Re: [PATCH v14 4/6] tracing: Allow user-space mapping of the ring-buffer

[Mathieu Desnoyers]

On 2024-02-05 11:34, Vincent Donnefort wrote:
> Currently, user-space extracts data from the ring-buffer via splice,
> which is handy for storage or network sharing. However, due to splice
> limitations, it is imposible to do real-time analysis without a copy.
>
> A solution for that problem is to let the user-space map the ring-buffer
> directly.
>
> The mapping is exposed via the per-CPU file trace_pipe_raw. The first
> element of the mapping is the meta-page. It is followed by each
> subbuffer constituting the ring-buffer, ordered by their unique page ID:
>
>    * Meta-page -- include/uapi/linux/trace_mmap.h for a description
>    * Subbuf ID 0
>    * Subbuf ID 1
>       ...
>
> It is therefore easy to translate a subbuf ID into an offset in the
> mapping:
>
>    reader_id = meta->reader->id;
>    reader_offset = meta->meta_page_size + reader_id * meta->subbuf_size;
>
> When new data is available, the mapper must call a newly introduced ioctl:
> TRACE_MMAP_IOCTL_GET_READER. This will update the Meta-page reader ID to
> point to the next reader containing unread data.
>
> Mapping will prevent snapshot and buffer size modifications.

How are the kernel linear mapping and the userspace mapping made coherent
on architectures with virtually aliasing data caches ?

Ref. https://lore.kernel.org/lkml/20240202210019.88022-1-mathieu.desnoyers@xxxxxxxxxxxx/T/#t

