relayfs: abolish prev_padding

JIRA: https://issues.redhat.com/browse/RHEL-116876

commit 2489e95
Author: Jason Xing <kernelxing@tencent.com>
Date:   Thu Jun 12 14:11:57 2025 +0800

    relayfs: abolish prev_padding

    Patch series "relayfs: misc changes", v5.

    The series mostly focuses on the error counters which helps every user
    debug their own kernel module.

    This patch (of 5):

    prev_padding represents the unused space of certain subbuffer.  If the
    content of a call of relay_write() exceeds the limit of the remainder of
    this subbuffer, it will skip storing in the rest space and record the
    start point as buf->prev_padding in relay_switch_subbuf().  Since the buf
    is a per-cpu big buffer, the point of prev_padding as a global value for
    the whole buffer instead of a single subbuffer (whose padding info is
    stored in buf->padding[]) seems meaningless from the real use cases, so we
    don't bother to record it any more.

    Link: https://lkml.kernel.org/r/20250612061201.34272-1-kerneljasonxing@gmail.com
    Link: https://lkml.kernel.org/r/20250612061201.34272-2-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>

Signed-off-by: Anusha Srivatsa <asrivats@redhat.com>

Author: anushasrivats

drivers/gpu/drm/i915/gt/uc/intel_guc_log.c

@@ -220,8 +220,7 @@ static int guc_action_control_log(struct intel_guc *guc, bool enable,
  */
 static int subbuf_start_callback(struct rchan_buf *buf,
 				 void *subbuf,
-				 void *prev_subbuf,
-				 size_t prev_padding)
+				 void *prev_subbuf)
 {
 	/*
 	 * Use no-overwrite mode by default, where relay will stop accepting

drivers/net/wwan/iosm/iosm_ipc_trace.c

@@ -51,8 +51,7 @@ static int ipc_trace_remove_buf_file_handler(struct dentry *dentry)
 }
 
 static int ipc_trace_subbuf_start_handler(struct rchan_buf *buf, void *subbuf,
-					  void *prev_subbuf,
-					  size_t prev_padding)
+					  void *prev_subbuf)
 {
 	if (relay_buf_full(buf)) {
 		pr_err_ratelimited("Relay_buf full dropping traces");

drivers/net/wwan/t7xx/t7xx_port_trace.c

@@ -33,7 +33,7 @@ static int t7xx_trace_remove_buf_file_handler(struct dentry *dentry)
 }
 
 static int t7xx_trace_subbuf_start_handler(struct rchan_buf *buf, void *subbuf,
-					   void *prev_subbuf, size_t prev_padding)
+					   void *prev_subbuf)
 {
 	if (relay_buf_full(buf)) {
 		pr_err_ratelimited("Relay_buf full dropping traces");

include/linux/relay.h

@@ -47,7 +47,6 @@ struct rchan_buf
 	unsigned int page_count;	/* number of current buffer pages */
 	unsigned int finalized;		/* buffer has been finalized */
 	size_t *padding;		/* padding counts per sub-buffer */
-	size_t prev_padding;		/* temporary variable */
 	size_t bytes_consumed;		/* bytes consumed in cur read subbuf */
 	size_t early_bytes;		/* bytes consumed before VFS inited */
 	unsigned int cpu;		/* this buf's cpu */
@@ -84,7 +83,6 @@ struct rchan_callbacks
 	 * @buf: the channel buffer containing the new sub-buffer
 	 * @subbuf: the start of the new sub-buffer
 	 * @prev_subbuf: the start of the previous sub-buffer
-	 * @prev_padding: unused space at the end of previous sub-buffer
 	 *
 	 * The client should return 1 to continue logging, 0 to stop
 	 * logging.
@@ -100,8 +98,7 @@ struct rchan_callbacks
 	 */
 	int (*subbuf_start) (struct rchan_buf *buf,
 			     void *subbuf,
-			     void *prev_subbuf,
-			     size_t prev_padding);
+			     void *prev_subbuf);
 
 	/*
 	 * create_buf_file - create file to represent a relay channel buffer

kernel/relay.c

@@ -250,13 +250,13 @@ EXPORT_SYMBOL_GPL(relay_buf_full);
  */
 
 static int relay_subbuf_start(struct rchan_buf *buf, void *subbuf,
-			      void *prev_subbuf, size_t prev_padding)
+			      void *prev_subbuf)
 {
 	if (!buf->chan->cb->subbuf_start)
 		return !relay_buf_full(buf);
 
 	return buf->chan->cb->subbuf_start(buf, subbuf,
-					   prev_subbuf, prev_padding);
+					   prev_subbuf);
 }
 
 /**
@@ -302,7 +302,7 @@ static void __relay_reset(struct rchan_buf *buf, unsigned int init)
 	for (i = 0; i < buf->chan->n_subbufs; i++)
 		buf->padding[i] = 0;
 
-	relay_subbuf_start(buf, buf->data, NULL, 0);
+	relay_subbuf_start(buf, buf->data, NULL);
 }
 
 /**
@@ -665,9 +665,11 @@ size_t relay_switch_subbuf(struct rchan_buf *buf, size_t length)
 		goto toobig;
 
 	if (buf->offset != buf->chan->subbuf_size + 1) {
-		buf->prev_padding = buf->chan->subbuf_size - buf->offset;
+		size_t prev_padding;
+
+		prev_padding = buf->chan->subbuf_size - buf->offset;
 		old_subbuf = buf->subbufs_produced % buf->chan->n_subbufs;
-		buf->padding[old_subbuf] = buf->prev_padding;
+		buf->padding[old_subbuf] = prev_padding;
 		buf->subbufs_produced++;
 		if (buf->dentry)
 			d_inode(buf->dentry)->i_size +=
@@ -692,7 +694,7 @@ size_t relay_switch_subbuf(struct rchan_buf *buf, size_t length)
 	new_subbuf = buf->subbufs_produced % buf->chan->n_subbufs;
 	new = buf->start + new_subbuf * buf->chan->subbuf_size;
 	buf->offset = 0;
-	if (!relay_subbuf_start(buf, new, old, buf->prev_padding)) {
+	if (!relay_subbuf_start(buf, new, old)) {
 		buf->offset = buf->chan->subbuf_size + 1;
 		return 0;
 	}

kernel/trace/blktrace.c

@@ -461,7 +461,7 @@ static const struct file_operations blk_msg_fops = {
  * the user space app in telling how many lost events there were.
  */
 static int blk_subbuf_start_callback(struct rchan_buf *buf, void *subbuf,
-				     void *prev_subbuf, size_t prev_padding)
+				     void *prev_subbuf)
 {
 	struct blk_trace *bt;