svcrdma: Persistently allocate and DMA-map Receive buffers

The current Receive path uses an array of pages which are allocated
and DMA mapped when each Receive WR is posted, and then handed off
to the upper layer in rqstp::rq_arg. The page flip releases unused
pages in the rq_pages pagelist. This mechanism introduces a
significant amount of overhead.

So instead, kmalloc the Receive buffer, and leave it DMA-mapped
while the transport remains connected. This confers a number of
benefits:

* Each Receive WR requires only one receive SGE, no matter how large
  the inline threshold is. This helps the server-side NFS/RDMA
  transport operate on less capable RDMA devices.

* The Receive buffer is left allocated and mapped all the time. This
  relieves svc_rdma_post_recv from the overhead of allocating and
  DMA-mapping a fresh buffer.

* svc_rdma_wc_receive no longer has to DMA unmap the Receive buffer.
  It has to DMA sync only the number of bytes that were received.

* svc_rdma_build_arg_xdr no longer has to free a page in rq_pages
  for each page in the Receive buffer, making it a constant-time
  function.

* The Receive buffer is now plugged directly into the rq_arg's
  head[0].iov_vec, and can be larger than a page without spilling
  over into rq_arg's page list. This enables simplification of
  the RDMA Read path in subsequent patches.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

include/linux/sunrpc/svc_rdma.h

@@ -148,12 +148,12 @@ struct svc_rdma_recv_ctxt {
 	struct list_head	rc_list;
 	struct ib_recv_wr	rc_recv_wr;
 	struct ib_cqe		rc_cqe;
+	struct ib_sge		rc_recv_sge;
+	void			*rc_recv_buf;
 	struct xdr_buf		rc_arg;
 	u32			rc_byte_len;
 	unsigned int		rc_page_count;
 	unsigned int		rc_hdr_count;
-	struct ib_sge		rc_sges[1 +
-					RPCRDMA_MAX_INLINE_THRESH / PAGE_SIZE];
 	struct page		*rc_pages[RPCSVC_MAXPAGES];
 };
 

net/sunrpc/xprtrdma/svc_rdma_recvfrom.c

@@ -117,6 +117,43 @@ svc_rdma_next_recv_ctxt(struct list_head *list)
 					rc_list);
 }
 
+static struct svc_rdma_recv_ctxt *
+svc_rdma_recv_ctxt_alloc(struct svcxprt_rdma *rdma)
+{
+	struct svc_rdma_recv_ctxt *ctxt;
+	dma_addr_t addr;
+	void *buffer;
+
+	ctxt = kmalloc(sizeof(*ctxt), 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_FROM_DEVICE);
+	if (ib_dma_mapping_error(rdma->sc_pd->device, addr))
+		goto fail2;
+
+	ctxt->rc_recv_wr.next = NULL;
+	ctxt->rc_recv_wr.wr_cqe = &ctxt->rc_cqe;
+	ctxt->rc_recv_wr.sg_list = &ctxt->rc_recv_sge;
+	ctxt->rc_recv_wr.num_sge = 1;
+	ctxt->rc_cqe.done = svc_rdma_wc_receive;
+	ctxt->rc_recv_sge.addr = addr;
+	ctxt->rc_recv_sge.length = rdma->sc_max_req_size;
+	ctxt->rc_recv_sge.lkey = rdma->sc_pd->local_dma_lkey;
+	ctxt->rc_recv_buf = buffer;
+	return ctxt;
+
+fail2:
+	kfree(buffer);
+fail1:
+	kfree(ctxt);
+fail0:
+	return NULL;
+}
+
 /**
  * svc_rdma_recv_ctxts_destroy - Release all recv_ctxt's for an xprt
  * @rdma: svcxprt_rdma being torn down
@@ -128,6 +165,11 @@ void svc_rdma_recv_ctxts_destroy(struct svcxprt_rdma *rdma)
 
 	while ((ctxt = svc_rdma_next_recv_ctxt(&rdma->sc_recv_ctxts))) {
 		list_del(&ctxt->rc_list);
+		ib_dma_unmap_single(rdma->sc_pd->device,
+				    ctxt->rc_recv_sge.addr,
+				    ctxt->rc_recv_sge.length,
+				    DMA_FROM_DEVICE);
+		kfree(ctxt->rc_recv_buf);
 		kfree(ctxt);
 	}
 }
@@ -145,32 +187,18 @@ svc_rdma_recv_ctxt_get(struct svcxprt_rdma *rdma)
 	spin_unlock(&rdma->sc_recv_lock);
 
 out:
-	ctxt->rc_recv_wr.num_sge = 0;
 	ctxt->rc_page_count = 0;
 	return ctxt;
 
 out_empty:
 	spin_unlock(&rdma->sc_recv_lock);
 
