SVCRDMA: Check num_sge when setting LAST_CTXT bit

The RDMACTXT_F_LAST_CTXT bit was getting set incorrectly
when the last chunk in the read-list spanned multiple pages. This
resulted in a kernel panic when the wrong context was used to
build the RPC iovec page list.

RDMA_READ is used to fetch RPC data from the client for
NFS_WRITE requests. A scatter-gather is used to map the
advertised client side buffer to the server-side iovec and
associated page list.

WR contexts are used to convey which scatter-gather entries are
handled by each WR. When the write data is large, a single RPC may
require multiple RDMA_READ requests so the contexts for a single RPC
are chained together in a linked list. The last context in this list
is marked with a bit RDMACTXT_F_LAST_CTXT so that when this WR completes,
the CQ handler code can enqueue the RPC for processing.

The code in rdma_read_xdr was setting this bit on the last two
contexts on this list when the last read-list chunk spanned multiple
pages. This caused the svc_rdma_recvfrom logic to incorrectly build
the RPC and caused the kernel to crash because the second-to-last
context doesn't contain the iovec page list.

Modified the condition that sets this bit so that it correctly detects
the last context for the RPC.

Signed-off-by: Tom Tucker <tom@opengridcomputing.com>
Tested-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

net/sunrpc/xprtrdma/svc_rdma_recvfrom.c

@@ -322,15 +322,6 @@ next_sge:
 		ctxt->direction = DMA_FROM_DEVICE;
 		clear_bit(RDMACTXT_F_READ_DONE, &ctxt->flags);
 		clear_bit(RDMACTXT_F_LAST_CTXT, &ctxt->flags);
-		if ((ch+1)->rc_discrim == 0) {
-			/*
-			 * Checked in sq_cq_reap to see if we need to
-			 * be enqueued
-			 */
-			set_bit(RDMACTXT_F_LAST_CTXT, &ctxt->flags);
-			ctxt->next = hdr_ctxt;
-			hdr_ctxt->next = head;
-		}
 
 		/* Prepare READ WR */
 		memset(&read_wr, 0, sizeof read_wr);
@@ -348,7 +339,17 @@ next_sge:
 		rdma_set_ctxt_sge(ctxt, &sge[ch_sge_ary[ch_no].start],
 				  &sgl_offset,
 				  read_wr.num_sge);
-
+		if (((ch+1)->rc_discrim == 0) &&
+		    (read_wr.num_sge == ch_sge_ary[ch_no].count)) {
+			/*
+			 * Mark the last RDMA_READ with a bit to
+			 * indicate all RPC data has been fetched from
+			 * the client and the RPC needs to be enqueued.
+			 */
+			set_bit(RDMACTXT_F_LAST_CTXT, &ctxt->flags);
+			ctxt->next = hdr_ctxt;
+			hdr_ctxt->next = head;
+		}
 		/* Post the read */
 		err = svc_rdma_send(xprt, &read_wr);
 		if (err) {