+}
+
+/*
+ * The application passes us an address range it wants to enable RDMA
+ * to/from. We map the area, and save the <R_Key,offset> pair
+ * in rm->m_rdma_cookie. This causes it to be sent along to the peer
+ * in an extension header.
+ */
+int rds_cmsg_rdma_map(struct rds_sock *rs, struct rds_message *rm,
+			  struct cmsghdr *cmsg)
+{
+	if (cmsg->cmsg_len < CMSG_LEN(sizeof(struct rds_get_mr_args))
+	 || rm->m_rdma_cookie != 0)
+		return -EINVAL;
+
+	return __rds_rdma_map(rs, CMSG_DATA(cmsg), &rm->m_rdma_cookie, &rm->m_rdma_mr);
+}