[PATCH v2 6/8] xprtrdma: Replace rpcrdma_mr_seg with xdr_buf cursor

[Chuck Lever <cel@kernel.org>]

From: Chuck Lever <chuck.lever@oracle.com>

The FRWR registration path converts data through three
representations: xdr_buf -> rpcrdma_mr_seg[] ->
scatterlist[] -> ib_map_mr_sg(). The rpcrdma_mr_seg
intermediate is a relic of when multiple registration
strategies existed (FMR, physical, FRWR). Only FRWR
remains, so the indirection serves no purpose.

Introduce struct rpcrdma_xdr_cursor to track position
within an xdr_buf during iterative MR registration.
The cursor is 16 bytes on the stack (a pointer and two
unsigned ints), replacing the 6240-byte rl_segments[260]
array that was embedded in each rpcrdma_req.

Rewrite frwr_map to populate scatterlist entries directly
from the xdr_buf regions (head kvec, page list, tail
kvec) via the cursor. The boundary logic for non-SG_GAPS
devices is simpler than before because the xdr_buf
structure guarantees that page-region entries after the
first start at offset 0, and that head/tail kvecs are
separate regions that naturally break at MR boundaries.

Rewrite the three chunk-encoding functions
(rpcrdma_encode_read_list, rpcrdma_encode_write_list,
rpcrdma_encode_reply_chunk) to use cursor-based iteration
instead of the two-pass convert-then-register approach.

Fix a pre-existing bug in rpcrdma_encode_write_list where
the write-pad statistics accumulator added mr->mr_length
from the last data MR rather than the write-pad MR. The
refactored code uses ep->re_write_pad_mr->mr_length.

Delete rpcrdma_convert_kvec, rpcrdma_convert_iovs, struct
rpcrdma_mr_seg, rl_segments, and RPCRDMA_MAX_IOV_SEGS.
Adapt the chunk tracepoints to take a bool is_last
parameter instead of the now-eliminated nsegs count.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 include/trace/events/rpcrdma.h  |  28 +++---
 net/sunrpc/xprtrdma/frwr_ops.c  | 116 ++++++++++++++++++-----
 net/sunrpc/xprtrdma/rpc_rdma.c  | 160 +++++++++++---------------------
 net/sunrpc/xprtrdma/xprt_rdma.h |  42 ++++++---
 4 files changed, 189 insertions(+), 157 deletions(-)

diff --git a/include/trace/events/rpcrdma.h b/include/trace/events/rpcrdma.h
index e6a72646c507..b79913048e1a 100644
--- a/include/trace/events/rpcrdma.h
+++ b/include/trace/events/rpcrdma.h
@@ -392,10 +392,10 @@ DECLARE_EVENT_CLASS(xprtrdma_rdch_event,
 		const struct rpc_task *task,
 		unsigned int pos,
 		struct rpcrdma_mr *mr,
-		int nsegs
+		bool is_last
 	),
 
-	TP_ARGS(task, pos, mr, nsegs),
+	TP_ARGS(task, pos, mr, is_last),
 
 	TP_STRUCT__entry(
 		__field(unsigned int, task_id)
@@ -405,7 +405,7 @@ DECLARE_EVENT_CLASS(xprtrdma_rdch_event,
 		__field(u32, handle)
 		__field(u32, length)
 		__field(u64, offset)
-		__field(int, nsegs)
+		__field(bool, is_last)
 	),
 
 	TP_fast_assign(
@@ -416,7 +416,7 @@ DECLARE_EVENT_CLASS(xprtrdma_rdch_event,
 		__entry->handle = mr->mr_handle;
 		__entry->length = mr->mr_length;
 		__entry->offset = mr->mr_offset;
-		__entry->nsegs = nsegs;
+		__entry->is_last = is_last;
 	),
 
 	TP_printk(SUNRPC_TRACE_TASK_SPECIFIER
@@ -424,7 +424,7 @@ DECLARE_EVENT_CLASS(xprtrdma_rdch_event,
 		__entry->task_id, __entry->client_id,
 		__entry->pos, __entry->length,
 		(unsigned long long)__entry->offset, __entry->handle,
-		__entry->nents < __entry->nsegs ? "more" : "last"
+		__entry->is_last ? "last" : "more"
 	)
 );
 
