[linux.git] / net / sunrpc / xprtrdma / svc_rdma_sendto.c

// SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause
/*
 * Copyright (c) 2016-2018 Oracle. All rights reserved.
 * Copyright (c) 2014 Open Grid Computing, Inc. All rights reserved.
 * Copyright (c) 2005-2006 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.
 *
 * Author: Tom Tucker <tom@opengridcomputing.com>
 */

/* Operation
 *
 * The main entry point is svc_rdma_sendto. This is called by the
 * RPC server when an RPC Reply is ready to be transmitted to a client.
 *
 * The passed-in svc_rqst contains a struct xdr_buf which holds an
 * XDR-encoded RPC Reply message. sendto must construct the RPC-over-RDMA
 * transport header, post all Write WRs needed for this Reply, then post
 * a Send WR conveying the transport header and the RPC message itself to
 * the client.
 *
 * svc_rdma_sendto must fully transmit the Reply before returning, as
 * the svc_rqst will be recycled as soon as sendto returns. Remaining
 * resources referred to by the svc_rqst are also recycled at that time.
 * Therefore any resources that must remain longer must be detached
 * from the svc_rqst and released later.
 *
 * Page Management
 *
 * The I/O that performs Reply transmission is asynchronous, and may
 * complete well after sendto returns. Thus pages under I/O must be
 * removed from the svc_rqst before sendto returns.
 *
 * The logic here depends on Send Queue and completion ordering. Since
 * the Send WR is always posted last, it will always complete last. Thus
 * when it completes, it is guaranteed that all previous Write WRs have
 * also completed.
 *
 * Write WRs are constructed and posted. Each Write segment gets its own
 * svc_rdma_rw_ctxt, allowing the Write completion handler to find and
 * DMA-unmap the pages under I/O for that Write segment. The Write
 * completion handler does not release any pages.
 *
 * When the Send WR is constructed, it also gets its own svc_rdma_send_ctxt.
 * The ownership of all of the Reply's pages are transferred into that
 * ctxt, the Send WR is posted, and sendto returns.
 *
 * The svc_rdma_send_ctxt is presented when the Send WR completes. The
 * Send completion handler finally releases the Reply's pages.
 *
 * This mechanism also assumes that completions on the transport's Send
 * Completion Queue do not run in parallel. Otherwise a Write completion
 * and Send completion running at the same time could release pages that
 * are still DMA-mapped.
 *
 * Error Handling
 *
 * - If the Send WR is posted successfully, it will either complete
 *   successfully, or get flushed. Either way, the Send completion
 *   handler releases the Reply's pages.
 * - If the Send WR cannot be not posted, the forward path releases
 *   the Reply's pages.
 *
 * This handles the case, without the use of page reference counting,
 * where two different Write segments send portions of the same page.
 */

#include <linux/spinlock.h>
#include <asm/unaligned.h>

#include <rdma/ib_verbs.h>
#include <rdma/rdma_cm.h>

#include <linux/sunrpc/debug.h>
#include <linux/sunrpc/rpc_rdma.h>
#include <linux/sunrpc/svc_rdma.h>

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

#define RPCDBG_FACILITY RPCDBG_SVCXPRT

static void svc_rdma_wc_send(struct ib_cq *cq, struct ib_wc *wc);

static inline struct svc_rdma_send_ctxt *
svc_rdma_next_send_ctxt(struct list_head *list)
{
        return list_first_entry_or_null(list, struct svc_rdma_send_ctxt,
                                        sc_list);
}

static struct svc_rdma_send_ctxt *
svc_rdma_send_ctxt_alloc(struct svcxprt_rdma *rdma)
{
        struct svc_rdma_send_ctxt *ctxt;
        dma_addr_t addr;
        void *buffer;
        size_t size;
        int i;

        size = sizeof(*ctxt);
        size += rdma->sc_max_send_sges * sizeof(struct ib_sge);
        ctxt = kmalloc(size, GFP_KERNEL);
        if (!ctxt)
                goto fail0;
        buffer = kmalloc(rdma->sc_max_req_size, GFP_KERNEL);
        if (!buffer)
                goto fail1;
        addr = ib_dma_map_single(rdma->sc_pd->device, buffer,
                                 rdma->sc_max_req_size, DMA_TO_DEVICE);
        if (ib_dma_mapping_error(rdma->sc_pd->device, addr))
                goto fail2;

