[PATCH RFC] nfsd: Fix NFSv4 READ on RDMA when using readv

[Chuck Lever <chuck.lever@xxxxxxxxxx>]

svcrdma expects that the READ payload falls precisely into the
xdr_buf's page vector. Adding "xdr->iov = NULL" forces
xdr_reserve_space() to always use pages from xdr->buf->pages when
calling nfsd_readv.

Also, the XDR padding is problematic. For NFS/RDMA Write chunks,
the padding needs to be in xdr->buf->tail so that the transport can
skip over it. However for NFS/TCP and the NFS/RDMA Reply chunks,
the padding has to be retained. Not yet sure how to add this.

Fixes: b04209806384 ("nfsd4: allow exotic read compounds")
Buglink: https://bugzilla.kernel.org/show_bug.cgi?id=198053
Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
---
Howdy Bruce-

I'm struggling with nfsd4_encode_readv().

- for NFS/RDMA Write chunks, the READ payload has to be in
  buf->pages. I've fixed that.

- xdr_reserve_space() calls don't need to explicitly align the
  @nbytes argument: xdr_reserve_space() already does this?

- the while loop probably won't work if a later READ in the COMPOUND
  doesn't start on a page boundary. This isn't a problem until we
  run into a Solaris client in forcedirectio mode.

- the XDR padding doesn't work for NFS/RDMA Write chunks, which are
  supposed to skip padding altogether.

Do you have suggestions? Thanks in advance.


 fs/nfsd/nfs4xdr.c |   17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
index d2dc4c0e22e8..14c68a136b4e 100644
--- a/fs/nfsd/nfs4xdr.c
+++ b/fs/nfsd/nfs4xdr.c
@@ -3519,17 +3519,14 @@ static __be32 nfsd4_encode_readv(struct nfsd4_compoundres *resp,
 	u32 zzz = 0;
 	int pad;
 
+	/* Ensure xdr_reserve_space behaves itself */
+	if (xdr->iov == xdr->buf->head) {
+		xdr->iov = NULL;
+		xdr->end = xdr->p;
+	}
+
 	len = maxcount;
 	v = 0;
-
-	thislen = min_t(long, len, ((void *)xdr->end - (void *)xdr->p));
-	p = xdr_reserve_space(xdr, (thislen+3)&~3);
-	WARN_ON_ONCE(!p);
-	resp->rqstp->rq_vec[v].iov_base = p;
-	resp->rqstp->rq_vec[v].iov_len = thislen;
-	v++;
-	len -= thislen;
-
 	while (len) {
 		thislen = min_t(long, len, PAGE_SIZE);
 		p = xdr_reserve_space(xdr, (thislen+3)&~3);
@@ -3548,7 +3545,7 @@ static __be32 nfsd4_encode_readv(struct nfsd4_compoundres *resp,
 	read->rd_length = maxcount;
 	if (nfserr)
 		return nfserr;
-	xdr_truncate_encode(xdr, starting_len + 8 + ((maxcount+3)&~3));
+	xdr_truncate_encode(xdr, starting_len + 8 + maxcount);
 
 	tmp = htonl(eof);
 	write_bytes_to_xdr_buf(xdr->buf, starting_len    , &tmp, 4);