xref: /linux/net/sunrpc/xprtrdma/rpc_rdma.c (revision 60e7870052f417d83965db144f70ae21fcfcf37f)

// SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause
/*
 * Copyright (c) 2014-2020, Oracle and/or its affiliates.
 * Copyright (c) 2003-2007 Network Appliance, Inc. 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 BSD-type
 * 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.
 *
 *      Neither the name of the Network Appliance, Inc. nor the names of
 *      its contributors may be used to endorse or promote products
 *      derived from this software without specific prior written
 *      permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

/*
 * rpc_rdma.c
 *
 * This file contains the guts of the RPC RDMA protocol, and
 * does marshaling/unmarshaling, etc. It is also where interfacing
 * to the Linux RPC framework lives.
 */

#include <linux/highmem.h>

#include <linux/sunrpc/svc_rdma.h>

#include "xprt_rdma.h"
#include <trace/events/rpcrdma.h>

/* Returns size of largest RPC-over-RDMA header in a Call message
 *
 * The largest Call header contains a full-size Read list and a
 * minimal Reply chunk.
 */
static unsigned int rpcrdma_max_call_header_size(unsigned int maxsegs)
{
	unsigned int size;

	/* Fixed header fields and list discriminators */
	size = RPCRDMA_HDRLEN_MIN;

	/* Maximum Read list size */
	size += maxsegs * rpcrdma_readchunk_maxsz * sizeof(__be32);

	/* Minimal Read chunk size */
	size += sizeof(__be32);	/* segment count */
	size += rpcrdma_segment_maxsz * sizeof(__be32);
	size += sizeof(__be32);	/* list discriminator */

	return size;
}

/* Returns size of largest RPC-over-RDMA header in a Reply message
 *
 * There is only one Write list or one Reply chunk per Reply
 * message.  The larger list is the Write list.
 */
static unsigned int rpcrdma_max_reply_header_size(unsigned int maxsegs)
{
	unsigned int size;

	/* Fixed header fields and list discriminators */
	size = RPCRDMA_HDRLEN_MIN;

	/* Maximum Write list size */
	size += sizeof(__be32);		/* segment count */
	size += maxsegs * rpcrdma_segment_maxsz * sizeof(__be32);
	size += sizeof(__be32);	/* list discriminator */

	return size;
}

/**
 * rpcrdma_set_max_header_sizes - Initialize inline payload sizes
 * @ep: endpoint to initialize
 *
 * The max_inline fields contain the maximum size of an RPC message
 * so the marshaling code doesn't have to repeat this calculation
 * for every RPC.
 */
void rpcrdma_set_max_header_sizes(struct rpcrdma_ep *ep)
{
	unsigned int maxsegs = ep->re_max_rdma_segs;

	ep->re_max_inline_send =
		ep->re_inline_send - rpcrdma_max_call_header_size(maxsegs);
	ep->re_max_inline_recv =
		ep->re_inline_recv - rpcrdma_max_reply_header_size(maxsegs);
}

/* The client can send a request inline as long as the RPCRDMA header
 * plus the RPC call fit under the transport's inline limit. If the
 * combined call message size exceeds that limit, the client must use
 * a Read chunk for this operation.
 *
 * A Read chunk is also required if sending the RPC call inline would
 * exceed this device's max_sge limit.
 */
static bool rpcrdma_args_inline(struct rpcrdma_xprt *r_xprt,
				struct rpc_rqst *rqst)
{
	struct xdr_buf *xdr = &rqst->rq_snd_buf;
	struct rpcrdma_ep *ep = r_xprt->rx_ep;
	unsigned int count, remaining, offset;

	if (xdr->len > ep->re_max_inline_send)
		return false;

	if (xdr->page_len) {
		remaining = xdr->page_len;
		offset = offset_in_page(xdr->page_base);
		count = RPCRDMA_MIN_SEND_SGES;
		while (remaining) {
			remaining -= min_t(unsigned int,
					   PAGE_SIZE - offset, remaining);
			offset = 0;
			if (++count > ep->re_attr.cap.max_send_sge)
				return false;
		}
	}

	return true;
}

/* The client can't know how large the actual reply will be. Thus it
 * plans for the largest possible reply for that particular ULP
 * operation. If the maximum combined reply message size exceeds that
 * limit, the client must provide a write list or a reply chunk for
 * this request.
 */
static bool rpcrdma_results_inline(struct rpcrdma_xprt *r_xprt,
				   struct rpc_rqst *rqst)
{
	return rqst->rq_rcv_buf.buflen <= r_xprt->rx_ep->re_max_inline_recv;
}

/* The client is required to provide a Reply chunk if the maximum
 * size of the non-payload part of the RPC Reply is larger than
 * the inline threshold.
 */
static bool
rpcrdma_nonpayload_inline(const struct rpcrdma_xprt *r_xprt,
			  const struct rpc_rqst *rqst)
{
	const struct xdr_buf *buf = &rqst->rq_rcv_buf;

	return (buf->head[0].iov_len + buf->tail[0].iov_len) <
		r_xprt->rx_ep->re_max_inline_recv;
}

/* ACL likes to be lazy in allocating pages. For TCP, these
 * pages can be allocated during receive processing. Not true
 * for RDMA, which must always provision receive buffers
 * up front.
 */
static noinline int
rpcrdma_alloc_sparse_pages(struct xdr_buf *buf)
{
	struct page **ppages;
	int len;

	len = buf->page_len;
	ppages = buf->pages + (buf->page_base >> PAGE_SHIFT);
	while (len > 0) {
		if (!*ppages)
			*ppages = alloc_page(GFP_NOWAIT);
		if (!*ppages)
			return -ENOBUFS;
		ppages++;
		len -= PAGE_SIZE;
	}

	return 0;
}

static void
rpcrdma_xdr_cursor_init(struct rpcrdma_xdr_cursor *cur,
			const struct xdr_buf *xdrbuf,
			unsigned int pos, enum rpcrdma_chunktype type)
{
	cur->xc_buf = xdrbuf;
	cur->xc_page_offset = 0;
	cur->xc_flags = 0;

	if (pos != 0)
		cur->xc_flags |= XC_HEAD_DONE;
	if (!xdrbuf->page_len)
		cur->xc_flags |= XC_PAGES_DONE;
	if (type == rpcrdma_readch || type == rpcrdma_writech ||
	    !xdrbuf->tail[0].iov_len)
		cur->xc_flags |= XC_TAIL_DONE;
}

static bool
rpcrdma_xdr_cursor_done(const struct rpcrdma_xdr_cursor *cur)
{
	return (cur->xc_flags & (XC_HEAD_DONE | XC_PAGES_DONE |
				 XC_TAIL_DONE)) ==
	       (XC_HEAD_DONE | XC_PAGES_DONE | XC_TAIL_DONE);
}

static int
encode_rdma_segment(struct xdr_stream *xdr, struct rpcrdma_mr *mr)
{
	__be32 *p;

	p = xdr_reserve_space(xdr, 4 * sizeof(*p));
	if (unlikely(!p))
		return -EMSGSIZE;

	xdr_encode_rdma_segment(p, mr->mr_handle, mr->mr_length, mr->mr_offset);
	return 0;
}

static int
encode_read_segment(struct xdr_stream *xdr, struct rpcrdma_mr *mr,
		    u32 position)
{
	__be32 *p;

	p = xdr_reserve_space(xdr, 6 * sizeof(*p));
	if (unlikely(!p))
		return -EMSGSIZE;