@@ -434,18 +434,18 @@ DECLARE_EVENT_CLASS(xprtrdma_rdch_event,
 					const struct rpc_task *task,	\
 					unsigned int pos,		\
 					struct rpcrdma_mr *mr,		\
-					int nsegs			\
+					bool is_last			\
 				),					\
-				TP_ARGS(task, pos, mr, nsegs))
+				TP_ARGS(task, pos, mr, is_last))
 
 DECLARE_EVENT_CLASS(xprtrdma_wrch_event,
 	TP_PROTO(
 		const struct rpc_task *task,
 		struct rpcrdma_mr *mr,
-		int nsegs
+		bool is_last
 	),
 
-	TP_ARGS(task, mr, nsegs),
+	TP_ARGS(task, mr, is_last),
 
 	TP_STRUCT__entry(
 		__field(unsigned int, task_id)
@@ -454,7 +454,7 @@ DECLARE_EVENT_CLASS(xprtrdma_wrch_event,
 		__field(u32, handle)
 		__field(u32, length)
 		__field(u64, offset)
-		__field(int, nsegs)
+		__field(bool, is_last)
 	),
 
 	TP_fast_assign(
@@ -464,7 +464,7 @@ DECLARE_EVENT_CLASS(xprtrdma_wrch_event,
 		__entry->handle = mr->mr_handle;
 		__entry->length = mr->mr_length;
 		__entry->offset = mr->mr_offset;
-		__entry->nsegs = nsegs;
+		__entry->is_last = is_last;
 	),
 
 	TP_printk(SUNRPC_TRACE_TASK_SPECIFIER
@@ -472,7 +472,7 @@ DECLARE_EVENT_CLASS(xprtrdma_wrch_event,
 		__entry->task_id, __entry->client_id,
 		__entry->length, (unsigned long long)__entry->offset,
 		__entry->handle,
-		__entry->nents < __entry->nsegs ? "more" : "last"
+		__entry->is_last ? "last" : "more"
 	)
 );
 
@@ -481,9 +481,9 @@ DECLARE_EVENT_CLASS(xprtrdma_wrch_event,
 				TP_PROTO(				\
 					const struct rpc_task *task,	\
 					struct rpcrdma_mr *mr,		\
-					int nsegs			\
+					bool is_last			\
 				),					\
-				TP_ARGS(task, mr, nsegs))
+				TP_ARGS(task, mr, is_last))
 
 TRACE_DEFINE_ENUM(DMA_BIDIRECTIONAL);
 TRACE_DEFINE_ENUM(DMA_TO_DEVICE);
diff --git a/net/sunrpc/xprtrdma/frwr_ops.c b/net/sunrpc/xprtrdma/frwr_ops.c
index 4331b0b65f4c..ef6a6ab9f940 100644
--- a/net/sunrpc/xprtrdma/frwr_ops.c
+++ b/net/sunrpc/xprtrdma/frwr_ops.c
@@ -268,10 +268,9 @@ int frwr_query_device(struct rpcrdma_ep *ep, const struct ib_device *device)
 }
 