        ctxt->sc_send_wr.next = NULL;
        ctxt->sc_send_wr.wr_cqe = &ctxt->sc_cqe;
        ctxt->sc_send_wr.sg_list = ctxt->sc_sges;
        ctxt->sc_send_wr.send_flags = IB_SEND_SIGNALED;
        ctxt->sc_cqe.done = svc_rdma_wc_send;
        ctxt->sc_xprt_buf = buffer;
        ctxt->sc_sges[0].addr = addr;

        for (i = 0; i < rdma->sc_max_send_sges; i++)
                ctxt->sc_sges[i].lkey = rdma->sc_pd->local_dma_lkey;
        return ctxt;

fail2:
        kfree(buffer);
fail1:
        kfree(ctxt);
fail0:
        return NULL;
}

/**
 * svc_rdma_send_ctxts_destroy - Release all send_ctxt's for an xprt
 * @rdma: svcxprt_rdma being torn down
 *
 */
void svc_rdma_send_ctxts_destroy(struct svcxprt_rdma *rdma)
{
        struct svc_rdma_send_ctxt *ctxt;

        while ((ctxt = svc_rdma_next_send_ctxt(&rdma->sc_send_ctxts))) {
                list_del(&ctxt->sc_list);
                ib_dma_unmap_single(rdma->sc_pd->device,
                                    ctxt->sc_sges[0].addr,
                                    rdma->sc_max_req_size,
                                    DMA_TO_DEVICE);
                kfree(ctxt->sc_xprt_buf);
                kfree(ctxt);
        }
}

/**
 * svc_rdma_send_ctxt_get - Get a free send_ctxt
 * @rdma: controlling svcxprt_rdma
 *
 * Returns a ready-to-use send_ctxt, or NULL if none are
 * available and a fresh one cannot be allocated.
 */
struct svc_rdma_send_ctxt *svc_rdma_send_ctxt_get(struct svcxprt_rdma *rdma)
{
        struct svc_rdma_send_ctxt *ctxt;

        spin_lock(&rdma->sc_send_lock);
        ctxt = svc_rdma_next_send_ctxt(&rdma->sc_send_ctxts);
        if (!ctxt)
                goto out_empty;
        list_del(&ctxt->sc_list);
        spin_unlock(&rdma->sc_send_lock);

out:
        ctxt->sc_send_wr.num_sge = 0;
        ctxt->sc_cur_sge_no = 0;
        ctxt->sc_page_count = 0;
        return ctxt;

out_empty:
        spin_unlock(&rdma->sc_send_lock);
        ctxt = svc_rdma_send_ctxt_alloc(rdma);
        if (!ctxt)
                return NULL;
        goto out;
}

/**
 * svc_rdma_send_ctxt_put - Return send_ctxt to free list
 * @rdma: controlling svcxprt_rdma
 * @ctxt: object to return to the free list
 *
 * Pages left in sc_pages are DMA unmapped and released.
 */
void svc_rdma_send_ctxt_put(struct svcxprt_rdma *rdma,
                            struct svc_rdma_send_ctxt *ctxt)
{
        struct ib_device *device = rdma->sc_cm_id->device;
        unsigned int i;

        /* The first SGE contains the transport header, which
         * remains mapped until @ctxt is destroyed.
         */
        for (i = 1; i < ctxt->sc_send_wr.num_sge; i++)
                ib_dma_unmap_page(device,
                                  ctxt->sc_sges[i].addr,
                                  ctxt->sc_sges[i].length,
                                  DMA_TO_DEVICE);

        for (i = 0; i < ctxt->sc_page_count; ++i)
                put_page(ctxt->sc_pages[i]);

        spin_lock(&rdma->sc_send_lock);
        list_add(&ctxt->sc_list, &rdma->sc_send_ctxts);
        spin_unlock(&rdma->sc_send_lock);
}