	*p++ = xdr_one;			/* Item present */
	xdr_encode_read_segment(p, position, mr->mr_handle, mr->mr_length,
				mr->mr_offset);
	return 0;
}

static int rpcrdma_mr_prepare(struct rpcrdma_xprt *r_xprt,
			      struct rpcrdma_req *req,
			      struct rpcrdma_xdr_cursor *cur,
			      bool writing, struct rpcrdma_mr **mr)
{
	*mr = rpcrdma_mr_pop(&req->rl_free_mrs);
	if (!*mr) {
		*mr = rpcrdma_mr_get(r_xprt);
		if (!*mr)
			goto out_getmr_err;
		(*mr)->mr_req = req;
	}

	rpcrdma_mr_push(*mr, &req->rl_registered);
	return frwr_map(r_xprt, cur, writing, req->rl_slot.rq_xid, *mr);

out_getmr_err:
	trace_xprtrdma_nomrs_err(r_xprt, req);
	xprt_wait_for_buffer_space(&r_xprt->rx_xprt);
	rpcrdma_mrs_refresh(r_xprt);
	return -EAGAIN;
}

/* Register and XDR encode the Read list. Supports encoding a list of read
 * segments that belong to a single read chunk.
 *
 * Encoding key for single-list chunks (HLOO = Handle32 Length32 Offset64):
 *
 *  Read chunklist (a linked list):
 *   N elements, position P (same P for all chunks of same arg!):
 *    1 - PHLOO - 1 - PHLOO - ... - 1 - PHLOO - 0
 *
 * Returns zero on success, or a negative errno if a failure occurred.
 * @xdr is advanced to the next position in the stream.
 *
 * Only a single @pos value is currently supported.
 */
static int rpcrdma_encode_read_list(struct rpcrdma_xprt *r_xprt,
				    struct rpcrdma_req *req,
				    struct rpc_rqst *rqst,
				    enum rpcrdma_chunktype rtype)
{
	struct xdr_stream *xdr = &req->rl_stream;
	struct rpcrdma_xdr_cursor cur;
	struct rpcrdma_mr *mr;
	unsigned int pos;
	int ret;

	if (rtype == rpcrdma_noch_pullup || rtype == rpcrdma_noch_mapped)
		goto done;

	pos = rqst->rq_snd_buf.head[0].iov_len;
	if (rtype == rpcrdma_areadch)
		pos = 0;
	rpcrdma_xdr_cursor_init(&cur, &rqst->rq_snd_buf, pos, rtype);

	do {
		ret = rpcrdma_mr_prepare(r_xprt, req, &cur, false, &mr);
		if (ret)
			return ret;

		if (encode_read_segment(xdr, mr, pos) < 0)
			return -EMSGSIZE;

		trace_xprtrdma_chunk_read(rqst->rq_task, pos, mr,
					  rpcrdma_xdr_cursor_done(&cur));
		r_xprt->rx_stats.read_chunk_count++;
	} while (!rpcrdma_xdr_cursor_done(&cur));

done:
	if (xdr_stream_encode_item_absent(xdr) < 0)
		return -EMSGSIZE;
	return 0;
}

/* Register and XDR encode the Write list. Supports encoding a list
 * containing one array of plain segments that belong to a single
 * write chunk.
 *
 * Encoding key for single-list chunks (HLOO = Handle32 Length32 Offset64):
 *
 *  Write chunklist (a list of (one) counted array):
 *   N elements:
 *    1 - N - HLOO - HLOO - ... - HLOO - 0
 *
 * Returns zero on success, or a negative errno if a failure occurred.
 * @xdr is advanced to the next position in the stream.
 *
 * Only a single Write chunk is currently supported.
 */
static int rpcrdma_encode_write_list(struct rpcrdma_xprt *r_xprt,
				     struct rpcrdma_req *req,
				     struct rpc_rqst *rqst,
				     enum rpcrdma_chunktype wtype)
{
	struct xdr_stream *xdr = &req->rl_stream;
	struct rpcrdma_ep *ep = r_xprt->rx_ep;
	struct rpcrdma_xdr_cursor cur;
	struct rpcrdma_mr *mr;
	int nchunks, ret;
	__be32 *segcount;

	if (wtype != rpcrdma_writech)
		goto done;

	rpcrdma_xdr_cursor_init(&cur, &rqst->rq_rcv_buf,
				rqst->rq_rcv_buf.head[0].iov_len, wtype);

	if (xdr_stream_encode_item_present(xdr) < 0)
		return -EMSGSIZE;
	segcount = xdr_reserve_space(xdr, sizeof(*segcount));
	if (unlikely(!segcount))
		return -EMSGSIZE;
	/* Actual value encoded below */

	nchunks = 0;
	do {
		ret = rpcrdma_mr_prepare(r_xprt, req, &cur, true, &mr);
		if (ret)
			return ret;

		if (encode_rdma_segment(xdr, mr) < 0)
			return -EMSGSIZE;

		trace_xprtrdma_chunk_write(rqst->rq_task, mr,
					   rpcrdma_xdr_cursor_done(&cur));
		r_xprt->rx_stats.write_chunk_count++;
		r_xprt->rx_stats.total_rdma_request += mr->mr_length;
		nchunks++;
	} while (!rpcrdma_xdr_cursor_done(&cur));

	if (xdr_pad_size(rqst->rq_rcv_buf.page_len)) {
		if (encode_rdma_segment(xdr, ep->re_write_pad_mr) < 0)
			return -EMSGSIZE;

		trace_xprtrdma_chunk_wp(rqst->rq_task, ep->re_write_pad_mr,
					true);
		r_xprt->rx_stats.write_chunk_count++;
		r_xprt->rx_stats.total_rdma_request +=
			ep->re_write_pad_mr->mr_length;
		nchunks++;
	}

	/* Update count of segments in this Write chunk */
	*segcount = cpu_to_be32(nchunks);

done:
	if (xdr_stream_encode_item_absent(xdr) < 0)
		return -EMSGSIZE;
	return 0;
}