 /**
- * frwr_map - Register a memory region
+ * frwr_map - Register a memory region from an xdr_buf cursor
  * @r_xprt: controlling transport
- * @seg: memory region co-ordinates
- * @nsegs: number of segments remaining
+ * @cur: cursor tracking position within the xdr_buf
  * @writing: true when RDMA Write will be used
  * @xid: XID of RPC using the registered memory
  * @mr: MR to fill in
@@ -279,34 +278,103 @@ int frwr_query_device(struct rpcrdma_ep *ep, const struct ib_device *device)
  * Prepare a REG_MR Work Request to register a memory region
  * for remote access via RDMA READ or RDMA WRITE.
  *
- * Returns the next segment or a negative errno pointer.
- * On success, @mr is filled in.
+ * Returns 0 on success (cursor advanced past consumed data,
+ * @mr populated) or a negative errno on failure.
  */
-struct rpcrdma_mr_seg *frwr_map(struct rpcrdma_xprt *r_xprt,
-				struct rpcrdma_mr_seg *seg,
-				int nsegs, bool writing, __be32 xid,
-				struct rpcrdma_mr *mr)
+int frwr_map(struct rpcrdma_xprt *r_xprt,
+	     struct rpcrdma_xdr_cursor *cur,
+	     bool writing, __be32 xid,
+	     struct rpcrdma_mr *mr)
 {
 	struct rpcrdma_ep *ep = r_xprt->rx_ep;
+	const struct xdr_buf *xdrbuf = cur->xc_buf;
+	bool sg_gaps = ep->re_mrtype == IB_MR_TYPE_SG_GAPS;
+	unsigned int max_depth = ep->re_max_fr_depth;
 	struct ib_reg_wr *reg_wr;
 	int i, n, dma_nents;
 	struct ib_mr *ibmr;
 	u8 key;
 
-	if (nsegs > ep->re_max_fr_depth)
-		nsegs = ep->re_max_fr_depth;
-	for (i = 0; i < nsegs;) {
-		sg_set_page(&mr->mr_sg[i], seg->mr_page,
-			    seg->mr_len, seg->mr_offset);
+	i = 0;
 
-		++seg;
-		++i;
-		if (ep->re_mrtype == IB_MR_TYPE_SG_GAPS)
-			continue;
-		if ((i < nsegs && seg->mr_offset) ||
-		    offset_in_page((seg-1)->mr_offset + (seg-1)->mr_len))
-			break;
+	/* Head kvec */
+	if (!(cur->xc_flags & XC_HEAD_DONE)) {
+		const struct kvec *head = &xdrbuf->head[0];
+
+		sg_set_page(&mr->mr_sg[i],
+			    virt_to_page(head->iov_base),
+			    head->iov_len,
+			    offset_in_page(head->iov_base));
+		cur->xc_flags |= XC_HEAD_DONE;
+		i++;
+		/* Without sg-gap support, each non-contiguous region
+		 * must be registered as a separate MR.  Returning
+		 * here after the head kvec causes the caller to
+		 * invoke frwr_map() again for the page list and
+		 * tail.
+		 */
+		if (!sg_gaps)
+			goto finish;
 	}
+
+	/* Page list */
+	if (!(cur->xc_flags & XC_PAGES_DONE) && xdrbuf->page_len) {
+		unsigned int page_base, remaining;
+		struct page **ppages;
+
+		remaining = xdrbuf->page_len - cur->xc_page_offset;
+		page_base = offset_in_page(xdrbuf->page_base +
+					   cur->xc_page_offset);
+		ppages = xdrbuf->pages +
+			 ((xdrbuf->page_base + cur->xc_page_offset)
+			  >> PAGE_SHIFT);
+
+		while (remaining > 0 && i < max_depth) {
+			unsigned int len;
+
+			len = min_t(unsigned int,
+				    PAGE_SIZE - page_base, remaining);
+			sg_set_page(&mr->mr_sg[i], *ppages,
+				    len, page_base);
+			cur->xc_page_offset += len;
+			i++;
+			ppages++;
+			remaining -= len;
+
+			if (!sg_gaps && remaining > 0 &&
+			    offset_in_page(page_base + len))
+				goto finish;
+			page_base = 0;
+		}
+		if (remaining == 0)
+			cur->xc_flags |= XC_PAGES_DONE;
+	} else if (!(cur->xc_flags & XC_PAGES_DONE)) {
+		cur->xc_flags |= XC_PAGES_DONE;
+	}
+
+	/* Tail kvec */
+	if (!(cur->xc_flags & XC_TAIL_DONE) && xdrbuf->tail[0].iov_len &&
+	    i < max_depth) {
+		const struct kvec *tail = &xdrbuf->tail[0];
+
+		if (!sg_gaps && i > 0) {
+			struct scatterlist *prev = &mr->mr_sg[i - 1];
+
+			if (offset_in_page(prev->offset + prev->length))
+				goto finish;
+		}
+		sg_set_page(&mr->mr_sg[i],
+			    virt_to_page(tail->iov_base),
+			    tail->iov_len,
+			    offset_in_page(tail->iov_base));
+		cur->xc_flags |= XC_TAIL_DONE;
+		i++;
+	} else if (!(cur->xc_flags & XC_TAIL_DONE) &&
+		   !xdrbuf->tail[0].iov_len) {
+		cur->xc_flags |= XC_TAIL_DONE;
+	}
+
+finish:
 	mr->mr_dir = rpcrdma_data_dir(writing);
 	mr->mr_nents = i;
 
