Re: [PATCH] libtracefs: Add ring buffer memory mapping APIs

[Vincent Donnefort <vdonnefort@google.com>]

[...]

> +/**
> + * trace_mmap - try to mmap the ring buffer
> + * @fd: The file descriptor to the trace_pipe_raw file
> + * @kbuf: The kbuffer to load the subbuffer to
> + *
> + * Will try to mmap the ring buffer if it is supported, and
> + * if not, will return NULL, otherwise it returns a descriptor
> + * to handle the mapping.
> + */
> +__hidden void *trace_mmap(int fd, struct kbuffer *kbuf)
> +{
> +	struct trace_mmap *tmap;
> +	int page_size;
> +	void *meta;
> +	void *data;
> +
> +	page_size = getpagesize();
> +	meta = mmap(NULL, page_size, PROT_READ, MAP_SHARED, fd, 0);
> +	if (meta == MAP_FAILED)
> +		return NULL;
> +
> +	tmap = calloc(1, sizeof(*tmap));
> +	if (!tmap) {
> +		munmap(meta, page_size);
> +		return NULL;
> +	}
> +
> +	tmap->kbuf = kbuffer_dup(kbuf);
> +	if (!tmap->kbuf) {
> +		munmap(meta, page_size);
> +		free(tmap);
> +	}
> +
> +	tmap->fd = fd;
> +
> +	tmap->map = meta;
> +	tmap->meta_len = tmap->map->meta_page_size;
> +
> +	if (tmap->meta_len > page_size) {
> +		munmap(meta, page_size);
> +		meta = mmap(NULL, tmap->meta_len, PROT_READ, MAP_SHARED, fd, 0);
> +		if (meta == MAP_FAILED) {
> +			kbuffer_free(tmap->kbuf);
> +			free(tmap);
> +			return NULL;
> +		}
> +		tmap->map = meta;
> +	}
> +
> +	tmap->data_pages = meta + tmap->meta_len;
> +
> +	tmap->data_len = tmap->map->subbuf_size * tmap->map->nr_subbufs;
> +
> +	tmap->data = mmap(NULL, tmap->data_len, PROT_READ, MAP_SHARED,
> +			  fd, tmap->meta_len);
> +	if (tmap->data == MAP_FAILED) {
> +		munmap(meta, tmap->meta_len);
> +		kbuffer_free(tmap->kbuf);
> +		free(tmap);
> +		return NULL;
> +	}
> +
> +	tmap->last_idx = tmap->map->reader.id;
> +
> +	data = tmap->data + tmap->map->subbuf_size * tmap->last_idx;
> +	kbuffer_load_subbuffer(kbuf, data);
> +
> +	/*
> +	 * The page could have left over data on it that was already
> +	 * consumed. Move the "read" forward in that case.
> +	 */
> +	if (tmap->map->reader.read) {
> +		int size = kbuffer_start_of_data(kbuf) + tmap->map->reader.read;
> +		char tmpbuf[size];
> +		kbuffer_read_buffer(kbuf, tmpbuf, size);

It does not seem to update the kbuf timestamp. To observe the problem I did:

### Create few events on the page

$ echo 0 > /sys/kernel/tracing/trace
$ <read ring-buffer>
$ cat /proc/uptime | awk '{print $1}' > /sys/kernel/debug/tracing/trace_marker
<...>-2305 279515.453542096     print: tracing_mark_write: 279515.33
<...>-2307 279522.090413680     print: tracing_mark_write: 279521.97
<...>-2309 279522.960932976     print: tracing_mark_write: 279522.85
$ <unmap and close ring-buffer>


### Re-map again the ring-buffer to trigger the fast-forward

$ <read ring-buffer>
  before fast-forward kbuf->timestamp=279515453542096
  after fast-forward kbuf->timestamp=279515453542096
$ cat /proc/uptime | awk '{print $1}' > /sys/kernel/debug/tracing/trace_marker
<...>-2312 279549.725524688     print: tracing_mark_write: 279557.12

The timestamp above is a few seconds off, which I believe might be due to an
outdated kbuf->timestamp.

> +	}
> +
> +	return tmap;
> +}
> +
> +__hidden void trace_unmap(void *mapping)
> +{
> +	struct trace_mmap *tmap = mapping;
> +
> +	munmap(tmap->data, tmap->data_len);
> +	munmap(tmap->map, tmap->meta_len);
> +	kbuffer_free(tmap->kbuf);
> +	free(tmap);
> +}
> +