/* Register and XDR encode the Reply chunk. Supports encoding an array
 * of plain segments that belong to a single write (reply) chunk.
 *
 * Encoding key for single-list chunks (HLOO = Handle32 Length32 Offset64):
 *
 *  Reply chunk (a counted array):
 *   N elements:
 *    1 - N - HLOO - HLOO - ... - HLOO
 *
 * Returns zero on success, or a negative errno if a failure occurred.
 * @xdr is advanced to the next position in the stream.
 */
static int rpcrdma_encode_reply_chunk(struct rpcrdma_xprt *r_xprt,
				      struct rpcrdma_req *req,
				      struct rpc_rqst *rqst,
				      enum rpcrdma_chunktype wtype)
{
	struct xdr_stream *xdr = &req->rl_stream;
	struct rpcrdma_xdr_cursor cur;
	struct rpcrdma_mr *mr;
	int nchunks, ret;
	__be32 *segcount;

	if (wtype != rpcrdma_replych) {
		if (xdr_stream_encode_item_absent(xdr) < 0)
			return -EMSGSIZE;
		return 0;
	}

	rpcrdma_xdr_cursor_init(&cur, &rqst->rq_rcv_buf, 0, wtype);

	if (xdr_stream_encode_item_present(xdr) < 0)
		return -EMSGSIZE;
	segcount = xdr_reserve_space(xdr, sizeof(*segcount));
	if (unlikely(!segcount))
		return -EMSGSIZE;
	/* Actual value encoded below */

	nchunks = 0;
	do {
		ret = rpcrdma_mr_prepare(r_xprt, req, &cur, true, &mr);
		if (ret)
			return ret;

		if (encode_rdma_segment(xdr, mr) < 0)
			return -EMSGSIZE;

		trace_xprtrdma_chunk_reply(rqst->rq_task, mr,
					   rpcrdma_xdr_cursor_done(&cur));
		r_xprt->rx_stats.reply_chunk_count++;
		r_xprt->rx_stats.total_rdma_request += mr->mr_length;
		nchunks++;
	} while (!rpcrdma_xdr_cursor_done(&cur));

	/* Update count of segments in the Reply chunk */
	*segcount = cpu_to_be32(nchunks);

	return 0;
}

static void rpcrdma_sendctx_dma_unmap(struct rpcrdma_sendctx *sc)
{
	struct rpcrdma_regbuf *rb = sc->sc_req->rl_sendbuf;
	struct ib_sge *sge;

	/* The first two SGEs contain the transport header and
	 * the inline buffer. These are always left mapped so
	 * they can be cheaply re-used.
	 */
	for (sge = &sc->sc_sges[2]; sc->sc_unmap_count;
	     ++sge, --sc->sc_unmap_count)
		ib_dma_unmap_page(rdmab_device(rb), sge->addr, sge->length,
				  DMA_TO_DEVICE);
}

/**
 * rpcrdma_sendctx_unmap - DMA-unmap Send buffer and release Send owner
 * @sc: sendctx containing SGEs to unmap
 *
 */
void rpcrdma_sendctx_unmap(struct rpcrdma_sendctx *sc)
{
	struct rpcrdma_req *req = sc->sc_req;

	rpcrdma_sendctx_dma_unmap(sc);
	sc->sc_req = NULL;
	req->rl_sendctx = NULL;
	rpcrdma_req_put(req);
}

/* No Send was posted. Release DMA mappings prepared for this
 * sendctx, but leave the request reference count alone.
 */
static void rpcrdma_sendctx_cancel(struct rpcrdma_sendctx *sc)
{
	struct rpcrdma_req *req = sc->sc_req;

	rpcrdma_sendctx_dma_unmap(sc);
	sc->sc_req = NULL;
	req->rl_sendctx = NULL;
}

/* Prepare an SGE for the RPC-over-RDMA transport header.
 */
static void rpcrdma_prepare_hdr_sge(struct rpcrdma_xprt *r_xprt,
				    struct rpcrdma_req *req, u32 len)
{
	struct rpcrdma_sendctx *sc = req->rl_sendctx;
	struct rpcrdma_regbuf *rb = req->rl_rdmabuf;
	struct ib_sge *sge = &sc->sc_sges[req->rl_wr.num_sge++];

	sge->addr = rdmab_addr(rb);
	sge->length = len;
	sge->lkey = rdmab_lkey(rb);

	ib_dma_sync_single_for_device(rdmab_device(rb), sge->addr, sge->length,
				      DMA_TO_DEVICE);
}

/* The head iovec is straightforward, as it is usually already
 * DMA-mapped. Sync the content that has changed.
 */
static bool rpcrdma_prepare_head_iov(struct rpcrdma_xprt *r_xprt,
				     struct rpcrdma_req *req, unsigned int len)
{
	struct rpcrdma_sendctx *sc = req->rl_sendctx;
	struct ib_sge *sge = &sc->sc_sges[req->rl_wr.num_sge++];
	struct rpcrdma_regbuf *rb = req->rl_sendbuf;

	if (!rpcrdma_regbuf_dma_map(r_xprt, rb))
		return false;

	sge->addr = rdmab_addr(rb);
	sge->length = len;
	sge->lkey = rdmab_lkey(rb);

	ib_dma_sync_single_for_device(rdmab_device(rb), sge->addr, sge->length,
				      DMA_TO_DEVICE);
	return true;
}

/* If there is a page list present, DMA map and prepare an
 * SGE for each page to be sent.
 */
static bool rpcrdma_prepare_pagelist(struct rpcrdma_req *req,
				     struct xdr_buf *xdr)
{
	struct rpcrdma_sendctx *sc = req->rl_sendctx;
	struct rpcrdma_regbuf *rb = req->rl_sendbuf;
	unsigned int page_base, len, remaining;
	struct page **ppages;
	struct ib_sge *sge;

	ppages = xdr->pages + (xdr->page_base >> PAGE_SHIFT);
	page_base = offset_in_page(xdr->page_base);
	remaining = xdr->page_len;
	while (remaining) {
		sge = &sc->sc_sges[req->rl_wr.num_sge++];
		len = min_t(unsigned int, PAGE_SIZE - page_base, remaining);
		sge->addr = ib_dma_map_page(rdmab_device(rb), *ppages,
					    page_base, len, DMA_TO_DEVICE);
		if (ib_dma_mapping_error(rdmab_device(rb), sge->addr))
			goto out_mapping_err;

		sge->length = len;
		sge->lkey = rdmab_lkey(rb);

		sc->sc_unmap_count++;
		ppages++;
		remaining -= len;
		page_base = 0;
	}

	return true;

out_mapping_err:
	trace_xprtrdma_dma_maperr(sge->addr);
	return false;
}

/* The tail iovec may include an XDR pad for the page list,
 * as well as additional content, and may not reside in the
 * same page as the head iovec.
 */
static bool rpcrdma_prepare_tail_iov(struct rpcrdma_req *req,
				     struct xdr_buf *xdr,
				     unsigned int page_base, unsigned int len)
{
	struct rpcrdma_sendctx *sc = req->rl_sendctx;
	struct ib_sge *sge = &sc->sc_sges[req->rl_wr.num_sge++];
	struct rpcrdma_regbuf *rb = req->rl_sendbuf;
	struct page *page = virt_to_page(xdr->tail[0].iov_base);