@@ -338,15 +406,15 @@ struct rpcrdma_mr_seg *frwr_map(struct rpcrdma_xprt *r_xprt,
 	mr->mr_offset = ibmr->iova;
 	trace_xprtrdma_mr_map(mr);
 
-	return seg;
+	return 0;
 
 out_dmamap_err:
 	trace_xprtrdma_frwr_sgerr(mr, i);
-	return ERR_PTR(-EIO);
+	return -EIO;
 
 out_mapmr_err:
 	trace_xprtrdma_frwr_maperr(mr, n);
-	return ERR_PTR(-EIO);
+	return -EIO;
 }
 
 /**
diff --git a/net/sunrpc/xprtrdma/rpc_rdma.c b/net/sunrpc/xprtrdma/rpc_rdma.c
index 3aac1456e23e..2ce50e8ce5fd 100644
--- a/net/sunrpc/xprtrdma/rpc_rdma.c
+++ b/net/sunrpc/xprtrdma/rpc_rdma.c
@@ -200,67 +200,27 @@ rpcrdma_alloc_sparse_pages(struct xdr_buf *buf)
 	return 0;
 }
 
-/* Convert @vec to a single SGL element.
- *
- * Returns pointer to next available SGE, and bumps the total number
- * of SGEs consumed.
- */
-static struct rpcrdma_mr_seg *
-rpcrdma_convert_kvec(struct kvec *vec, struct rpcrdma_mr_seg *seg,
-		     unsigned int *n)
+static void
+rpcrdma_xdr_cursor_init(struct rpcrdma_xdr_cursor *cur,
+			const struct xdr_buf *xdrbuf,
+			unsigned int pos, enum rpcrdma_chunktype type)
 {
-	seg->mr_page = virt_to_page(vec->iov_base);
-	seg->mr_offset = offset_in_page(vec->iov_base);
-	seg->mr_len = vec->iov_len;
-	++seg;
-	++(*n);
-	return seg;
+	cur->xc_buf = xdrbuf;
+	cur->xc_page_offset = 0;
+	cur->xc_flags = 0;
+
+	if (pos != 0)
+		cur->xc_flags |= XC_HEAD_DONE;
+	if (type == rpcrdma_readch || type == rpcrdma_writech)
+		cur->xc_flags |= XC_TAIL_DONE;
 }
 
