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

  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.

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
  

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@efficios.com/T/#t

Thanks,

Mathieu

> 
> Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
> 
> 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;
  

On Mon, Feb 05, 2024 at 11:55:08AM -0500, Mathieu Desnoyers wrote:
> 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@efficios.com/T/#t

Hi Mathieu,

Thanks for the pointer.

We are in the exact same problem as DAX. We do modify the data through the
kernel linear mapping while user-space can read it through its own. I should
probably return an error when used with any of the arch ARM || SPARC || MIPS,
until cpu_dcache_is_aliasing() introduces a fine-grain differentiation.

> 
> Thanks,
> 
> Mathieu
> 
> > 
> > Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
> > 
> > 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
>
  

On 2024-02-05 13:34, Vincent Donnefort wrote:
> On Mon, Feb 05, 2024 at 11:55:08AM -0500, Mathieu Desnoyers wrote:
[...]

>>
>> 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@efficios.com/T/#t
> 
> Hi Mathieu,
> 
> Thanks for the pointer.
> 
> We are in the exact same problem as DAX. We do modify the data through the
> kernel linear mapping while user-space can read it through its own. I should
> probably return an error when used with any of the arch ARM || SPARC || MIPS,
> until cpu_dcache_is_aliasing() introduces a fine-grain differentiation.

You might want to use LTTng's ring buffer approach instead. See

https://github.com/lttng/lttng-modules/blob/master/src/lib/ringbuffer/ring_buffer_frontend.c#L1202

lib_ring_buffer_flush_read_subbuf_dcache()

Basically, whenever user-space grabs a sub-buffer for reading (through
lttng-modules's LTTNG_KERNEL_ABI_RING_BUFFER_GET_SUBBUF ioctl), lttng
calls flush_dcache_page() on all pages of this subbuffer (I should
really change this for a call to flush_dcache_folio() which would be
more efficient).

Note that doing this is not very efficient on architectures which have
coherent data caches and incoherent dcache vs icache: in that case,
we issue the flush_dcache_page uselessly. I plan on using the new
cpu_dcache_is_aliasing() check once/if it makes it way upstream to
remove those useless flushes on architectures which define
ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE, but do not virtually alias the
data cache.

The equivalent of LTTng's "get subbuf" operation would be
the new TRACE_MMAP_IOCTL_GET_READER ioctl in ftrace AFAIU.

Thanks,

Mathieu
  

On Mon, Feb 05, 2024 at 01:44:47PM -0500, Mathieu Desnoyers wrote:
> On 2024-02-05 13:34, Vincent Donnefort wrote:
> > On Mon, Feb 05, 2024 at 11:55:08AM -0500, Mathieu Desnoyers wrote:
> [...]
> 
> > > 
> > > 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@efficios.com/T/#t
> > 
> > Hi Mathieu,
> > 
> > Thanks for the pointer.
> > 
> > We are in the exact same problem as DAX. We do modify the data through the
> > kernel linear mapping while user-space can read it through its own. I should
> > probably return an error when used with any of the arch ARM || SPARC || MIPS,
> > until cpu_dcache_is_aliasing() introduces a fine-grain differentiation.
> 
> You might want to use LTTng's ring buffer approach instead. See
> 
> https://github.com/lttng/lttng-modules/blob/master/src/lib/ringbuffer/ring_buffer_frontend.c#L1202
> 
> lib_ring_buffer_flush_read_subbuf_dcache()

Thanks!

> 
> Basically, whenever user-space grabs a sub-buffer for reading (through
> lttng-modules's LTTNG_KERNEL_ABI_RING_BUFFER_GET_SUBBUF ioctl), lttng
> calls flush_dcache_page() on all pages of this subbuffer (I should
> really change this for a call to flush_dcache_folio() which would be
> more efficient).
> 
> Note that doing this is not very efficient on architectures which have
> coherent data caches and incoherent dcache vs icache: in that case,
> we issue the flush_dcache_page uselessly. I plan on using the new
> cpu_dcache_is_aliasing() check once/if it makes it way upstream to
> remove those useless flushes on architectures which define
> ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE, but do not virtually alias the
> data cache.

I believe the aim is to use the mapping by default in libtracefs and fallback to
splice whenever not available...  But for those arch, I guess that might be a
mistake. Wonder if then it isn't just better to return ENOTSUPP?

> 
> The equivalent of LTTng's "get subbuf" operation would be
> the new TRACE_MMAP_IOCTL_GET_READER ioctl in ftrace AFAIU.

That is correct!

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

Hi Vincent,

kernel test robot noticed the following build warnings:

[auto build test WARNING on e2412e51fdea837b50ce31fea8e5dfc885237f3a]

url:    https://github.com/intel-lab-lkp/linux/commits/Vincent-Donnefort/ring-buffer-Zero-ring-buffer-sub-buffers/20240206-004413
base:   e2412e51fdea837b50ce31fea8e5dfc885237f3a
patch link:    https://lore.kernel.org/r/20240205163410.2296552-5-vdonnefort%40google.com
patch subject: [PATCH v14 4/6] tracing: Allow user-space mapping of the ring-buffer
config: arc-defconfig (https://download.01.org/0day-ci/archive/20240206/202402061809.T2cv9J8W-lkp@intel.com/config)
compiler: arc-elf-gcc (GCC) 13.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20240206/202402061809.T2cv9J8W-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202402061809.T2cv9J8W-lkp@intel.com/

All warnings (new ones prefixed by >>):

   kernel/trace/trace.c: In function 'tracing_buffers_mmap_close':
>> kernel/trace/trace.c:8722:29: warning: unused variable 'tr' [-Wunused-variable]
    8722 |         struct trace_array *tr = iter->tr;
         |                             ^~
   kernel/trace/trace.c: In function 'tracing_buffers_mmap':
   kernel/trace/trace.c:8752:29: warning: unused variable 'tr' [-Wunused-variable]
    8752 |         struct trace_array *tr = iter->tr;
         |                             ^~


vim +/tr +8722 kernel/trace/trace.c

  8717	
  8718	static void tracing_buffers_mmap_close(struct vm_area_struct *vma)
  8719	{
  8720		struct ftrace_buffer_info *info = vma->vm_file->private_data;
  8721		struct trace_iterator *iter = &info->iter;
> 8722		struct trace_array *tr = iter->tr;
  8723	
  8724		ring_buffer_unmap(iter->array_buffer->buffer, iter->cpu_file);
  8725