	sge->addr = ib_dma_map_page(rdmab_device(rb), page, page_base, len,
				    DMA_TO_DEVICE);
	if (ib_dma_mapping_error(rdmab_device(rb), sge->addr))
		goto out_mapping_err;

	sge->length = len;
	sge->lkey = rdmab_lkey(rb);
	++sc->sc_unmap_count;
	return true;

out_mapping_err:
	trace_xprtrdma_dma_maperr(sge->addr);
	return false;
}

/* Copy the tail to the end of the head buffer.
 */
static void rpcrdma_pullup_tail_iov(struct rpcrdma_xprt *r_xprt,
				    struct rpcrdma_req *req,
				    struct xdr_buf *xdr)
{
	unsigned char *dst;

	dst = (unsigned char *)xdr->head[0].iov_base;
	dst += xdr->head[0].iov_len + xdr->page_len;
	memmove(dst, xdr->tail[0].iov_base, xdr->tail[0].iov_len);
	r_xprt->rx_stats.pullup_copy_count += xdr->tail[0].iov_len;
}

/* Copy pagelist content into the head buffer.
 */
static void rpcrdma_pullup_pagelist(struct rpcrdma_xprt *r_xprt,
				    struct rpcrdma_req *req,
				    struct xdr_buf *xdr)
{
	unsigned int len, page_base, remaining;
	struct page **ppages;
	unsigned char *src, *dst;

	dst = (unsigned char *)xdr->head[0].iov_base;
	dst += xdr->head[0].iov_len;
	ppages = xdr->pages + (xdr->page_base >> PAGE_SHIFT);
	page_base = offset_in_page(xdr->page_base);
	remaining = xdr->page_len;
	while (remaining) {
		src = page_address(*ppages);
		src += page_base;
		len = min_t(unsigned int, PAGE_SIZE - page_base, remaining);
		memcpy(dst, src, len);
		r_xprt->rx_stats.pullup_copy_count += len;

		ppages++;
		dst += len;
		remaining -= len;
		page_base = 0;
	}
}

/* Copy the contents of @xdr into @rl_sendbuf and DMA sync it.
 * When the head, pagelist, and tail are small, a pull-up copy
 * is considerably less costly than DMA mapping the components
 * of @xdr.
 *
 * Assumptions:
 *  - the caller has already verified that the total length
 *    of the RPC Call body will fit into @rl_sendbuf.
 */
static bool rpcrdma_prepare_noch_pullup(struct rpcrdma_xprt *r_xprt,
					struct rpcrdma_req *req,
					struct xdr_buf *xdr)
{
	if (unlikely(xdr->tail[0].iov_len))
		rpcrdma_pullup_tail_iov(r_xprt, req, xdr);

	if (unlikely(xdr->page_len))
		rpcrdma_pullup_pagelist(r_xprt, req, xdr);

	/* The whole RPC message resides in the head iovec now */
	return rpcrdma_prepare_head_iov(r_xprt, req, xdr->len);
}

static bool rpcrdma_prepare_noch_mapped(struct rpcrdma_xprt *r_xprt,
					struct rpcrdma_req *req,
					struct xdr_buf *xdr)
{
	struct kvec *tail = &xdr->tail[0];

	if (!rpcrdma_prepare_head_iov(r_xprt, req, xdr->head[0].iov_len))
		return false;
	if (xdr->page_len)
		if (!rpcrdma_prepare_pagelist(req, xdr))
			return false;
	if (tail->iov_len)
		if (!rpcrdma_prepare_tail_iov(req, xdr,
					      offset_in_page(tail->iov_base),
					      tail->iov_len))
			return false;

	return true;
}

static bool rpcrdma_prepare_readch(struct rpcrdma_xprt *r_xprt,
				   struct rpcrdma_req *req,
				   struct xdr_buf *xdr)
{
	if (!rpcrdma_prepare_head_iov(r_xprt, req, xdr->head[0].iov_len))
		return false;

	/* If there is a Read chunk, the page list is being handled
	 * via explicit RDMA, and thus is skipped here.
	 */

	/* Do not include the tail if it is only an XDR pad */
	if (xdr->tail[0].iov_len > 3) {
		unsigned int page_base, len;

		/* If the content in the page list is an odd length,
		 * xdr_write_pages() adds a pad at the beginning of
		 * the tail iovec. Force the tail's non-pad content to
		 * land at the next XDR position in the Send message.
		 */
		page_base = offset_in_page(xdr->tail[0].iov_base);
		len = xdr->tail[0].iov_len;
		page_base += len & 3;
		len -= len & 3;
		if (!rpcrdma_prepare_tail_iov(req, xdr, page_base, len))
			return false;
	}

	return true;
}

/**
 * rpcrdma_prepare_send_sges - Construct SGEs for a Send WR
 * @r_xprt: controlling transport
 * @req: context of RPC Call being marshalled
 * @hdrlen: size of transport header, in bytes
 * @xdr: xdr_buf containing RPC Call
 * @rtype: chunk type being encoded
 *
 * Returns 0 on success; otherwise a negative errno is returned.
 */
inline int rpcrdma_prepare_send_sges(struct rpcrdma_xprt *r_xprt,
				     struct rpcrdma_req *req, u32 hdrlen,
				     struct xdr_buf *xdr,
				     enum rpcrdma_chunktype rtype)
{
	struct rpcrdma_sendctx *sc;
	int ret;

	ret = -EAGAIN;
	req->rl_sendctx = rpcrdma_sendctx_get_locked(r_xprt);
	if (!req->rl_sendctx)
		goto out_nosc;
	req->rl_sendctx->sc_unmap_count = 0;
	req->rl_sendctx->sc_req = req;
	req->rl_wr.wr_cqe = &req->rl_sendctx->sc_cqe;
	req->rl_wr.sg_list = req->rl_sendctx->sc_sges;
	req->rl_wr.num_sge = 0;
	req->rl_wr.opcode = IB_WR_SEND;

	rpcrdma_prepare_hdr_sge(r_xprt, req, hdrlen);

	ret = -EIO;
	switch (rtype) {
	case rpcrdma_noch_pullup:
		if (!rpcrdma_prepare_noch_pullup(r_xprt, req, xdr))
			goto out_unmap;
		break;
	case rpcrdma_noch_mapped:
		if (!rpcrdma_prepare_noch_mapped(r_xprt, req, xdr))
			goto out_unmap;
		break;
	case rpcrdma_readch:
		if (!rpcrdma_prepare_readch(r_xprt, req, xdr))
			goto out_unmap;
		break;
	case rpcrdma_areadch:
		break;
	default:
		goto out_unmap;
	}