/**
 * svc_rdma_wc_send - Invoked by RDMA provider for each polled Send WC
 * @cq: Completion Queue context
 * @wc: Work Completion object
 *
 * NB: The svc_xprt/svcxprt_rdma is pinned whenever it's possible that
 * the Send completion handler could be running.
 */
static void svc_rdma_wc_send(struct ib_cq *cq, struct ib_wc *wc)
{
        struct svcxprt_rdma *rdma = cq->cq_context;
        struct ib_cqe *cqe = wc->wr_cqe;
        struct svc_rdma_send_ctxt *ctxt;

        trace_svcrdma_wc_send(wc);

        atomic_inc(&rdma->sc_sq_avail);
        wake_up(&rdma->sc_send_wait);

        ctxt = container_of(cqe, struct svc_rdma_send_ctxt, sc_cqe);
        svc_rdma_send_ctxt_put(rdma, ctxt);

        if (unlikely(wc->status != IB_WC_SUCCESS)) {
                set_bit(XPT_CLOSE, &rdma->sc_xprt.xpt_flags);
                svc_xprt_enqueue(&rdma->sc_xprt);
                if (wc->status != IB_WC_WR_FLUSH_ERR)
                        pr_err("svcrdma: Send: %s (%u/0x%x)\n",
                               ib_wc_status_msg(wc->status),
                               wc->status, wc->vendor_err);
        }

        svc_xprt_put(&rdma->sc_xprt);
}

/**
 * svc_rdma_send - Post a single Send WR
 * @rdma: transport on which to post the WR
 * @wr: prepared Send WR to post
 *
 * Returns zero the Send WR was posted successfully. Otherwise, a
 * negative errno is returned.
 */
int svc_rdma_send(struct svcxprt_rdma *rdma, struct ib_send_wr *wr)
{
        int ret;

        might_sleep();

        /* If the SQ is full, wait until an SQ entry is available */
        while (1) {
                if ((atomic_dec_return(&rdma->sc_sq_avail) < 0)) {
                        atomic_inc(&rdma_stat_sq_starve);
                        trace_svcrdma_sq_full(rdma);
                        atomic_inc(&rdma->sc_sq_avail);
                        wait_event(rdma->sc_send_wait,
                                   atomic_read(&rdma->sc_sq_avail) > 1);
                        if (test_bit(XPT_CLOSE, &rdma->sc_xprt.xpt_flags))
                                return -ENOTCONN;
                        trace_svcrdma_sq_retry(rdma);
                        continue;
                }

                svc_xprt_get(&rdma->sc_xprt);
                ret = ib_post_send(rdma->sc_qp, wr, NULL);
                trace_svcrdma_post_send(wr, ret);
                if (ret) {
                        set_bit(XPT_CLOSE, &rdma->sc_xprt.xpt_flags);
                        svc_xprt_put(&rdma->sc_xprt);
                        wake_up(&rdma->sc_send_wait);
                }
                break;
        }
        return ret;
}

static u32 xdr_padsize(u32 len)
{
        return (len & 3) ? (4 - (len & 3)) : 0;
}

/* Returns length of transport header, in bytes.
 */
static unsigned int svc_rdma_reply_hdr_len(__be32 *rdma_resp)
{
        unsigned int nsegs;
        __be32 *p;

        p = rdma_resp;

        /* RPC-over-RDMA V1 replies never have a Read list. */
        p += rpcrdma_fixed_maxsz + 1;

        /* Skip Write list. */
        while (*p++ != xdr_zero) {
                nsegs = be32_to_cpup(p++);
                p += nsegs * rpcrdma_segment_maxsz;
        }

        /* Skip Reply chunk. */
        if (*p++ != xdr_zero) {
                nsegs = be32_to_cpup(p++);
                p += nsegs * rpcrdma_segment_maxsz;
        }

        return (unsigned long)p - (unsigned long)rdma_resp;
}

/* One Write chunk is copied from Call transport header to Reply
 * transport header. Each segment's length field is updated to
 * reflect number of bytes consumed in the segment.
 *
 * Returns number of segments in this chunk.
 */
static unsigned int xdr_encode_write_chunk(__be32 *dst, __be32 *src,
                                           unsigned int remaining)
{
        unsigned int i, nsegs;
        u32 seg_len;

        /* Write list discriminator */
        *dst++ = *src++;

