[RFC PATCH 8/9] cifs: Build the RDMA SGE list directly from an iterator

[David Howells]

In the depths of the cifs RDMA code, extract part of an iov iterator
directly into an SGE list without going through an intermediate
scatterlist.

Note that this doesn't support extraction from an IOBUF- or UBUF-type
iterator (ie. user-supplied buffer).  The assumption is that the higher
layers will extract those to a BVEC-type iterator first and do whatever is
required to stop the pages from going away.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---

 fs/cifs/smbdirect.c |   89 ++++++++++++++++++---------------------------------
 1 file changed, 32 insertions(+), 57 deletions(-)

diff --git a/fs/cifs/smbdirect.c b/fs/cifs/smbdirect.c
index 7f1a1e190650..c4465d6a2fc6 100644
--- a/fs/cifs/smbdirect.c
+++ b/fs/cifs/smbdirect.c
@@ -828,16 +828,16 @@ static int smbd_post_send(struct smbd_connection *info,
 	return rc;
 }
 
-static int smbd_post_send_sgl(struct smbd_connection *info,
-	struct scatterlist *sgl, int data_length, int remaining_data_length)
+static int smbd_post_send_iter(struct smbd_connection *info,
+			       struct iov_iter *iter,
+			       int *_remaining_data_length)
 {
-	int num_sgs;
 	int i, rc;
 	int header_length;
+	int data_length;
 	struct smbd_request *request;
 	struct smbd_data_transfer *packet;
 	int new_credits;
-	struct scatterlist *sg;
 
 wait_credit:
 	/* Wait for send credits. A SMBD packet needs one credit */
@@ -881,6 +881,30 @@ static int smbd_post_send_sgl(struct smbd_connection *info,
 	}
 
 	request->info = info;
+	memset(request->sge, 0, sizeof(request->sge));
+
+	/* Fill in the data payload to find out how much data we can add */
+	if (iter) {
+		struct smb_extract_to_rdma extract = {
+			.nr_sge		= 1,
+			.max_sge	= SMBDIRECT_MAX_SEND_SGE,
+			.sge		= request->sge,
+			.device		= info->id->device,
+			.local_dma_lkey	= info->pd->local_dma_lkey,
+			.direction	= DMA_TO_DEVICE,
+		};
+
+		rc = smb_extract_iter_to_rdma(iter, *_remaining_data_length,
+					      &extract);
+		if (rc < 0)
+			goto err_dma;
+		data_length = rc;
+		request->num_sge = extract.nr_sge;
+		*_remaining_data_length -= data_length;
+	} else {
+		data_length = 0;
+		request->num_sge = 1;
+	}
 
 	/* Fill in the packet header */
 	packet = smbd_request_payload(request);
@@ -902,7 +926,7 @@ static int smbd_post_send_sgl(struct smbd_connection *info,
 	else
 		packet->data_offset = cpu_to_le32(24);
 	packet->data_length = cpu_to_le32(data_length);
-	packet->remaining_data_length = cpu_to_le32(remaining_data_length);
+	packet->remaining_data_length = cpu_to_le32(*_remaining_data_length);
 	packet->padding = 0;
 
 	log_outgoing(INFO, "credits_requested=%d credits_granted=%d data_offset=%d data_length=%d remaining_data_length=%d\n",
@@ -918,7 +942,6 @@ static int smbd_post_send_sgl(struct smbd_connection *info,
 	if (!data_length)
 		header_length = offsetof(struct smbd_data_transfer, padding);
 
-	request->num_sge = 1;
 	request->sge[0].addr = ib_dma_map_single(info->id->device,
 						 (void *)packet,
 						 header_length,
@@ -932,23 +955,6 @@ static int smbd_post_send_sgl(struct smbd_connection *info,
 	request->sge[0].length = header_length;
 	request->sge[0].lkey = info->pd->local_dma_lkey;
 
-	/* Fill in the packet data payload */
-	num_sgs = sgl ? sg_nents(sgl) : 0;
-	for_each_sg(sgl, sg, num_sgs, i) {
-		request->sge[i+1].addr =
-			ib_dma_map_page(info->id->device, sg_page(sg),
-			       sg->offset, sg->length, DMA_TO_DEVICE);
-		if (ib_dma_mapping_error(
-				info->id->device, request->sge[i+1].addr)) {
-			rc = -EIO;
-			request->sge[i+1].addr = 0;
-			goto err_dma;
-		}
-		request->sge[i+1].length = sg->length;
-		request->sge[i+1].lkey = info->pd->local_dma_lkey;
-		request->num_sge++;
-	}
-
 	rc = smbd_post_send(info, request);
 	if (!rc)
 		return 0;
@@ -987,8 +993,10 @@ static int smbd_post_send_sgl(struct smbd_connection *info,
  */
 static int smbd_post_send_empty(struct smbd_connection *info)
 {
+	int remaining_data_length = 0;
+
 	info->count_send_empty++;
-	return smbd_post_send_sgl(info, NULL, 0, 0);
+	return smbd_post_send_iter(info, NULL, &remaining_data_length);
 }
 
 /*
@@ -1932,39 +1940,6 @@ int smbd_recv(struct smbd_connection *info, struct msghdr *msg)
 	return rc;
 }
 
-/*
- * Send the contents of an iterator
- * @iter: The iterator to send
- * @_remaining_data_length: remaining data to send in this payload
- */
-static int smbd_post_send_iter(struct smbd_connection *info,
-			       struct iov_iter *iter,
-			       int *_remaining_data_length)
-{
-	struct scatterlist sgl[SMBDIRECT_MAX_SEND_SGE - 1];
-	unsigned int max_payload = info->max_send_size - sizeof(struct smbd_data_transfer);
-	ssize_t rc;
-
-	do {
-		struct sg_table sgtable = { .sgl = sgl };
-		size_t maxlen = min_t(size_t, *_remaining_data_length, max_payload);
-
-		sg_init_table(sgtable.sgl, ARRAY_SIZE(sgl));
-		rc = netfs_extract_iter_to_sg(iter, maxlen,
-					      &sgtable, ARRAY_SIZE(sgl));
-		if (rc < 0)
-			break;
-		if (WARN_ON_ONCE(sgtable.nents == 0))
-			return -EIO;
-
-		sg_mark_end(&sgl[sgtable.nents - 1]);
-		*_remaining_data_length -= rc;
-		rc = smbd_post_send_sgl(info, sgl, rc, *_remaining_data_length);
-	} while (rc == 0 && iov_iter_count(iter) > 0);
-
-	return rc;
-}
-
 /*
  * Send data to transport
  * Each rqst is transported as a SMBDirect payload