	/* The Send-side owner releases this reference when the
	 * Send has completed.
	 */
	kref_get(&req->rl_kref);
	return 0;

out_unmap:
	sc = req->rl_sendctx;
	rpcrdma_sendctx_cancel(sc);
	rpcrdma_sendctx_unget_locked(r_xprt, sc);
out_nosc:
	trace_xprtrdma_prepsend_failed(&req->rl_slot, ret);
	return ret;
}

/**
 * rpcrdma_marshal_req - Marshal and send one RPC request
 * @r_xprt: controlling transport
 * @rqst: RPC request to be marshaled
 *
 * For the RPC in "rqst", this function:
 *  - Chooses the transfer mode (eg., RDMA_MSG or RDMA_NOMSG)
 *  - Registers Read, Write, and Reply chunks
 *  - Constructs the transport header
 *  - Posts a Send WR to send the transport header and request
 *
 * Returns:
 *	%0 if the RPC was sent successfully,
 *	%-ENOTCONN if the connection was lost,
 *	%-EAGAIN if the caller should call again with the same arguments,
 *	%-ENOBUFS if the caller should call again after a delay,
 *	%-EMSGSIZE if the transport header is too small,
 *	%-EIO if a permanent problem occurred while marshaling.
 */
int
rpcrdma_marshal_req(struct rpcrdma_xprt *r_xprt, struct rpc_rqst *rqst)
{
	struct rpcrdma_req *req = rpcr_to_rdmar(rqst);
	struct xdr_stream *xdr = &req->rl_stream;
	enum rpcrdma_chunktype rtype, wtype;
	struct xdr_buf *buf = &rqst->rq_snd_buf;
	bool ddp_allowed;
	__be32 *p;
	int ret;

	if (unlikely(rqst->rq_rcv_buf.flags & XDRBUF_SPARSE_PAGES)) {
		ret = rpcrdma_alloc_sparse_pages(&rqst->rq_rcv_buf);
		if (ret)
			return ret;
	}

	rpcrdma_set_xdrlen(&req->rl_hdrbuf, 0);
	xdr_init_encode(xdr, &req->rl_hdrbuf, rdmab_data(req->rl_rdmabuf),
			rqst);

	/* Fixed header fields */
	ret = -EMSGSIZE;
	p = xdr_reserve_space(xdr, 4 * sizeof(*p));
	if (!p)
		goto out_err;
	*p++ = rqst->rq_xid;
	*p++ = rpcrdma_version;
	*p++ = r_xprt->rx_buf.rb_max_requests;

	/* When the ULP employs a GSS flavor that guarantees integrity
	 * or privacy, direct data placement of individual data items
	 * is not allowed.
	 */
	ddp_allowed = !test_bit(RPCAUTH_AUTH_DATATOUCH,
				&rqst->rq_cred->cr_auth->au_flags);

	/*
	 * Chunks needed for results?
	 *
	 * o If the expected result is under the inline threshold, all ops
	 *   return as inline.
	 * o Large read ops return data as write chunk(s), header as
	 *   inline.
	 * o Large non-read ops return as a single reply chunk.
	 */
	if (rpcrdma_results_inline(r_xprt, rqst))
		wtype = rpcrdma_noch;
	else if ((ddp_allowed && rqst->rq_rcv_buf.flags & XDRBUF_READ) &&
		 rpcrdma_nonpayload_inline(r_xprt, rqst))
		wtype = rpcrdma_writech;
	else
		wtype = rpcrdma_replych;

	/*
	 * Chunks needed for arguments?
	 *
	 * o If the total request is under the inline threshold, all ops
	 *   are sent as inline.
	 * o Large write ops transmit data as read chunk(s), header as
	 *   inline.
	 * o Large non-write ops are sent with the entire message as a
	 *   single read chunk (protocol 0-position special case).
	 *
	 * This assumes that the upper layer does not present a request
	 * that both has a data payload, and whose non-data arguments
	 * by themselves are larger than the inline threshold.
	 */
	if (rpcrdma_args_inline(r_xprt, rqst)) {
		*p++ = rdma_msg;
		rtype = buf->len < rdmab_length(req->rl_sendbuf) ?
			rpcrdma_noch_pullup : rpcrdma_noch_mapped;
	} else if (ddp_allowed && buf->flags & XDRBUF_WRITE) {
		*p++ = rdma_msg;
		rtype = rpcrdma_readch;
	} else {
		r_xprt->rx_stats.nomsg_call_count++;
		*p++ = rdma_nomsg;
		rtype = rpcrdma_areadch;
	}

	/* This implementation supports the following combinations
	 * of chunk lists in one RPC-over-RDMA Call message:
	 *
	 *   - Read list
	 *   - Write list
	 *   - Reply chunk
	 *   - Read list + Reply chunk
	 *
	 * It might not yet support the following combinations:
	 *
	 *   - Read list + Write list
	 *
	 * It does not support the following combinations:
	 *
	 *   - Write list + Reply chunk
	 *   - Read list + Write list + Reply chunk
	 *
	 * This implementation supports only a single chunk in each
	 * Read or Write list. Thus for example the client cannot
	 * send a Call message with a Position Zero Read chunk and a
	 * regular Read chunk at the same time.
	 */
	ret = rpcrdma_encode_read_list(r_xprt, req, rqst, rtype);
	if (ret)
		goto out_err;
	ret = rpcrdma_encode_write_list(r_xprt, req, rqst, wtype);
	if (ret)
		goto out_err;
	ret = rpcrdma_encode_reply_chunk(r_xprt, req, rqst, wtype);
	if (ret)
		goto out_err;

	ret = rpcrdma_prepare_send_sges(r_xprt, req, req->rl_hdrbuf.len,
					buf, rtype);
	if (ret)
		goto out_err;

	trace_xprtrdma_marshal(req, rtype, wtype);
	return 0;

out_err:
	trace_xprtrdma_marshal_failed(rqst, ret);
	r_xprt->rx_stats.failed_marshal_count++;
	frwr_reset(req);
	return ret;
}

static void __rpcrdma_update_cwnd_locked(struct rpc_xprt *xprt,
					 struct rpcrdma_buffer *buf,
					 u32 grant)
{
	buf->rb_credits = grant;
	xprt->cwnd = grant << RPC_CWNDSHIFT;
}

static void rpcrdma_update_cwnd(struct rpcrdma_xprt *r_xprt, u32 grant)
{
	struct rpc_xprt *xprt = &r_xprt->rx_xprt;

	spin_lock(&xprt->transport_lock);
	__rpcrdma_update_cwnd_locked(xprt, &r_xprt->rx_buf, grant);
	spin_unlock(&xprt->transport_lock);
}