Thanks,

Mathieu

> Signed-off-by: Vincent Donnefort <vdonnefort@xxxxxxxxxx>
>
> diff --git a/include/uapi/linux/trace_mmap.h b/include/uapi/linux/trace_mmap.h
> index 182e05a3004a..7330249257e7 100644
> --- a/include/uapi/linux/trace_mmap.h
> +++ b/include/uapi/linux/trace_mmap.h
> @@ -43,4 +43,6 @@ struct trace_buffer_meta {
>   	__u64	Reserved2;
>   };
>   
> +#define TRACE_MMAP_IOCTL_GET_READER		_IO('T', 0x1)
> +
>   #endif /* _TRACE_MMAP_H_ */
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 4ebf4d0bd14c..36b62cf2fb3f 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -1175,6 +1175,12 @@ static void tracing_snapshot_instance_cond(struct trace_array *tr,
>   		return;
>   	}
>   
> +	if (tr->mapped) {
> +		trace_array_puts(tr, "*** BUFFER MEMORY MAPPED ***\n");
> +		trace_array_puts(tr, "*** Can not use snapshot (sorry) ***\n");
> +		return;
> +	}
> +
>   	local_irq_save(flags);
>   	update_max_tr(tr, current, smp_processor_id(), cond_data);
>   	local_irq_restore(flags);
> @@ -1307,7 +1313,7 @@ static int tracing_arm_snapshot_locked(struct trace_array *tr)
>   	lockdep_assert_held(&trace_types_lock);
>   
>   	spin_lock(&tr->snapshot_trigger_lock);
> -	if (tr->snapshot == UINT_MAX) {
> +	if (tr->snapshot == UINT_MAX || tr->mapped) {
>   		spin_unlock(&tr->snapshot_trigger_lock);
>   		return -EBUSY;
>   	}
> @@ -6533,7 +6539,7 @@ static void tracing_set_nop(struct trace_array *tr)
>   {
>   	if (tr->current_trace == &nop_trace)
>   		return;
> -	
> +
>   	tr->current_trace->enabled--;
>   
>   	if (tr->current_trace->reset)
> @@ -8652,15 +8658,31 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
>   	return ret;
>   }
>   
> -/* An ioctl call with cmd 0 to the ring buffer file will wake up all waiters */
>   static long tracing_buffers_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
>   {
>   	struct ftrace_buffer_info *info = file->private_data;
>   	struct trace_iterator *iter = &info->iter;
> +	int err;
> +
> +	if (cmd == TRACE_MMAP_IOCTL_GET_READER) {
> +		if (!(file->f_flags & O_NONBLOCK)) {
> +			err = ring_buffer_wait(iter->array_buffer->buffer,
> +					       iter->cpu_file,
> +					       iter->tr->buffer_percent);
> +			if (err)
> +				return err;
> +		}
>   
> -	if (cmd)
> -		return -ENOIOCTLCMD;
> +		return ring_buffer_map_get_reader(iter->array_buffer->buffer,
> +						  iter->cpu_file);
> +	} else if (cmd) {
> +		return -ENOTTY;
> +	}
>   
> +	/*
> +	 * An ioctl call with cmd 0 to the ring buffer file will wake up all
> +	 * waiters
> +	 */
>   	mutex_lock(&trace_types_lock);
>   
>   	iter->wait_index++;
> @@ -8673,6 +8695,97 @@ static long tracing_buffers_ioctl(struct file *file, unsigned int cmd, unsigned
>   	return 0;
>   }
>   
> +static vm_fault_t tracing_buffers_mmap_fault(struct vm_fault *vmf)
> +{
> +	struct ftrace_buffer_info *info = vmf->vma->vm_file->private_data;
> +	struct trace_iterator *iter = &info->iter;
> +	vm_fault_t ret = VM_FAULT_SIGBUS;
> +	struct page *page;
> +
> +	page = ring_buffer_map_fault(iter->array_buffer->buffer, iter->cpu_file,
> +				     vmf->pgoff);
> +	if (!page)
> +		return ret;
> +
> +	get_page(page);
> +	vmf->page = page;
> +	vmf->page->mapping = vmf->vma->vm_file->f_mapping;
> +	vmf->page->index = vmf->pgoff;
> +
> +	return 0;
> +}
> +
> +static void tracing_buffers_mmap_close(struct vm_area_struct *vma)
> +{
> +	struct ftrace_buffer_info *info = vma->vm_file->private_data;
> +	struct trace_iterator *iter = &info->iter;
> +	struct trace_array *tr = iter->tr;
> +
> +	ring_buffer_unmap(iter->array_buffer->buffer, iter->cpu_file);
> +
> +#ifdef CONFIG_TRACER_MAX_TRACE
> +	spin_lock(&tr->snapshot_trigger_lock);
> +	if (!WARN_ON(!tr->mapped))
> +		tr->mapped--;
> +	spin_unlock(&tr->snapshot_trigger_lock);
> +#endif
> +}
> +
> +static void tracing_buffers_mmap_open(struct vm_area_struct *vma)
> +{
> +	struct ftrace_buffer_info *info = vma->vm_file->private_data;
> +	struct trace_iterator *iter = &info->iter;
> +
> +	WARN_ON(ring_buffer_map(iter->array_buffer->buffer, iter->cpu_file));
> +}
> +
> +static const struct vm_operations_struct tracing_buffers_vmops = {
> +	.open		= tracing_buffers_mmap_open,
> +	.close		= tracing_buffers_mmap_close,
> +	.fault		= tracing_buffers_mmap_fault,
> +};
> +
> +static int tracing_buffers_mmap(struct file *filp, struct vm_area_struct *vma)
> +{
> +	struct ftrace_buffer_info *info = filp->private_data;
> +	struct trace_iterator *iter = &info->iter;
> +	struct trace_array *tr = iter->tr;
> +	int ret = 0;
> +
> +	if (vma->vm_flags & VM_WRITE)
> +		return -EPERM;
> +
> +	vm_flags_mod(vma, VM_DONTCOPY | VM_DONTDUMP, VM_MAYWRITE);
> +	vma->vm_ops = &tracing_buffers_vmops;
> +
> +#ifdef CONFIG_TRACER_MAX_TRACE
> +	/*
> +	 * We hold mmap_lock here. lockdep would be unhappy if we would now take
> +	 * trace_types_lock. Instead use the specific snapshot_trigger_lock.
> +	 */
> +	spin_lock(&tr->snapshot_trigger_lock);
> +	if (tr->snapshot || tr->mapped == UINT_MAX) {
> +		spin_unlock(&tr->snapshot_trigger_lock);
> +		return -EBUSY;
> +	}
> +	tr->mapped++;
> +	spin_unlock(&tr->snapshot_trigger_lock);
> +
> +	/* Wait for update_max_tr() to observe iter->tr->mapped */
> +	if (tr->mapped == 1)
> +		synchronize_rcu();
> +#endif
> +	ret = ring_buffer_map(iter->array_buffer->buffer, iter->cpu_file);
> +#ifdef CONFIG_TRACER_MAX_TRACE
> +	if (ret) {
> +		spin_lock(&tr->snapshot_trigger_lock);
> +		iter->tr->mapped--;
> +		spin_unlock(&tr->snapshot_trigger_lock);
> +	}
> +#endif
> +	return ret;
> +}
> +
>   static const struct file_operations tracing_buffers_fops = {
>   	.open		= tracing_buffers_open,
>   	.read		= tracing_buffers_read,
> @@ -8681,6 +8794,7 @@ static const struct file_operations tracing_buffers_fops = {
>   	.splice_read	= tracing_buffers_splice_read,
>   	.unlocked_ioctl = tracing_buffers_ioctl,
>   	.llseek		= no_llseek,
> +	.mmap		= tracing_buffers_mmap,
>   };
>   
>   static ssize_t
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index bd312e9afe25..8a96e7a89e6b 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -336,6 +336,7 @@ struct trace_array {
>   	bool			allocated_snapshot;
>   	spinlock_t		snapshot_trigger_lock;
>   	unsigned int		snapshot;
> +	unsigned int		mapped;
>   	unsigned long		max_latency;
>   #ifdef CONFIG_FSNOTIFY
>   	struct dentry		*d_max_latency;

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com