        /* number of segments in this chunk */
        nsegs = be32_to_cpup(src);
        *dst++ = *src++;

        for (i = nsegs; i; i--) {
                /* segment's RDMA handle */
                *dst++ = *src++;

                /* bytes returned in this segment */
                seg_len = be32_to_cpu(*src);
                if (remaining >= seg_len) {
                        /* entire segment was consumed */
                        *dst = *src;
                        remaining -= seg_len;
                } else {
                        /* segment only partly filled */
                        *dst = cpu_to_be32(remaining);
                        remaining = 0;
                }
                dst++; src++;

                /* segment's RDMA offset */
                *dst++ = *src++;
                *dst++ = *src++;
        }

        return nsegs;
}

/* The client provided a Write list in the Call message. Fill in
 * the segments in the first Write chunk in the Reply's transport
 * header with the number of bytes consumed in each segment.
 * Remaining chunks are returned unused.
 *
 * Assumptions:
 *  - Client has provided only one Write chunk
 */
static void svc_rdma_xdr_encode_write_list(__be32 *rdma_resp, __be32 *wr_ch,
                                           unsigned int consumed)
{
        unsigned int nsegs;
        __be32 *p, *q;

        /* RPC-over-RDMA V1 replies never have a Read list. */
        p = rdma_resp + rpcrdma_fixed_maxsz + 1;

        q = wr_ch;
        while (*q != xdr_zero) {
                nsegs = xdr_encode_write_chunk(p, q, consumed);
                q += 2 + nsegs * rpcrdma_segment_maxsz;
                p += 2 + nsegs * rpcrdma_segment_maxsz;
                consumed = 0;
        }

        /* Terminate Write list */
        *p++ = xdr_zero;

        /* Reply chunk discriminator; may be replaced later */
        *p = xdr_zero;
}

/* The client provided a Reply chunk in the Call message. Fill in
 * the segments in the Reply chunk in the Reply message with the
 * number of bytes consumed in each segment.
 *
 * Assumptions:
 * - Reply can always fit in the provided Reply chunk
 */
static void svc_rdma_xdr_encode_reply_chunk(__be32 *rdma_resp, __be32 *rp_ch,
                                            unsigned int consumed)
{
        __be32 *p;

        /* Find the Reply chunk in the Reply's xprt header.
         * RPC-over-RDMA V1 replies never have a Read list.
         */
        p = rdma_resp + rpcrdma_fixed_maxsz + 1;

        /* Skip past Write list */
        while (*p++ != xdr_zero)
                p += 1 + be32_to_cpup(p) * rpcrdma_segment_maxsz;

        xdr_encode_write_chunk(p, rp_ch, consumed);
}

/* Parse the RPC Call's transport header.
 */
static void svc_rdma_get_write_arrays(__be32 *rdma_argp,
                                      __be32 **write, __be32 **reply)
{
        __be32 *p;

        p = rdma_argp + rpcrdma_fixed_maxsz;

        /* Read list */
        while (*p++ != xdr_zero)
                p += 5;

        /* Write list */
        if (*p != xdr_zero) {
                *write = p;
                while (*p++ != xdr_zero)
                        p += 1 + be32_to_cpu(*p) * 4;
        } else {
                *write = NULL;
                p++;
        }

        /* Reply chunk */
        if (*p != xdr_zero)
                *reply = p;
        else
                *reply = NULL;
}

/* RPC-over-RDMA Version One private extension: Remote Invalidation.
 * Responder's choice: requester signals it can handle Send With
 * Invalidate, and responder chooses one rkey to invalidate.
 *
 * Find a candidate rkey to invalidate when sending a reply.  Picks the
 * first R_key it finds in the chunk lists.
 *
 * Returns zero if RPC's chunk lists are empty.
 */
static u32 svc_rdma_get_inv_rkey(__be32 *rdma_argp,
                                 __be32 *wr_lst, __be32 *rp_ch)
{
        __be32 *p;

        p = rdma_argp + rpcrdma_fixed_maxsz;
        if (*p != xdr_zero)
                p += 2;
        else if (wr_lst && be32_to_cpup(wr_lst + 1))
                p = wr_lst + 2;
        else if (rp_ch && be32_to_cpup(rp_ch + 1))
                p = rp_ch + 2;
        else
                return 0;
        return be32_to_cpup(p);
}