-/* Convert @xdrbuf into SGEs no larger than a page each. As they
- * are registered, these SGEs are then coalesced into RDMA segments
- * when the selected memreg mode supports it.
- *
- * Returns positive number of SGEs consumed, or a negative errno.
- */
-
-static int
-rpcrdma_convert_iovs(struct rpcrdma_xprt *r_xprt, struct xdr_buf *xdrbuf,
-		     unsigned int pos, enum rpcrdma_chunktype type,
-		     struct rpcrdma_mr_seg *seg)
+static bool
+rpcrdma_xdr_cursor_done(const struct rpcrdma_xdr_cursor *cur)
 {
-	unsigned long page_base;
-	unsigned int len, n;
-	struct page **ppages;
-
-	n = 0;
-	if (pos == 0)
-		seg = rpcrdma_convert_kvec(&xdrbuf->head[0], seg, &n);
-
-	len = xdrbuf->page_len;
-	ppages = xdrbuf->pages + (xdrbuf->page_base >> PAGE_SHIFT);
-	page_base = offset_in_page(xdrbuf->page_base);
-	while (len) {
-		seg->mr_page = *ppages;
-		seg->mr_offset = page_base;
-		seg->mr_len = min_t(u32, PAGE_SIZE - page_base, len);
-		len -= seg->mr_len;
-		++ppages;
-		++seg;
-		++n;
-		page_base = 0;
-	}
-
-	if (type == rpcrdma_readch || type == rpcrdma_writech)
-		goto out;
-
-	if (xdrbuf->tail[0].iov_len)
-		rpcrdma_convert_kvec(&xdrbuf->tail[0], seg, &n);
-
-out:
-	if (unlikely(n > RPCRDMA_MAX_SEGS))
-		return -EIO;
-	return n;
+	return (cur->xc_flags & (XC_HEAD_DONE | XC_PAGES_DONE |
+				 XC_TAIL_DONE)) ==
+	       (XC_HEAD_DONE | XC_PAGES_DONE | XC_TAIL_DONE);
 }
 
 static int
@@ -292,11 +252,10 @@ encode_read_segment(struct xdr_stream *xdr, struct rpcrdma_mr *mr,
 	return 0;
 }
 
-static struct rpcrdma_mr_seg *rpcrdma_mr_prepare(struct rpcrdma_xprt *r_xprt,
-						 struct rpcrdma_req *req,
-						 struct rpcrdma_mr_seg *seg,
-						 int nsegs, bool writing,
-						 struct rpcrdma_mr **mr)
+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) {
@@ -307,13 +266,13 @@ static struct rpcrdma_mr_seg *rpcrdma_mr_prepare(struct rpcrdma_xprt *r_xprt,
 	}
 
 	rpcrdma_mr_push(*mr, &req->rl_registered);
-	return frwr_map(r_xprt, seg, nsegs, writing, req->rl_slot.rq_xid, *mr);
+	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 ERR_PTR(-EAGAIN);
+	return -EAGAIN;
 }
 
 /* Register and XDR encode the Read list. Supports encoding a list of read
@@ -336,10 +295,10 @@ static int rpcrdma_encode_read_list(struct rpcrdma_xprt *r_xprt,
 				    enum rpcrdma_chunktype rtype)
 {
 	struct xdr_stream *xdr = &req->rl_stream;
-	struct rpcrdma_mr_seg *seg;
+	struct rpcrdma_xdr_cursor cur;
 	struct rpcrdma_mr *mr;
 	unsigned int pos;
-	int nsegs;
+	int ret;
 
 	if (rtype == rpcrdma_noch_pullup || rtype == rpcrdma_noch_mapped)
 		goto done;
@@ -347,24 +306,20 @@ static int rpcrdma_encode_read_list(struct rpcrdma_xprt *r_xprt,
 	pos = rqst->rq_snd_buf.head[0].iov_len;
 	if (rtype == rpcrdma_areadch)
 		pos = 0;
-	seg = req->rl_segments;
-	nsegs = rpcrdma_convert_iovs(r_xprt, &rqst->rq_snd_buf, pos,
-				     rtype, seg);
-	if (nsegs < 0)
-		return nsegs;
+	rpcrdma_xdr_cursor_init(&cur, &rqst->rq_snd_buf, pos, rtype);
 
 	do {
-		seg = rpcrdma_mr_prepare(r_xprt, req, seg, nsegs, false, &mr);
-		if (IS_ERR(seg))
-			return PTR_ERR(seg);
+		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, nsegs);
+		trace_xprtrdma_chunk_read(rqst->rq_task, pos, mr,
+					  rpcrdma_xdr_cursor_done(&cur));
 		r_xprt->rx_stats.read_chunk_count++;
-		nsegs -= mr->mr_nents;
-	} while (nsegs);
+	} while (!rpcrdma_xdr_cursor_done(&cur));
 
 done:
 	if (xdr_stream_encode_item_absent(xdr) < 0)
@@ -394,20 +349,16 @@ static int rpcrdma_encode_write_list(struct rpcrdma_xprt *r_xprt,
 {
 	struct xdr_stream *xdr = &req->rl_stream;
 	struct rpcrdma_ep *ep = r_xprt->rx_ep;
-	struct rpcrdma_mr_seg *seg;
+	struct rpcrdma_xdr_cursor cur;
 	struct rpcrdma_mr *mr;
-	int nsegs, nchunks;
+	int nchunks, ret;
 	__be32 *segcount;
 
 	if (wtype != rpcrdma_writech)
 		goto done;
 