/**
 * rpcrdma_reset_cwnd - Reset the xprt's congestion window
 * @r_xprt: controlling transport instance
 *
 * Prepare @r_xprt for the next connection by reinitializing
 * its credit grant to one (see RFC 8166, Section 3.3.3).
 */
void rpcrdma_reset_cwnd(struct rpcrdma_xprt *r_xprt)
{
	struct rpc_xprt *xprt = &r_xprt->rx_xprt;

	spin_lock(&xprt->transport_lock);
	xprt->cong = 0;
	__rpcrdma_update_cwnd_locked(xprt, &r_xprt->rx_buf, 1);
	spin_unlock(&xprt->transport_lock);
}

/**
 * rpcrdma_inline_fixup - Scatter inline received data into rqst's iovecs
 * @rqst: controlling RPC request
 * @srcp: points to RPC message payload in receive buffer
 * @copy_len: remaining length of receive buffer content
 * @pad: Write chunk pad bytes needed (zero for pure inline)
 *
 * The upper layer has set the maximum number of bytes it can
 * receive in each component of rq_rcv_buf. These values are set in
 * the head.iov_len, page_len, tail.iov_len, and buflen fields.
 *
 * Unlike the TCP equivalent (xdr_partial_copy_from_skb), in
 * many cases this function simply updates iov_base pointers in
 * rq_rcv_buf to point directly to the received reply data, to
 * avoid copying reply data.
 *
 * Returns the count of bytes which had to be memcopied.
 */
static unsigned long
rpcrdma_inline_fixup(struct rpc_rqst *rqst, char *srcp, int copy_len, int pad)
{
	unsigned long fixup_copy_count;
	int i, npages, curlen;
	char *destp;
	struct page **ppages;
	int page_base;

	/* The head iovec is redirected to the RPC reply message
	 * in the receive buffer, to avoid a memcopy.
	 */
	rqst->rq_rcv_buf.head[0].iov_base = srcp;
	rqst->rq_private_buf.head[0].iov_base = srcp;

	/* The contents of the receive buffer that follow
	 * head.iov_len bytes are copied into the page list.
	 */
	curlen = rqst->rq_rcv_buf.head[0].iov_len;
	if (curlen > copy_len)
		curlen = copy_len;
	srcp += curlen;
	copy_len -= curlen;

	ppages = rqst->rq_rcv_buf.pages +
		(rqst->rq_rcv_buf.page_base >> PAGE_SHIFT);
	page_base = offset_in_page(rqst->rq_rcv_buf.page_base);
	fixup_copy_count = 0;
	if (copy_len && rqst->rq_rcv_buf.page_len) {
		int pagelist_len;

		pagelist_len = rqst->rq_rcv_buf.page_len;
		if (pagelist_len > copy_len)
			pagelist_len = copy_len;
		npages = PAGE_ALIGN(page_base + pagelist_len) >> PAGE_SHIFT;
		for (i = 0; i < npages; i++) {
			curlen = PAGE_SIZE - page_base;
			if (curlen > pagelist_len)
				curlen = pagelist_len;

			destp = kmap_atomic(ppages[i]);
			memcpy(destp + page_base, srcp, curlen);
			flush_dcache_page(ppages[i]);
			kunmap_atomic(destp);
			srcp += curlen;
			copy_len -= curlen;
			fixup_copy_count += curlen;
			pagelist_len -= curlen;
			if (!pagelist_len)
				break;
			page_base = 0;
		}

		/* Implicit padding for the last segment in a Write
		 * chunk is inserted inline at the front of the tail
		 * iovec. The upper layer ignores the content of
		 * the pad. Simply ensure inline content in the tail
		 * that follows the Write chunk is properly aligned.
		 */
		if (pad)
			srcp -= pad;
	}

	/* The tail iovec is redirected to the remaining data
	 * in the receive buffer, to avoid a memcopy.
	 */
	if (copy_len || pad) {
		rqst->rq_rcv_buf.tail[0].iov_base = srcp;
		rqst->rq_private_buf.tail[0].iov_base = srcp;
	}

	if (fixup_copy_count)
		trace_xprtrdma_fixup(rqst, fixup_copy_count);
	return fixup_copy_count;
}

/* By convention, backchannel calls arrive via rdma_msg type
 * messages, and never populate the chunk lists. This makes
 * the RPC/RDMA header small and fixed in size, so it is
 * straightforward to check the RPC header's direction field.
 */
static bool
rpcrdma_is_bcall(struct rpcrdma_xprt *r_xprt, struct rpcrdma_rep *rep)
#if defined(CONFIG_SUNRPC_BACKCHANNEL)
{
	struct rpc_xprt *xprt = &r_xprt->rx_xprt;
	struct xdr_stream *xdr = &rep->rr_stream;
	__be32 *p;

	if (rep->rr_proc != rdma_msg)
		return false;

	/* Peek at stream contents without advancing. */
	p = xdr_inline_decode(xdr, 0);
	if ((char *)xdr->end - (char *)p < 5 * XDR_UNIT)
		return false;

	/* Chunk lists */
	if (xdr_item_is_present(p++))
		return false;
	if (xdr_item_is_present(p++))
		return false;
	if (xdr_item_is_present(p++))
		return false;

	/* RPC header */
	if (*p++ != rep->rr_xid)
		return false;
	if (*p != cpu_to_be32(RPC_CALL))
		return false;

	/* No bc service. */
	if (xprt->bc_serv == NULL)
		return false;

	/* Now that we are sure this is a backchannel call,
	 * advance to the RPC header.
	 */
	p = xdr_inline_decode(xdr, 3 * sizeof(*p));
	if (unlikely(!p))
		return false;

	rpcrdma_bc_receive_call(r_xprt, rep);
	return true;
}
#else	/* CONFIG_SUNRPC_BACKCHANNEL */
{
	return false;
}
#endif	/* CONFIG_SUNRPC_BACKCHANNEL */

static int decode_rdma_segment(struct xdr_stream *xdr, u32 *length)
{
	u32 handle;
	u64 offset;
	__be32 *p;

	p = xdr_inline_decode(xdr, 4 * sizeof(*p));
	if (unlikely(!p))
		return -EIO;

	xdr_decode_rdma_segment(p, &handle, length, &offset);
	trace_xprtrdma_decode_seg(handle, *length, offset);
	return 0;
}

static int decode_write_chunk(struct xdr_stream *xdr, u32 *length)
{
	u32 segcount, seglength;
	__be32 *p;

	p = xdr_inline_decode(xdr, sizeof(*p));
	if (unlikely(!p))
		return -EIO;

	*length = 0;
	segcount = be32_to_cpup(p);
	while (segcount--) {
		if (decode_rdma_segment(xdr, &seglength))
			return -EIO;
		*length += seglength;
	}

	return 0;
}

/* In RPC-over-RDMA Version One replies, a Read list is never
 * expected. This decoder is a stub that returns an error if
 * a Read list is present.
 */
static int decode_read_list(struct xdr_stream *xdr)
{
	__be32 *p;

	p = xdr_inline_decode(xdr, sizeof(*p));
	if (unlikely(!p))
		return -EIO;
	if (unlikely(xdr_item_is_present(p)))
		return -EIO;
	return 0;
}