static int svc_rdma_dma_map_page(struct svcxprt_rdma *rdma,
                                 struct svc_rdma_send_ctxt *ctxt,
                                 struct page *page,
                                 unsigned long offset,
                                 unsigned int len)
{
        struct ib_device *dev = rdma->sc_cm_id->device;
        dma_addr_t dma_addr;

        dma_addr = ib_dma_map_page(dev, page, offset, len, DMA_TO_DEVICE);
        if (ib_dma_mapping_error(dev, dma_addr))
                goto out_maperr;

        ctxt->sc_sges[ctxt->sc_cur_sge_no].addr = dma_addr;
        ctxt->sc_sges[ctxt->sc_cur_sge_no].length = len;
        ctxt->sc_send_wr.num_sge++;
        return 0;

out_maperr:
        trace_svcrdma_dma_map_page(rdma, page);
        return -EIO;
}

/* ib_dma_map_page() is used here because svc_rdma_dma_unmap()
 * handles DMA-unmap and it uses ib_dma_unmap_page() exclusively.
 */
static int svc_rdma_dma_map_buf(struct svcxprt_rdma *rdma,
                                struct svc_rdma_send_ctxt *ctxt,
                                unsigned char *base,
                                unsigned int len)
{
        return svc_rdma_dma_map_page(rdma, ctxt, virt_to_page(base),
                                     offset_in_page(base), len);
}

/**
 * svc_rdma_sync_reply_hdr - DMA sync the transport header buffer
 * @rdma: controlling transport
 * @ctxt: send_ctxt for the Send WR
 * @len: length of transport header
 *
 */
void svc_rdma_sync_reply_hdr(struct svcxprt_rdma *rdma,
                             struct svc_rdma_send_ctxt *ctxt,
                             unsigned int len)
{
        ctxt->sc_sges[0].length = len;
        ctxt->sc_send_wr.num_sge++;
        ib_dma_sync_single_for_device(rdma->sc_pd->device,
                                      ctxt->sc_sges[0].addr, len,
                                      DMA_TO_DEVICE);
}

/* svc_rdma_map_reply_msg - Map the buffer holding RPC message
 * @rdma: controlling transport
 * @ctxt: send_ctxt for the Send WR
 * @xdr: prepared xdr_buf containing RPC message
 * @wr_lst: pointer to Call header's Write list, or NULL
 *
 * Load the xdr_buf into the ctxt's sge array, and DMA map each
 * element as it is added.
 *
 * Returns zero on success, or a negative errno on failure.
 */
int svc_rdma_map_reply_msg(struct svcxprt_rdma *rdma,
                           struct svc_rdma_send_ctxt *ctxt,
                           struct xdr_buf *xdr, __be32 *wr_lst)
{
        unsigned int len, remaining;
        unsigned long page_off;
        struct page **ppages;
        unsigned char *base;
        u32 xdr_pad;
        int ret;

        if (++ctxt->sc_cur_sge_no >= rdma->sc_max_send_sges)
                return -EIO;
        ret = svc_rdma_dma_map_buf(rdma, ctxt,
                                   xdr->head[0].iov_base,
                                   xdr->head[0].iov_len);
        if (ret < 0)
                return ret;

        /* If a Write chunk is present, the xdr_buf's page list
         * is not included inline. However the Upper Layer may
         * have added XDR padding in the tail buffer, and that
         * should not be included inline.
         */
        if (wr_lst) {
                base = xdr->tail[0].iov_base;
                len = xdr->tail[0].iov_len;
                xdr_pad = xdr_padsize(xdr->page_len);

                if (len && xdr_pad) {
                        base += xdr_pad;
                        len -= xdr_pad;
                }

                goto tail;
        }

        ppages = xdr->pages + (xdr->page_base >> PAGE_SHIFT);
        page_off = xdr->page_base & ~PAGE_MASK;
        remaining = xdr->page_len;
        while (remaining) {
                len = min_t(u32, PAGE_SIZE - page_off, remaining);