-	ctxt = kmalloc(sizeof(*ctxt), GFP_KERNEL);
+	ctxt = svc_rdma_recv_ctxt_alloc(rdma);
 	if (!ctxt)
 		return NULL;
 	goto out;
 }
 
-static void svc_rdma_recv_ctxt_unmap(struct svcxprt_rdma *rdma,
-				     struct svc_rdma_recv_ctxt *ctxt)
-{
-	struct ib_device *device = rdma->sc_cm_id->device;
-	int i;
-
-	for (i = 0; i < ctxt->rc_recv_wr.num_sge; i++)
-		ib_dma_unmap_page(device,
-				  ctxt->rc_sges[i].addr,
-				  ctxt->rc_sges[i].length,
-				  DMA_FROM_DEVICE);
-}
-
 /**
  * svc_rdma_recv_ctxt_put - Return recv_ctxt to free list
  * @rdma: controlling svcxprt_rdma
@@ -191,46 +219,14 @@ void svc_rdma_recv_ctxt_put(struct svcxprt_rdma *rdma,
 
 static int svc_rdma_post_recv(struct svcxprt_rdma *rdma)
 {
-	struct ib_device *device = rdma->sc_cm_id->device;
 	struct svc_rdma_recv_ctxt *ctxt;
 	struct ib_recv_wr *bad_recv_wr;
-	int sge_no, buflen, ret;
-	struct page *page;
-	dma_addr_t pa;
+	int ret;
 
 	ctxt = svc_rdma_recv_ctxt_get(rdma);
 	if (!ctxt)
 		return -ENOMEM;
 
-	buflen = 0;
-	ctxt->rc_cqe.done = svc_rdma_wc_receive;
-	for (sge_no = 0; buflen < rdma->sc_max_req_size; sge_no++) {
-		if (sge_no >= rdma->sc_max_sge) {
-			pr_err("svcrdma: Too many sges (%d)\n", sge_no);
-			goto err_put_ctxt;
-		}
-
-		page = alloc_page(GFP_KERNEL);
-		if (!page)
-			goto err_put_ctxt;
-		ctxt->rc_pages[sge_no] = page;
-		ctxt->rc_page_count++;
-
-		pa = ib_dma_map_page(device, ctxt->rc_pages[sge_no],
-				     0, PAGE_SIZE, DMA_FROM_DEVICE);
-		if (ib_dma_mapping_error(device, pa))
-			goto err_put_ctxt;
-		ctxt->rc_sges[sge_no].addr = pa;
-		ctxt->rc_sges[sge_no].length = PAGE_SIZE;
-		ctxt->rc_sges[sge_no].lkey = rdma->sc_pd->local_dma_lkey;
-		ctxt->rc_recv_wr.num_sge++;
-
-		buflen += PAGE_SIZE;
-	}
-	ctxt->rc_recv_wr.next = NULL;
-	ctxt->rc_recv_wr.sg_list = &ctxt->rc_sges[0];
-	ctxt->rc_recv_wr.wr_cqe = &ctxt->rc_cqe;
-
 	svc_xprt_get(&rdma->sc_xprt);
 	ret = ib_post_recv(rdma->sc_qp, &ctxt->rc_recv_wr, &bad_recv_wr);
 	trace_svcrdma_post_recv(&ctxt->rc_recv_wr, ret);
@@ -238,12 +234,7 @@ static int svc_rdma_post_recv(struct svcxprt_rdma *rdma)
 		goto err_post;
 	return 0;
 
-err_put_ctxt:
-	svc_rdma_recv_ctxt_unmap(rdma, ctxt);
-	svc_rdma_recv_ctxt_put(rdma, ctxt);
-	return -ENOMEM;
 err_post:
-	svc_rdma_recv_ctxt_unmap(rdma, ctxt);
 	svc_rdma_recv_ctxt_put(rdma, ctxt);
 	svc_xprt_put(&rdma->sc_xprt);
 	return ret;
@@ -289,7 +280,6 @@ static void svc_rdma_wc_receive(struct ib_cq *cq, struct ib_wc *wc)
 
