NFS: Trace short NFS READs

A short read can generate an -EIO error without there being an error
on the wire. This tracepoint acts as an eyecatcher when there is no
obvious I/O error.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/nfstrace.h

@@ -961,6 +961,53 @@ TRACE_EVENT(nfs_readpage_done,
 		)
 );
 
+TRACE_EVENT(nfs_readpage_short,
+		TP_PROTO(
+			const struct rpc_task *task,
+			const struct nfs_pgio_header *hdr
+		),
+
+		TP_ARGS(task, hdr),
+
+		TP_STRUCT__entry(
+			__field(dev_t, dev)
+			__field(u32, fhandle)
+			__field(u64, fileid)
+			__field(loff_t, offset)
+			__field(u32, arg_count)
+			__field(u32, res_count)
+			__field(bool, eof)
+			__field(int, status)
+		),
+
+		TP_fast_assign(
+			const struct inode *inode = hdr->inode;
+			const struct nfs_inode *nfsi = NFS_I(inode);
+			const struct nfs_fh *fh = hdr->args.fh ?
+						  hdr->args.fh : &nfsi->fh;
+
+			__entry->status = task->tk_status;
+			__entry->offset = hdr->args.offset;
+			__entry->arg_count = hdr->args.count;
+			__entry->res_count = hdr->res.count;
+			__entry->eof = hdr->res.eof;
+			__entry->dev = inode->i_sb->s_dev;
+			__entry->fileid = nfsi->fileid;
+			__entry->fhandle = nfs_fhandle_hash(fh);
+		),
+
+		TP_printk(
+			"fileid=%02x:%02x:%llu fhandle=0x%08x "
+			"offset=%lld count=%u res=%u status=%d%s",
+			MAJOR(__entry->dev), MINOR(__entry->dev),
+			(unsigned long long)__entry->fileid,
+			__entry->fhandle,
+			(long long)__entry->offset, __entry->arg_count,
+			__entry->res_count, __entry->status,
+			__entry->eof ? " eof" : ""
+		)
+);
+
 TRACE_DEFINE_ENUM(NFS_UNSTABLE);
 TRACE_DEFINE_ENUM(NFS_DATA_SYNC);
 TRACE_DEFINE_ENUM(NFS_FILE_SYNC);

fs/nfs/read.c

@@ -264,6 +264,8 @@ static void nfs_readpage_retry(struct rpc_task *task,
 
 	/* This is a short read! */
 	nfs_inc_stats(hdr->inode, NFSIOS_SHORTREAD);
+	trace_nfs_readpage_short(task, hdr);
+
 	/* Has the server at least made some progress? */
 	if (resp->count == 0) {
 		nfs_set_pgio_error(hdr, -EIO, argp->offset);