NFS: Introduce trace events triggered by page writeback errors

Try to capture the reason for the writeback path tagging an error on
a page.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/nfstrace.h

@@ -989,6 +989,51 @@ TRACE_EVENT(nfs_writeback_done,
 		)
 );
 
+DECLARE_EVENT_CLASS(nfs_page_error_class,
+		TP_PROTO(
+			const struct nfs_page *req,
+			int error
+		),
+
+		TP_ARGS(req, error),
+
+		TP_STRUCT__entry(
+			__field(const void *, req)
+			__field(pgoff_t, index)
+			__field(unsigned int, offset)
+			__field(unsigned int, pgbase)
+			__field(unsigned int, bytes)
+			__field(int, error)
+		),
+
+		TP_fast_assign(
+			__entry->req = req;
+			__entry->index = req->wb_index;
+			__entry->offset = req->wb_offset;
+			__entry->pgbase = req->wb_pgbase;
+			__entry->bytes = req->wb_bytes;
+			__entry->error = error;
+		),
+
+		TP_printk(
+			"req=%p index=%lu offset=%u pgbase=%u bytes=%u error=%d",
+			__entry->req, __entry->index, __entry->offset,
+			__entry->pgbase, __entry->bytes, __entry->error
+		)
+);
+
+#define DEFINE_NFS_PAGEERR_EVENT(name) \
+	DEFINE_EVENT(nfs_page_error_class, name, \
+			TP_PROTO( \
+				const struct nfs_page *req, \
+				int error \
+			), \
+			TP_ARGS(req, error))
+
+DEFINE_NFS_PAGEERR_EVENT(nfs_write_error);
+DEFINE_NFS_PAGEERR_EVENT(nfs_comp_error);
+DEFINE_NFS_PAGEERR_EVENT(nfs_commit_error);
+
 TRACE_EVENT(nfs_initiate_commit,
 		TP_PROTO(
 			const struct nfs_commit_data *data

fs/nfs/write.c

@@ -593,6 +593,7 @@ nfs_lock_and_join_requests(struct page *page)
 static void nfs_write_error(struct nfs_page *req, int error)
 {
 	nfs_set_pageerror(page_file_mapping(req->wb_page));
+	trace_nfs_write_error(req, error);
 	nfs_mapping_set_error(req->wb_page, error);
 	nfs_inode_remove_request(req);
 	nfs_end_page_writeback(req);
@@ -999,6 +1000,7 @@ static void nfs_write_completion(struct nfs_pgio_header *hdr)
 		if (test_bit(NFS_IOHDR_ERROR, &hdr->flags) &&
 		    (hdr->good_bytes < bytes)) {
 			nfs_set_pageerror(page_file_mapping(req->wb_page));
+			trace_nfs_comp_error(req, hdr->error);
 			nfs_mapping_set_error(req->wb_page, hdr->error);
 			goto remove_req;
 		}
@@ -1847,6 +1849,7 @@ static void nfs_commit_release_pages(struct nfs_commit_data *data)
 			(long long)req_offset(req));
 		if (status < 0) {
 			if (req->wb_page) {
+				trace_nfs_commit_error(req, status);
 				nfs_mapping_set_error(req->wb_page, status);
 				nfs_inode_remove_request(req);
 			}