/* Supports only one Write chunk in the Write list
 */
static int decode_write_list(struct xdr_stream *xdr, u32 *length)
{
	u32 chunklen;
	bool first;
	__be32 *p;

	*length = 0;
	first = true;
	do {
		p = xdr_inline_decode(xdr, sizeof(*p));
		if (unlikely(!p))
			return -EIO;
		if (xdr_item_is_absent(p))
			break;
		if (!first)
			return -EIO;

		if (decode_write_chunk(xdr, &chunklen))
			return -EIO;
		*length += chunklen;
		first = false;
	} while (true);
	return 0;
}

static int decode_reply_chunk(struct xdr_stream *xdr, u32 *length)
{
	__be32 *p;

	p = xdr_inline_decode(xdr, sizeof(*p));
	if (unlikely(!p))
		return -EIO;

	*length = 0;
	if (xdr_item_is_present(p))
		if (decode_write_chunk(xdr, length))
			return -EIO;
	return 0;
}

static int
rpcrdma_decode_msg(struct rpcrdma_xprt *r_xprt, struct rpcrdma_rep *rep,
		   struct rpc_rqst *rqst)
{
	struct xdr_stream *xdr = &rep->rr_stream;
	u32 writelist, replychunk, rpclen;
	char *base;

	/* Decode the chunk lists */
	if (decode_read_list(xdr))
		return -EIO;
	if (decode_write_list(xdr, &writelist))
		return -EIO;
	if (decode_reply_chunk(xdr, &replychunk))
		return -EIO;

	/* RDMA_MSG sanity checks */
	if (unlikely(replychunk))
		return -EIO;

	/* Build the RPC reply's Payload stream in rqst->rq_rcv_buf */
	base = (char *)xdr_inline_decode(xdr, 0);
	rpclen = xdr_stream_remaining(xdr);
	r_xprt->rx_stats.fixup_copy_count +=
		rpcrdma_inline_fixup(rqst, base, rpclen, writelist & 3);

	r_xprt->rx_stats.total_rdma_reply += writelist;
	return rpclen + xdr_align_size(writelist);
}

static noinline int
rpcrdma_decode_nomsg(struct rpcrdma_xprt *r_xprt, struct rpcrdma_rep *rep)
{
	struct xdr_stream *xdr = &rep->rr_stream;
	u32 writelist, replychunk;

	/* Decode the chunk lists */
	if (decode_read_list(xdr))
		return -EIO;
	if (decode_write_list(xdr, &writelist))
		return -EIO;
	if (decode_reply_chunk(xdr, &replychunk))
		return -EIO;

	/* RDMA_NOMSG sanity checks */
	if (unlikely(writelist))
		return -EIO;
	if (unlikely(!replychunk))
		return -EIO;

	/* Reply chunk buffer already is the reply vector */
	r_xprt->rx_stats.total_rdma_reply += replychunk;
	return replychunk;
}

static noinline int
rpcrdma_decode_error(struct rpcrdma_xprt *r_xprt, struct rpcrdma_rep *rep,
		     struct rpc_rqst *rqst)
{
	struct xdr_stream *xdr = &rep->rr_stream;
	__be32 *p;

	p = xdr_inline_decode(xdr, sizeof(*p));
	if (unlikely(!p))
		return -EIO;

	switch (*p) {
	case err_vers:
		p = xdr_inline_decode(xdr, 2 * sizeof(*p));
		if (!p)
			break;
		trace_xprtrdma_err_vers(rqst, p, p + 1);
		break;
	case err_chunk:
		trace_xprtrdma_err_chunk(rqst);
		break;
	default:
		trace_xprtrdma_err_unrecognized(rqst, p);
	}

	return -EIO;
}

/**
 * rpcrdma_unpin_rqst - Release rqst without completing it
 * @rep: RPC/RDMA Receive context
 *
 * This is done when a connection is lost so that a Reply
 * can be dropped and its matching Call can be subsequently
 * retransmitted on a new connection.
 */
void rpcrdma_unpin_rqst(struct rpcrdma_rep *rep)
{
	struct rpc_xprt *xprt = &rep->rr_rxprt->rx_xprt;
	struct rpc_rqst *rqst = rep->rr_rqst;
	struct rpcrdma_req *req = rpcr_to_rdmar(rqst);

	req->rl_reply = NULL;
	rep->rr_rqst = NULL;

	spin_lock(&xprt->queue_lock);
	xprt_unpin_rqst(rqst);
	spin_unlock(&xprt->queue_lock);
}

/**
 * rpcrdma_complete_rqst - Pass completed rqst back to RPC
 * @rep: RPC/RDMA Receive context
 *
 * Reconstruct the RPC reply and complete the transaction
 * while @rqst is still pinned to ensure the rep, rqst, and
 * rq_task pointers remain stable.
 */
void rpcrdma_complete_rqst(struct rpcrdma_rep *rep)
{
	struct rpcrdma_xprt *r_xprt = rep->rr_rxprt;
	struct rpc_xprt *xprt = &r_xprt->rx_xprt;
	struct rpc_rqst *rqst = rep->rr_rqst;
	int status;

	/* I3: rl_registered has been drained by frwr_unmap before
	 * complete_rqst runs.
	 */
	WARN_ON_ONCE(!list_empty(&rpcr_to_rdmar(rqst)->rl_registered));

	switch (rep->rr_proc) {
	case rdma_msg:
		status = rpcrdma_decode_msg(r_xprt, rep, rqst);
		break;
	case rdma_nomsg:
		status = rpcrdma_decode_nomsg(r_xprt, rep);
		break;
	case rdma_error:
		status = rpcrdma_decode_error(r_xprt, rep, rqst);
		break;
	default:
		status = -EIO;
	}
	if (status < 0)
		goto out_badheader;

out:
	spin_lock(&xprt->queue_lock);
	xprt_complete_rqst(rqst->rq_task, status);
	xprt_unpin_rqst(rqst);
	spin_unlock(&xprt->queue_lock);
	return;

out_badheader:
	trace_xprtrdma_reply_hdr_err(rep);
	r_xprt->rx_stats.bad_reply_count++;
	rqst->rq_task->tk_status = status;
	status = 0;
	goto out;
}

