eventfs: Shortcut eventfs_iterate() by skipping entries already read

As the ei->entries array is fixed for the duration of the eventfs_inode,
it can be used to skip over already read entries in eventfs_iterate().

That is, if ctx->pos is greater than zero, there's no reason in doing the
loop across the ei->entries array for the entries less than ctx->pos.
Instead, start the lookup of the entries at the current ctx->pos.

Link: https://lore.kernel.org/all/CAHk-=wiKwDUDv3+jCsv-uacDcHDVTYsXtBR9=6sGM5mqX+DhOg@mail.gmail.com/
Link: https://lore.kernel.org/linux-trace-kernel/20240104220048.494956957@goodmis.org

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

fs/tracefs/event_inode.c

@@ -746,21 +746,15 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
 	if (!ei || !ei_dentry)
 		goto out;
 
-	ret = 0;
-
 	/*
 	 * Need to create the dentries and inodes to have a consistent
 	 * inode number.
 	 */
-	for (i = 0; i < ei->nr_entries; i++) {
-		void *cdata = ei->data;
-
-		if (c > 0) {
-			c--;
-			continue;
-		}
+	ret = 0;
 
-		ctx->pos++;
+	/* Start at 'c' to jump over already read entries */
+	for (i = c; i < ei->nr_entries; i++, ctx->pos++) {
+		void *cdata = ei->data;
 
 		entry = &ei->entries[i];
 		name = entry->name;
@@ -769,7 +763,7 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
 		/* If ei->is_freed then just bail here, nothing more to do */
 		if (ei->is_freed) {
 			mutex_unlock(&eventfs_mutex);
-			goto out_dec;
+			goto out;
 		}
 		r = entry->callback(name, &mode, &cdata, &fops);
 		mutex_unlock(&eventfs_mutex);
@@ -778,14 +772,17 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
 
 		dentry = create_file_dentry(ei, i, ei_dentry, name, mode, cdata, fops);
 		if (!dentry)
-			goto out_dec;
+			goto out;
 		ino = dentry->d_inode->i_ino;
 		dput(dentry);
 
 		if (!dir_emit(ctx, name, strlen(name), ino, DT_REG))
-			goto out_dec;
+			goto out;
 	}
 
+	/* Subtract the skipped entries above */
+	c -= min((unsigned int)c, (unsigned int)ei->nr_entries);
+
 	list_for_each_entry_srcu(ei_child, &ei->children, list,
 				 srcu_read_lock_held(&eventfs_srcu)) {