[...]
> +/**
> + * 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);
> +}
> +
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