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

From: Vincent Donnefort <vdonnefort@google.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Linux Trace Devel <linux-trace-devel@vger.kernel.org>
Subject: Re: [PATCH] libtracefs: Add ring buffer memory mapping APIs
Date: Tue, 23 Jan 2024 09:52:55 +0000	[thread overview]
Message-ID: <Za-Md51snPcIoYFn@google.com> (raw)
In-Reply-To: <20240105152906.743d7e03@gandalf.local.home>

[...]

> +/**
> + * 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);
> +	}

We're fast-forwarding tmap->kbuf here. But in tracefs_cpu_read_buf(), we are
using tcpu->kbuf. So overall, it seems this has no effect on what will be read
later.

Sorry, not sure how I missed that when I worked with those changes before.

[...]