[PATCH v6 1/2] trace-cmd library: Use the real trace buffer page size

["Tzvetomir Stoyanov (VMware)" <tz.stoyanov@gmail.com>]

When new output handler is created, it assumes that the trace buffer page
size is equal to the system memory page size. This assumption is valid
for the current ftrace implementation, but it may change in the future.
The newly introduced traceevent library API should be used to get the
real trace buffer page size, bases on the information from the
"events/header_page" ftrace file.

This commit depends on:
 [PATCH] libtraceevent: A new API for trace page size
 https://lore.kernel.org/linux-trace-devel/20211001062338.2389024-1-tz.stoyanov@gmail.com/

Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
---
 lib/trace-cmd/trace-output.c | 37 +++++++++++++++++++++++++++++++++++-
 1 file changed, 36 insertions(+), 1 deletion(-)

diff --git a/lib/trace-cmd/trace-output.c b/lib/trace-cmd/trace-output.c
index e7915aa6..cd40d312 100644
--- a/lib/trace-cmd/trace-output.c
+++ b/lib/trace-cmd/trace-output.c
@@ -1299,6 +1299,41 @@ static int write_compression_header(struct tracecmd_output *handle)
 	return 0;
 }
 
+static int get_trace_page_size(struct tracecmd_output *handle)
+{
+	struct tracefs_instance *instance;
+	struct tep_handle *tep = NULL;
+	int psize, size;
+	char *buff = NULL;
+
+	/* In case of an error, return user space page size */
+	psize = getpagesize();
+
+	instance = tracefs_instance_alloc(find_tracing_dir(handle), NULL);
+	if (!instance)
+		goto out;
+
+	buff = tracefs_instance_file_read(instance, "events/header_page", &size);
+	if (!buff)
+		goto out;
+
+	tep = tep_alloc();
+	if (!tep)
+		goto out;
+
+	if (tep_parse_header_page(tep, buff, size, sizeof(long long)))
+		goto out;
+
+	psize = tep_get_sub_buffer_size(tep);
+
+out:
+	tracefs_instance_free(instance);
+	tep_free(tep);
+	free(buff);
+
+	return psize;
+}
+
 /**
  * tracecmd_output_create_fd - allocate new output handle to a trace file
  * @fd: File descriptor for the handle to write to.
@@ -1323,7 +1358,7 @@ struct tracecmd_output *tracecmd_output_create_fd(int fd)
 
 	handle->file_version = FILE_VERSION_DEFAULT;
 
-	handle->page_size = getpagesize();
+	handle->page_size = get_trace_page_size(handle);
 	handle->big_endian = tracecmd_host_bigendian();
 
 	list_head_init(&handle->options);
-- 
2.34.1