-	seg = req->rl_segments;
-	nsegs = rpcrdma_convert_iovs(r_xprt, &rqst->rq_rcv_buf,
-				     rqst->rq_rcv_buf.head[0].iov_len,
-				     wtype, seg);
-	if (nsegs < 0)
-		return nsegs;
+	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;
@@ -418,30 +369,30 @@ static int rpcrdma_encode_write_list(struct rpcrdma_xprt *r_xprt,
 
 	nchunks = 0;
 	do {
-		seg = rpcrdma_mr_prepare(r_xprt, req, seg, nsegs, true, &mr);
-		if (IS_ERR(seg))
-			return PTR_ERR(seg);
+		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, nsegs);
+		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++;
-		nsegs -= mr->mr_nents;
-	} while (nsegs);
+	} 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,
-					nsegs);
+					true);
 		r_xprt->rx_stats.write_chunk_count++;
-		r_xprt->rx_stats.total_rdma_request += mr->mr_length;
+		r_xprt->rx_stats.total_rdma_request +=
+			ep->re_write_pad_mr->mr_length;
 		nchunks++;
-		nsegs -= mr->mr_nents;
 	}
 
 	/* Update count of segments in this Write chunk */
@@ -471,9 +422,9 @@ static int rpcrdma_encode_reply_chunk(struct rpcrdma_xprt *r_xprt,
 				      enum rpcrdma_chunktype wtype)
 {
 	struct xdr_stream *xdr = &req->rl_stream;
-	struct rpcrdma_mr_seg *seg;
+	struct rpcrdma_xdr_cursor cur;
 	struct rpcrdma_mr *mr;
-	int nsegs, nchunks;
+	int nchunks, ret;
 	__be32 *segcount;
 
 	if (wtype != rpcrdma_replych) {
@@ -482,10 +433,7 @@ static int rpcrdma_encode_reply_chunk(struct rpcrdma_xprt *r_xprt,
 		return 0;
 	}
 
-	seg = req->rl_segments;
-	nsegs = rpcrdma_convert_iovs(r_xprt, &rqst->rq_rcv_buf, 0, wtype, seg);
-	if (nsegs < 0)
-		return nsegs;
+	rpcrdma_xdr_cursor_init(&cur, &rqst->rq_rcv_buf, 0, wtype);
 
 	if (xdr_stream_encode_item_present(xdr) < 0)
 		return -EMSGSIZE;
@@ -496,19 +444,19 @@ static int rpcrdma_encode_reply_chunk(struct rpcrdma_xprt *r_xprt,
 
 	nchunks = 0;
 	do {
-		seg = rpcrdma_mr_prepare(r_xprt, req, seg, nsegs, true, &mr);
-		if (IS_ERR(seg))
-			return PTR_ERR(seg);
+		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, nsegs);
+		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++;
-		nsegs -= mr->mr_nents;
-	} while (nsegs);
+	} while (!rpcrdma_xdr_cursor_done(&cur));
 
 	/* Update count of segments in the Reply chunk */
 	*segcount = cpu_to_be32(nchunks);
diff --git a/net/sunrpc/xprtrdma/xprt_rdma.h b/net/sunrpc/xprtrdma/xprt_rdma.h
index 8147d2b41494..37bba72065e8 100644
--- a/net/sunrpc/xprtrdma/xprt_rdma.h
+++ b/net/sunrpc/xprtrdma/xprt_rdma.h
@@ -283,19 +283,36 @@ struct rpcrdma_mr {
  * registered or invalidated. Must handle a Reply chunk:
  */
 enum {
-	RPCRDMA_MAX_IOV_SEGS	= 3,
+	RPCRDMA_MAX_IOV_SEGS	= 3,	/* head, page-boundary, tail */
 	RPCRDMA_MAX_DATA_SEGS	= ((1 * 1024 * 1024) / PAGE_SIZE) + 1,
 	RPCRDMA_MAX_SEGS	= RPCRDMA_MAX_DATA_SEGS +
 				  RPCRDMA_MAX_IOV_SEGS,
 };
 