                if (++ctxt->sc_cur_sge_no >= rdma->sc_max_send_sges)
                        return -EIO;
                ret = svc_rdma_dma_map_page(rdma, ctxt, *ppages++,
                                            page_off, len);
                if (ret < 0)
                        return ret;

                remaining -= len;
                page_off = 0;
        }

        base = xdr->tail[0].iov_base;
        len = xdr->tail[0].iov_len;
tail:
        if (len) {
                if (++ctxt->sc_cur_sge_no >= rdma->sc_max_send_sges)
                        return -EIO;
                ret = svc_rdma_dma_map_buf(rdma, ctxt, base, len);
                if (ret < 0)
                        return ret;
        }

        return 0;
}

/* The svc_rqst and all resources it owns are released as soon as
 * svc_rdma_sendto returns. Transfer pages under I/O to the ctxt
 * so they are released by the Send completion handler.
 */
static void svc_rdma_save_io_pages(struct svc_rqst *rqstp,
                                   struct svc_rdma_send_ctxt *ctxt)
{
        int i, pages = rqstp->rq_next_page - rqstp->rq_respages;

        ctxt->sc_page_count += pages;
        for (i = 0; i < pages; i++) {
                ctxt->sc_pages[i] = rqstp->rq_respages[i];
                rqstp->rq_respages[i] = NULL;
        }

        /* Prevent svc_xprt_release from releasing pages in rq_pages */
        rqstp->rq_next_page = rqstp->rq_respages;
}

/* Prepare the portion of the RPC Reply that will be transmitted
 * via RDMA Send. The RPC-over-RDMA transport header is prepared
 * in sc_sges[0], and the RPC xdr_buf is prepared in following sges.
 *
 * Depending on whether a Write list or Reply chunk is present,
 * the server may send all, a portion of, or none of the xdr_buf.
 * In the latter case, only the transport header (sc_sges[0]) is
 * transmitted.
 *
 * RDMA Send is the last step of transmitting an RPC reply. Pages
 * involved in the earlier RDMA Writes are here transferred out
 * of the rqstp and into the ctxt's page array. These pages are
 * DMA unmapped by each Write completion, but the subsequent Send
 * completion finally releases these pages.
 *
 * Assumptions:
 * - The Reply's transport header will never be larger than a page.
 */
static int svc_rdma_send_reply_msg(struct svcxprt_rdma *rdma,
                                   struct svc_rdma_send_ctxt *ctxt,
                                   __be32 *rdma_argp,
                                   struct svc_rqst *rqstp,
                                   __be32 *wr_lst, __be32 *rp_ch)
{
        int ret;

        if (!rp_ch) {
                ret = svc_rdma_map_reply_msg(rdma, ctxt,
                                             &rqstp->rq_res, wr_lst);
                if (ret < 0)
                        return ret;
        }

        svc_rdma_save_io_pages(rqstp, ctxt);

        ctxt->sc_send_wr.opcode = IB_WR_SEND;
        if (rdma->sc_snd_w_inv) {
                ctxt->sc_send_wr.ex.invalidate_rkey =
                        svc_rdma_get_inv_rkey(rdma_argp, wr_lst, rp_ch);
                if (ctxt->sc_send_wr.ex.invalidate_rkey)
                        ctxt->sc_send_wr.opcode = IB_WR_SEND_WITH_INV;
        }
        dprintk("svcrdma: posting Send WR with %u sge(s)\n",
                ctxt->sc_send_wr.num_sge);
        return svc_rdma_send(rdma, &ctxt->sc_send_wr);
}

/* Given the client-provided Write and Reply chunks, the server was not
 * able to form a complete reply. Return an RDMA_ERROR message so the
 * client can retire this RPC transaction. As above, the Send completion
 * routine releases payload pages that were part of a previous RDMA Write.
 *
 * Remote Invalidation is skipped for simplicity.
 */
static int svc_rdma_send_error_msg(struct svcxprt_rdma *rdma,
                                   struct svc_rdma_send_ctxt *ctxt,
                                   struct svc_rqst *rqstp)
{
        __be32 *p;
        int ret;

        p = ctxt->sc_xprt_buf;
        trace_svcrdma_err_chunk(*p);
        p += 3;
        *p++ = rdma_error;
        *p   = err_chunk;
        svc_rdma_sync_reply_hdr(rdma, ctxt, RPCRDMA_HDRLEN_ERR);