 	/* WARNING: Only wc->wr_cqe and wc->status are reliable */
 	ctxt = container_of(cqe, struct svc_rdma_recv_ctxt, rc_cqe);
-	svc_rdma_recv_ctxt_unmap(rdma, ctxt);
 
 	if (wc->status != IB_WC_SUCCESS)
 		goto flushed;
@@ -299,6 +289,10 @@ static void svc_rdma_wc_receive(struct ib_cq *cq, struct ib_wc *wc)
 
 	/* All wc fields are now known to be valid */
 	ctxt->rc_byte_len = wc->byte_len;
+	ib_dma_sync_single_for_cpu(rdma->sc_pd->device,
+				   ctxt->rc_recv_sge.addr,
+				   wc->byte_len, DMA_FROM_DEVICE);
+
 	spin_lock(&rdma->sc_rq_dto_lock);
 	list_add_tail(&ctxt->rc_list, &rdma->sc_rq_dto_q);
 	spin_unlock(&rdma->sc_rq_dto_lock);
@@ -339,64 +333,22 @@ void svc_rdma_flush_recv_queues(struct svcxprt_rdma *rdma)
 	}
 }
 
-/*
- * Replace the pages in the rq_argpages array with the pages from the SGE in
- * the RDMA_RECV completion. The SGL should contain full pages up until the
- * last one.
- */
 static void svc_rdma_build_arg_xdr(struct svc_rqst *rqstp,
 				   struct svc_rdma_recv_ctxt *ctxt)
 {
-	struct page *page;
-	int sge_no;
-	u32 len;
-
-	/* The reply path assumes the Call's transport header resides
-	 * in rqstp->rq_pages[0].
-	 */
-	page = ctxt->rc_pages[0];
-	put_page(rqstp->rq_pages[0]);
-	rqstp->rq_pages[0] = page;
-
-	/* Set up the XDR head */
-	rqstp->rq_arg.head[0].iov_base = page_address(page);
-	rqstp->rq_arg.head[0].iov_len =
-		min_t(size_t, ctxt->rc_byte_len, ctxt->rc_sges[0].length);
-	rqstp->rq_arg.len = ctxt->rc_byte_len;
-	rqstp->rq_arg.buflen = ctxt->rc_byte_len;
-
-	/* Compute bytes past head in the SGL */
-	len = ctxt->rc_byte_len - rqstp->rq_arg.head[0].iov_len;
-
-	/* If data remains, store it in the pagelist */
-	rqstp->rq_arg.page_len = len;
-	rqstp->rq_arg.page_base = 0;
-
-	sge_no = 1;
-	while (len && sge_no < ctxt->rc_recv_wr.num_sge) {
-		page = ctxt->rc_pages[sge_no];
-		put_page(rqstp->rq_pages[sge_no]);
-		rqstp->rq_pages[sge_no] = page;
-		len -= min_t(u32, len, ctxt->rc_sges[sge_no].length);
-		sge_no++;
-	}
-	ctxt->rc_hdr_count = sge_no;
-	rqstp->rq_respages = &rqstp->rq_pages[sge_no];
+	struct xdr_buf *arg = &rqstp->rq_arg;
+
+	arg->head[0].iov_base = ctxt->rc_recv_buf;
+	arg->head[0].iov_len = ctxt->rc_byte_len;
+	arg->tail[0].iov_base = NULL;
+	arg->tail[0].iov_len = 0;
+	arg->page_len = 0;
+	arg->page_base = 0;
+	arg->buflen = ctxt->rc_byte_len;
+	arg->len = ctxt->rc_byte_len;
+
+	rqstp->rq_respages = &rqstp->rq_pages[0];
 	rqstp->rq_next_page = rqstp->rq_respages + 1;
-
-	/* If not all pages were used from the SGL, free the remaining ones */
-	while (sge_no < ctxt->rc_recv_wr.num_sge) {
-		page = ctxt->rc_pages[sge_no++];
-		put_page(page);
-	}
-
-	/* @ctxt's pages have all been released or moved to @rqstp->rq_pages.
-	 */
-	ctxt->rc_page_count = 0;
-
-	/* Set up tail */
-	rqstp->rq_arg.tail[0].iov_base = NULL;
-	rqstp->rq_arg.tail[0].iov_len = 0;
 }
 
 /* This accommodates the largest possible Write chunk,

net/sunrpc/xprtrdma/svc_rdma_rw.c

@@ -718,15 +718,14 @@ static int svc_rdma_build_normal_read_chunk(struct svc_rqst *rqstp,
 	struct svc_rdma_recv_ctxt *head = info->ri_readctxt;
 	int ret;
 