/* Reply-side ownership invariants
 *
 * I1 (Receive WR ownership).  A struct rpcrdma_rep is owned by the
 *    HCA between ib_post_recv() and the matching Receive completion.
 *    After ib_dma_sync_single_for_cpu() in rpcrdma_wc_receive() it is
 *    owned by the CPU until rpcrdma_rep_put() returns it to
 *    rb_free_reps; a rep on rb_free_reps is not re-posted until
 *    rpcrdma_post_recvs() pulls it off.  Asserted: rpcrdma_post_recvs()
 *    WARNs that a pulled rep has rr_rqst == NULL.
 *
 * I2 (rep attachment).  While req->rl_reply == rep, the rep cannot be
 *    re-posted.  rpcrdma_reply_put() NULLs req->rl_reply before handing
 *    the rep to rpcrdma_rep_put().  Asserted: rpcrdma_reply_put() WARNs
 *    that rl_reply is NULL after the put.
 *
 * I3 (Registered-MR fence).  On entry to rpcrdma_complete_rqst() every
 *    MR that was on req->rl_registered has had its rkey invalidated
 *    (remotely via IB_WC_WITH_INVALIDATE or locally via IB_WR_LOCAL_INV)
 *    and its pages ib_dma_unmap_sg()'d.  The LocalInv chain is posted
 *    on a single QP; strong send-queue ordering makes the last
 *    completion (frwr_wc_localinv_done) observe the
 *    ib_dma_unmap_sg() that ran from each earlier completion's
 *    frwr_mr_put() before complete_rqst is called.  The inline
 *    frwr_reminv() path unmaps its one MR synchronously before
 *    rpcrdma_reply_handler() reaches complete_rqst.  Asserted:
 *    rpcrdma_complete_rqst() WARNs that rl_registered is empty.
 *
 * I4 (Send-buffer release).  req->rl_kref carries two unconditional
 *    owners while a Send is outstanding: the RPC-layer reference (set
 *    at xprt_rdma_alloc_slot / xprt_rdma_bc_rqst_get / rpcrdma_req_release
 *    pool-entry) and the Send-side reference (kref_get() in
 *    rpcrdma_prepare_send_sges()).  rpcrdma_req_release() runs only
 *    after both have dropped, so the req does not return to its free
 *    pool until rpcrdma_sendctx_unmap() has fired -- the HCA has
 *    released the send buffer before the req can be reused.  Asserted:
 *    rpcrdma_req_release() WARNs that rl_sendctx is NULL.
 *
 * I5 (req lifecycle).  A req is owned by the RPC layer between slot
 *    acquisition and the matching xprt_rdma_free_slot() (or, for the
 *    backchannel, xprt_rdma_bc_free_rqst()).  While owned, rl_kref >= 1.
 *    The pools (rb_send_bufs, bc_pa_list, backlog wake target) never
 *    contain a req with outstanding Send-side or Reply-side work.
 *
 * Non-hazards.  The following claims have been raised by adversarial
 * review and are each closed by the invariants above:
 *
 *   * "Reply completes the RPC while the HCA still holds the send
 *     buffer" -- excluded by I4.  The Send-side kref reference is held
 *     until rpcrdma_sendctx_unmap() runs from Send completion.
 *
 *   * "Signal-driven release races the in-flight Send" -- same
 *     resolution.  xprt_rdma_free() does not touch rl_kref; the
 *     Send-side reference keeps the req out of its pool until Send
 *     completion fires.
 *
 *   * "Receive completion races rep reuse" -- excluded by I1.  A rep
 *     is on rb_free_reps only after rpcrdma_rep_put() has been called
 *     and rpcrdma_post_recvs() owns the next transition back to the HCA.
 *
 *   * "Pages still DMA-mapped when call_decode reads them" -- excluded
 *     by I3.  The matching ib_dma_unmap_sg() for every MR has run on
 *     the same CPU thread that calls rpcrdma_complete_rqst().
 */

/**
 * rpcrdma_reply_handler - Process received RPC/RDMA messages
 * @rep: Incoming rpcrdma_rep object to process
 *
 * Errors must result in the RPC task either being awakened, or
 * allowed to timeout, to discover the errors at that time.
 */
void rpcrdma_reply_handler(struct rpcrdma_rep *rep)
{
	struct rpcrdma_xprt *r_xprt = rep->rr_rxprt;
	struct rpc_xprt *xprt = &r_xprt->rx_xprt;
	struct rpcrdma_buffer *buf = &r_xprt->rx_buf;
	struct rpcrdma_req *req;
	struct rpc_rqst *rqst;
	u32 credits;
	__be32 *p;

	/* Any data means we had a useful conversation, so
	 * then we don't need to delay the next reconnect.
	 */
	if (xprt->reestablish_timeout)
		xprt->reestablish_timeout = 0;

	/* Fixed transport header fields */
	xdr_init_decode(&rep->rr_stream, &rep->rr_hdrbuf,
			rep->rr_hdrbuf.head[0].iov_base, NULL);
	p = xdr_inline_decode(&rep->rr_stream, 4 * sizeof(*p));
	if (unlikely(!p))
		goto out_shortreply;
	rep->rr_xid = *p++;
	rep->rr_vers = *p++;
	credits = be32_to_cpu(*p++);
	rep->rr_proc = *p++;

	/* The credit grant from the wire is not trustworthy;
	 * sanitize it before any code path consumes it.
	 */
	if (credits == 0)
		credits = 1;	/* don't deadlock */
	else if (credits > r_xprt->rx_ep->re_max_requests)
		credits = r_xprt->rx_ep->re_max_requests;

	if (rep->rr_vers != rpcrdma_version)
		goto out_badversion;

	if (rpcrdma_is_bcall(r_xprt, rep))
		return;

	/* Match incoming rpcrdma_rep to an rpcrdma_req to
	 * get context for handling any incoming chunks.
	 */
	spin_lock(&xprt->queue_lock);
	rqst = xprt_lookup_rqst(xprt, rep->rr_xid);
	if (!rqst)
		goto out_norqst;
	xprt_pin_rqst(rqst);
	spin_unlock(&xprt->queue_lock);

	if (buf->rb_credits != credits)
		rpcrdma_update_cwnd(r_xprt, credits);

	req = rpcr_to_rdmar(rqst);
	if (unlikely(req->rl_reply))
		rpcrdma_rep_put(buf, req->rl_reply);
	req->rl_reply = rep;
	rep->rr_rqst = rqst;

	trace_xprtrdma_reply(rqst->rq_task, rep, credits);

	if (rep->rr_wc_flags & IB_WC_WITH_INVALIDATE)
		frwr_reminv(rep, &req->rl_registered);
	if (!list_empty(&req->rl_registered))
		frwr_unmap_async(r_xprt, req);
		/* LocalInv completion will complete the RPC */
	else
		rpcrdma_complete_rqst(rep);

out_post:
	rpcrdma_post_recvs(r_xprt,
			   credits + (buf->rb_bc_srv_max_requests << 1));
	return;

out_norqst:
	spin_unlock(&xprt->queue_lock);
	trace_xprtrdma_reply_rqst_err(rep);
	rpcrdma_rep_put(buf, rep);
	goto out_post;

out_badversion:
	trace_xprtrdma_reply_vers_err(rep);
	rpcrdma_rep_put(buf, rep);
	credits = buf->rb_credits;
	goto out_post;

out_shortreply:
	trace_xprtrdma_reply_short_err(rep);
	rpcrdma_rep_put(buf, rep);
	credits = buf->rb_credits;
	goto out_post;
}