-/* Arguments for DMA mapping and registration */
-struct rpcrdma_mr_seg {
-	u32		mr_len;		/* length of segment */
-	struct page	*mr_page;	/* underlying struct page */
-	u64		mr_offset;	/* IN: page offset, OUT: iova */
+/**
+ * struct rpcrdma_xdr_cursor - tracks position within an xdr_buf
+ *     for iterative MR registration
+ * @xc_buf: the xdr_buf being iterated
+ * @xc_page_offset: byte offset into the page region consumed so far
+ * @xc_flags: combination of XC_* bits
+ *
+ * Each XC_*_DONE flag indicates that this region has no
+ * remaining MR registration work.  That condition holds both when the region
+ * has already been registered by a prior frwr_map() call and
+ * when the region is excluded from this chunk type (pre-set
+ * at init time by rpcrdma_xdr_cursor_init()).  frwr_map()
+ * treats the two cases identically: skip the region.
+ */
+struct rpcrdma_xdr_cursor {
+	const struct xdr_buf		*xc_buf;
+	unsigned int			xc_page_offset;
+	unsigned int			xc_flags;
 };
 
+#define XC_HEAD_DONE	BIT(0)
+#define XC_PAGES_DONE	BIT(1)
+#define XC_TAIL_DONE	BIT(2)
+
 /* The Send SGE array is provisioned to send a maximum size
  * inline request:
  * - RPC-over-RDMA header
@@ -330,7 +347,6 @@ struct rpcrdma_req {
 
 	struct list_head	rl_free_mrs;
 	struct list_head	rl_registered;
-	struct rpcrdma_mr_seg	rl_segments[RPCRDMA_MAX_SEGS];
 };
 
 static inline struct rpcrdma_req *
@@ -450,8 +466,8 @@ rpcrdma_portstr(const struct rpcrdma_xprt *r_xprt)
 }
 
 /* Setting this to 0 ensures interoperability with early servers.
- * Setting this to 1 enhances certain unaligned read/write performance.
- * Default is 0, see sysctl entry and rpc_rdma.c rpcrdma_convert_iovs() */
+ * Setting this to 1 enhances unaligned read/write performance.
+ * Default is 0, see sysctl entry and rpc_rdma.c */
 extern int xprt_rdma_pad_optimize;
 
 /* This setting controls the hunt for a supported memory
@@ -535,10 +551,10 @@ void frwr_reset(struct rpcrdma_req *req);
 int frwr_query_device(struct rpcrdma_ep *ep, const struct ib_device *device);
 int frwr_mr_init(struct rpcrdma_xprt *r_xprt, struct rpcrdma_mr *mr);
 void frwr_mr_release(struct rpcrdma_mr *mr);
-struct rpcrdma_mr_seg *frwr_map(struct rpcrdma_xprt *r_xprt,
-				struct rpcrdma_mr_seg *seg,
-				int nsegs, bool writing, __be32 xid,
-				struct rpcrdma_mr *mr);
+int frwr_map(struct rpcrdma_xprt *r_xprt,
+	     struct rpcrdma_xdr_cursor *cur,
+	     bool writing, __be32 xid,
+	     struct rpcrdma_mr *mr);
 int frwr_send(struct rpcrdma_xprt *r_xprt, struct rpcrdma_req *req);
 void frwr_reminv(struct rpcrdma_rep *rep, struct list_head *mrs);
 void frwr_unmap_sync(struct rpcrdma_xprt *r_xprt, struct rpcrdma_req *req);
-- 
2.53.0