relayfs: support a counter tracking if data is too big to write

It really doesn't matter if the user/admin knows what the last too big
value is.  Record how many times this case is triggered would be helpful.

Solve the existing issue where relay_reset() doesn't restore the value.

Store the counter in the per-cpu buffer structure instead of the global
buffer structure.  It also solves the racy condition which is likely to
happen when a few of per-cpu buffers encounter the too big data case and
then access the global field last_toobig without lock protection.

Remove the printk in relay_close() since kernel module can directly call
relay_stats() as they want.

Link: https://lkml.kernel.org/r/20250612061201.34272-6-kerneljasonxing@gmail.com
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Yushan Zhou <katrinzhou@tencent.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 13 insertions, 10 deletions

diff --git a/include/linux/relay.h b/include/linux/relay.h
index 5310967f9d74..6772a7075840 100644
--- a/include/linux/relay.h
+++ b/include/linux/relay.h
@@ -33,13 +33,15 @@
  */
 enum {
 	RELAY_STATS_BUF_FULL = (1 << 0),
+	RELAY_STATS_WRT_BIG = (1 << 1),
 
-	RELAY_STATS_LAST = RELAY_STATS_BUF_FULL,
+	RELAY_STATS_LAST = RELAY_STATS_WRT_BIG,
 };
 
 struct rchan_buf_stats
 {
 	unsigned int full_count;	/* counter for buffer full */
+	unsigned int big_count;		/* counter for too big to write */
 };
 
 /*
@@ -79,7 +81,6 @@ struct rchan
 	const struct rchan_callbacks *cb; /* client callbacks */
 	struct kref kref;		/* channel refcount */
 	void *private_data;		/* for user-defined data */
-	size_t last_toobig;		/* tried to log event > subbuf size */
 	struct rchan_buf * __percpu *buf; /* per-cpu channel buffers */
 	int is_global;			/* One global buffer ? */
 	struct list_head list;		/* for channel list */
diff --git a/kernel/relay.c b/kernel/relay.c
index 2fc27c0e771e..8d915fe98198 100644
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -303,6 +303,7 @@ static void __relay_reset(struct rchan_buf *buf, unsigned int init)
 	buf->data = buf->start;
 	buf->offset = 0;
 	buf->stats.full_count = 0;
+	buf->stats.big_count = 0;
 
 	for (i = 0; i < buf->chan->n_subbufs; i++)
 		buf->padding[i] = 0;
@@ -602,7 +603,7 @@ size_t relay_switch_subbuf(struct rchan_buf *buf, size_t length)
 	return length;
 
 toobig:
-	buf->chan->last_toobig = length;
+	buf->stats.big_count++;
 	return 0;
 }
 EXPORT_SYMBOL_GPL(relay_switch_subbuf);
@@ -662,11 +663,6 @@ void relay_close(struct rchan *chan)
 			if ((buf = *per_cpu_ptr(chan->buf, i)))
 				relay_close_buf(buf);
 
-	if (chan->last_toobig)
-		printk(KERN_WARNING "relay: one or more items not logged "
-		       "[item size (%zd) > sub-buffer size (%zd)]\n",
-		       chan->last_toobig, chan->subbuf_size);
-
 	list_del(&chan->list);
 	kref_put(&chan->kref, relay_destroy_channel);
 	mutex_unlock(&relay_channels_mutex);
@@ -719,11 +715,17 @@ size_t relay_stats(struct rchan *chan, int flags)
 		rbuf = *per_cpu_ptr(chan->buf, 0);
 		if (flags & RELAY_STATS_BUF_FULL)
 			count = rbuf->stats.full_count;
+		else if (flags & RELAY_STATS_WRT_BIG)
+			count = rbuf->stats.big_count;
 	} else {
 		for_each_online_cpu(i) {
 			rbuf = *per_cpu_ptr(chan->buf, i);
-			if (rbuf && flags & RELAY_STATS_BUF_FULL)
-				count += rbuf->stats.full_count;
+			if (rbuf) {
+				if (flags & RELAY_STATS_BUF_FULL)
+					count += rbuf->stats.full_count;
+				else if (flags & RELAY_STATS_WRT_BIG)
+					count += rbuf->stats.big_count;
+			}
 		}
 	}