-	info->ri_pageno = head->rc_hdr_count;
-	info->ri_pageoff = 0;
-
 	ret = svc_rdma_build_read_chunk(rqstp, info, p);
 	if (ret < 0)
 		goto out;
 
 	trace_svcrdma_encode_read(info->ri_chunklen, info->ri_position);
 
+	head->rc_hdr_count = 0;
+
 	/* Split the Receive buffer between the head and tail
 	 * buffers at Read chunk's position. XDR roundup of the
 	 * chunk is not included in either the pagelist or in
@@ -775,9 +774,6 @@ static int svc_rdma_build_pz_read_chunk(struct svc_rqst *rqstp,
 	struct svc_rdma_recv_ctxt *head = info->ri_readctxt;
 	int ret;
 
-	info->ri_pageno = head->rc_hdr_count - 1;
-	info->ri_pageoff = offset_in_page(head->rc_byte_len);
-
 	ret = svc_rdma_build_read_chunk(rqstp, info, p);
 	if (ret < 0)
 		goto out;
@@ -787,20 +783,13 @@ static int svc_rdma_build_pz_read_chunk(struct svc_rqst *rqstp,
 	head->rc_arg.len += info->ri_chunklen;
 	head->rc_arg.buflen += info->ri_chunklen;
 
-	if (head->rc_arg.buflen <= head->rc_sges[0].length) {
-		/* Transport header and RPC message fit entirely
-		 * in page where head iovec resides.
-		 */
-		head->rc_arg.head[0].iov_len = info->ri_chunklen;
-	} else {
-		/* Transport header and part of RPC message reside
-		 * in the head iovec's page.
-		 */
-		head->rc_arg.head[0].iov_len =
-			head->rc_sges[0].length - head->rc_byte_len;
-		head->rc_arg.page_len =
-			info->ri_chunklen - head->rc_arg.head[0].iov_len;
-	}
+	head->rc_hdr_count = 1;
+	head->rc_arg.head[0].iov_base = page_address(head->rc_pages[0]);
+	head->rc_arg.head[0].iov_len = min_t(size_t, PAGE_SIZE,
+					     info->ri_chunklen);
+
+	head->rc_arg.page_len = info->ri_chunklen -
+				head->rc_arg.head[0].iov_len;
 
 out:
 	return ret;
@@ -834,7 +823,6 @@ int svc_rdma_recv_read_chunk(struct svcxprt_rdma *rdma, struct svc_rqst *rqstp,
 	 * head->rc_arg. Pages involved with RDMA Read I/O are
 	 * transferred there.
 	 */
-	head->rc_page_count = head->rc_hdr_count;
 	head->rc_arg.head[0] = rqstp->rq_arg.head[0];
 	head->rc_arg.tail[0] = rqstp->rq_arg.tail[0];
 	head->rc_arg.pages = head->rc_pages;
@@ -847,6 +835,8 @@ int svc_rdma_recv_read_chunk(struct svcxprt_rdma *rdma, struct svc_rqst *rqstp,
 	if (!info)
 		return -ENOMEM;
 	info->ri_readctxt = head;
+	info->ri_pageno = 0;
+	info->ri_pageoff = 0;
 
 	info->ri_position = be32_to_cpup(p + 1);
 	if (info->ri_position)

net/sunrpc/xprtrdma/svc_rdma_sendto.c

@@ -629,10 +629,7 @@ int svc_rdma_sendto(struct svc_rqst *rqstp)
 	struct page *res_page;
 	int ret;
 
-	/* Find the call's chunk lists to decide how to send the reply.
-	 * Receive places the Call's xprt header at the start of page 0.
-	 */
-	rdma_argp = page_address(rqstp->rq_pages[0]);
+	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,

net/sunrpc/xprtrdma/svc_rdma_transport.c

@@ -669,7 +669,7 @@ static struct svc_xprt *svc_rdma_accept(struct svc_xprt *xprt)
 	qp_attr.cap.max_send_wr = newxprt->sc_sq_depth - ctxts;
 	qp_attr.cap.max_recv_wr = rq_depth;
 	qp_attr.cap.max_send_sge = newxprt->sc_max_sge;
-	qp_attr.cap.max_recv_sge = newxprt->sc_max_sge;
+	qp_attr.cap.max_recv_sge = 1;
 	qp_attr.sq_sig_type = IB_SIGNAL_REQ_WR;
 	qp_attr.qp_type = IB_QPT_RC;
 	qp_attr.send_cq = newxprt->sc_sq_cq;