[PATCH v4 3/4] NFSD: Relocate the xdr_reserve_space_vec() call site

[Chuck Lever <cel@kernel.org>]

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

In order to detect when a direct READ is possible, we need the send
buffer's .page_len to be zero when there is nothing in the buffer's
.pages array yet.

However, when xdr_reserve_space_vec() extends the size of the
xdr_stream to accommodate a READ payload, it adds to the send
buffer's .page_len.

It should be safe to reserve the stream space /after/ the VFS read
operation completes. This is, for example, how an NFSv3 READ works:
the VFS read goes into the rq_bvec, and is then added to the send
xdr_stream later by svcxdr_encode_opaque_pages().

Nit: we could probably get rid of the xdr_truncate_encode(), now
that maxcount reflects the actual count of bytes returned by the
file system.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 fs/nfsd/nfs4xdr.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
index 41f0d54d6e1b..3a5c7a0e2db8 100644
--- a/fs/nfsd/nfs4xdr.c
+++ b/fs/nfsd/nfs4xdr.c
@@ -4475,18 +4475,30 @@ static __be32 nfsd4_encode_readv(struct nfsd4_compoundres *resp,
 	__be32 zero = xdr_zero;
 	__be32 nfserr;
 
-	if (xdr_reserve_space_vec(xdr, maxcount) < 0)
-		return nfserr_resource;
-
 	nfserr = nfsd_iter_read(resp->rqstp, read->rd_fhp, read->rd_nf,
 				read->rd_offset, &maxcount, base,
 				&read->rd_eof);
 	read->rd_length = maxcount;
 	if (nfserr)
 		return nfserr;
+
+	/*
+	 * svcxdr_encode_opaque_pages() is not used here because
+	 * we don't want to encode subsequent results in this
+	 * COMPOUND into the xdr->buf's tail, but rather those
+	 * results should follow the NFS READ payload in the
+	 * buf's pages.
+	 */
+	if (xdr_reserve_space_vec(xdr, maxcount) < 0)
+		return nfserr_resource;
+
+	/*
+	 * Mark the buffer location of the NFS READ payload so that
+	 * direct placement-capable transports send only the
+	 * payload bytes out-of-band.
+	 */
 	if (svc_encode_result_payload(resp->rqstp, starting_len, maxcount))
 		return nfserr_io;
-	xdr_truncate_encode(xdr, starting_len + xdr_align_size(maxcount));
 
 	write_bytes_to_xdr_buf(xdr->buf, starting_len + maxcount, &zero,
 			       xdr_pad_size(maxcount));
-- 
2.51.0