NFSD: Add an nfsd_file_fsync tracepoint

Add a tracepoint to capture the number of filecache-triggered fsync
calls and which files needed it. Also, record when an fsync triggers
a write verifier reset.

Examples:

<...>-97 [007] 262.505611: nfsd_file_free: inode=0xffff888171e08140 ref=0 flags=GC may=WRITE nf_file=0xffff8881373d2400
<...>-97 [007] 262.505612: nfsd_file_fsync: inode=0xffff888171e08140 ref=0 flags=GC may=WRITE nf_file=0xffff8881373d2400 ret=0
<...>-97 [007] 262.505623: nfsd_file_free: inode=0xffff888171e08dc0 ref=0 flags=GC may=WRITE nf_file=0xffff8881373d1e00
<...>-97 [007] 262.505624: nfsd_file_fsync: inode=0xffff888171e08dc0 ref=0 flags=GC may=WRITE nf_file=0xffff8881373d1e00 ret=0

Signed-off-by: Chuck Lever <[e-mail]>
Reviewed-by: Jeff Layton <[e-mail]>

fs/nfsd/filecache.c

···
 nfsd_file_fsync(struct nfsd_file *nf)
 {
 	struct file *file = nf->nf_file;
+	int ret;
 
 	if (!file || !(file->f_mode & FMODE_WRITE))
 		return;
-	if (vfs_fsync(file, 1) != 0)
+	ret = vfs_fsync(file, 1);
+	trace_nfsd_file_fsync(nf, ret);
+	if (ret)
 		nfsd_reset_write_verifier(net_generic(nf->nf_net, nfsd_net_id));
 }
 

fs/nfsd/trace.h

···
 DEFINE_NFSD_FILE_LRUWALK_EVENT(nfsd_file_gc_removed);
 DEFINE_NFSD_FILE_LRUWALK_EVENT(nfsd_file_shrinker_removed);
 
+TRACE_EVENT(nfsd_file_fsync,
+	TP_PROTO(
+		const struct nfsd_file *nf,
+		int ret
+	),
+	TP_ARGS(nf, ret),
+	TP_STRUCT__entry(
+		__field(void *, nf_inode)
+		__field(int, nf_ref)
+		__field(int, ret)
+		__field(unsigned long, nf_flags)
+		__field(unsigned char, nf_may)
+		__field(struct file *, nf_file)
+	),
+	TP_fast_assign(
+		__entry->nf_inode = nf->nf_inode;
+		__entry->nf_ref = refcount_read(&nf->nf_ref);
+		__entry->ret = ret;
+		__entry->nf_flags = nf->nf_flags;
+		__entry->nf_may = nf->nf_may;
+		__entry->nf_file = nf->nf_file;
+	),
+	TP_printk("inode=%p ref=%d flags=%s may=%s nf_file=%p ret=%d",
+		__entry->nf_inode,
+		__entry->nf_ref,
+		show_nf_flags(__entry->nf_flags),
+		show_nfsd_may_flags(__entry->nf_may),
+		__entry->nf_file, __entry->ret
+	)
+);
+
 #include "cache.h"
 
 TRACE_DEFINE_ENUM(RC_DROPIT);