        svc_rdma_save_io_pages(rqstp, ctxt);

        ctxt->sc_send_wr.opcode = IB_WR_SEND;
        ret = svc_rdma_send(rdma, &ctxt->sc_send_wr);
        if (ret) {
                svc_rdma_send_ctxt_put(rdma, ctxt);
                return ret;
        }

        return 0;
}

void svc_rdma_prep_reply_hdr(struct svc_rqst *rqstp)
{
}

/**
 * svc_rdma_sendto - Transmit an RPC reply
 * @rqstp: processed RPC request, reply XDR already in ::rq_res
 *
 * Any resources still associated with @rqstp are released upon return.
 * If no reply message was possible, the connection is closed.
 *
 * Returns:
 *      %0 if an RPC reply has been successfully posted,
 *      %-ENOMEM if a resource shortage occurred (connection is lost),
 *      %-ENOTCONN if posting failed (connection is lost).
 */
int svc_rdma_sendto(struct svc_rqst *rqstp)
{
        struct svc_xprt *xprt = rqstp->rq_xprt;
        struct svcxprt_rdma *rdma =
                container_of(xprt, struct svcxprt_rdma, sc_xprt);
        struct svc_rdma_recv_ctxt *rctxt = rqstp->rq_xprt_ctxt;
        __be32 *p, *rdma_argp, *rdma_resp, *wr_lst, *rp_ch;
        struct xdr_buf *xdr = &rqstp->rq_res;
        struct svc_rdma_send_ctxt *sctxt;
        int ret;

        rdma_argp = rctxt->rc_recv_buf;
        svc_rdma_get_write_arrays(rdma_argp, &wr_lst, &rp_ch);

        /* Create the RDMA response header. xprt->xpt_mutex,
         * acquired in svc_send(), serializes RPC replies. The
         * code path below that inserts the credit grant value
         * into each transport header runs only inside this
         * critical section.
         */
        ret = -ENOMEM;
        sctxt = svc_rdma_send_ctxt_get(rdma);
        if (!sctxt)
                goto err0;
        rdma_resp = sctxt->sc_xprt_buf;

        p = rdma_resp;
        *p++ = *rdma_argp;
        *p++ = *(rdma_argp + 1);
        *p++ = rdma->sc_fc_credits;
        *p++ = rp_ch ? rdma_nomsg : rdma_msg;

        /* Start with empty chunks */
        *p++ = xdr_zero;
        *p++ = xdr_zero;
        *p   = xdr_zero;

        if (wr_lst) {
                /* XXX: Presume the client sent only one Write chunk */
                ret = svc_rdma_send_write_chunk(rdma, wr_lst, xdr);
                if (ret < 0)
                        goto err2;
                svc_rdma_xdr_encode_write_list(rdma_resp, wr_lst, ret);
        }
        if (rp_ch) {
                ret = svc_rdma_send_reply_chunk(rdma, rp_ch, wr_lst, xdr);
                if (ret < 0)
                        goto err2;
                svc_rdma_xdr_encode_reply_chunk(rdma_resp, rp_ch, ret);
        }

        svc_rdma_sync_reply_hdr(rdma, sctxt, svc_rdma_reply_hdr_len(rdma_resp));
        ret = svc_rdma_send_reply_msg(rdma, sctxt, rdma_argp, rqstp,
                                      wr_lst, rp_ch);
        if (ret < 0)
                goto err1;
        ret = 0;

out:
        rqstp->rq_xprt_ctxt = NULL;
        svc_rdma_recv_ctxt_put(rdma, rctxt);
        return ret;

 err2:
        if (ret != -E2BIG && ret != -EINVAL)
                goto err1;

        ret = svc_rdma_send_error_msg(rdma, sctxt, rqstp);
        if (ret < 0)
                goto err1;
        ret = 0;
        goto out;

 err1:
        svc_rdma_send_ctxt_put(rdma, sctxt);
 err0:
        trace_svcrdma_send_failed(rqstp, ret);
        set_bit(XPT_CLOSE, &xprt->xpt_flags);
        ret = -ENOTCONN;
        goto out;
}