NFSv4.2: Rework scratch handling for READ_PLUS

Instead of using a tiny, static scratch buffer, we should use a kmalloc()-ed
buffer that is allocated when checking for read plus usage. This lets us
use the buffer before decoding any part of the READ_PLUS operation
instead of setting it right before segment decoding, meaning it should
be a little more robust.
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/nfs42xdr.c

@@ -1122,7 +1122,6 @@ static int decode_read_plus(struct xdr_stream *xdr, struct nfs_pgio_res *res)
 	uint32_t segments;
 	struct read_plus_segment *segs;
 	int status, i;
-	char scratch_buf[16];
 	__be32 *p;
 
 	status = decode_op_hdr(xdr, OP_READ_PLUS);
@@ -1143,7 +1142,6 @@ static int decode_read_plus(struct xdr_stream *xdr, struct nfs_pgio_res *res)
 	if (!segs)
 		return -ENOMEM;
 
-	xdr_set_scratch_buffer(xdr, &scratch_buf, sizeof(scratch_buf));
 	status = -EIO;
 	for (i = 0; i < segments; i++) {
 		status = decode_read_plus_segment(xdr, &segs[i]);
@@ -1348,6 +1346,8 @@ static int nfs4_xdr_dec_read_plus(struct rpc_rqst *rqstp,
 	struct compound_hdr hdr;
 	int status;
 
+	xdr_set_scratch_buffer(xdr, res->scratch, sizeof(res->scratch));
+
 	status = decode_compound_hdr(xdr, &hdr);
 	if (status)
 		goto out;

fs/nfs/nfs4proc.c

@@ -5439,6 +5439,8 @@ static bool nfs4_read_plus_not_supported(struct rpc_task *task,
 
 static int nfs4_read_done(struct rpc_task *task, struct nfs_pgio_header *hdr)
 {
+	if (hdr->res.scratch)
+		kfree(hdr->res.scratch);
 	if (!nfs4_sequence_done(task, &hdr->res.seq_res))
 		return -EAGAIN;
 	if (nfs4_read_stateid_changed(task, &hdr->args))
@@ -5452,17 +5454,22 @@ static int nfs4_read_done(struct rpc_task *task, struct nfs_pgio_header *hdr)
 }
 
 #if defined CONFIG_NFS_V4_2 && defined CONFIG_NFS_V4_2_READ_PLUS
-static void nfs42_read_plus_support(struct nfs_pgio_header *hdr,
+static bool nfs42_read_plus_support(struct nfs_pgio_header *hdr,
 				    struct rpc_message *msg)
 {
 	/* Note: We don't use READ_PLUS with pNFS yet */
-	if (nfs_server_capable(hdr->inode, NFS_CAP_READ_PLUS) && !hdr->ds_clp)
+	if (nfs_server_capable(hdr->inode, NFS_CAP_READ_PLUS) && !hdr->ds_clp) {
 		msg->rpc_proc = &nfs4_procedures[NFSPROC4_CLNT_READ_PLUS];
+		hdr->res.scratch = kmalloc(32, GFP_KERNEL);
+		return hdr->res.scratch != NULL;
+	}
+	return false;
 }
 #else
-static void nfs42_read_plus_support(struct nfs_pgio_header *hdr,
+static bool nfs42_read_plus_support(struct nfs_pgio_header *hdr,
 				    struct rpc_message *msg)
 {
+	return false;
 }
 #endif /* CONFIG_NFS_V4_2 */
 
@@ -5472,8 +5479,8 @@ static void nfs4_proc_read_setup(struct nfs_pgio_header *hdr,
 	hdr->timestamp   = jiffies;
 	if (!hdr->pgio_done_cb)
 		hdr->pgio_done_cb = nfs4_read_done_cb;
+	if (!nfs42_read_plus_support(hdr, msg))
 		msg->rpc_proc = &nfs4_procedures[NFSPROC4_CLNT_READ];
-	nfs42_read_plus_support(hdr, msg);
 	nfs4_init_sequence(&hdr->args.seq_args, &hdr->res.seq_res, 0, 0);
 }
 

include/linux/nfs_xdr.h

@@ -670,6 +670,7 @@ struct nfs_pgio_res {
 		struct {
 			unsigned int		replen;		/* used by read */
 			int			eof;		/* used by read */
+			void *			scratch;	/* used by read */
 		};
 		struct {
 			struct nfs_writeverf *	verf;		/* used by write */