Date: Tue, 30 Jan 2024 16:22:06 +0000
From: Vincent Donnefort <vdonnefort@...gle.com>
To: Masami Hiramatsu <mhiramat@...nel.org>, g@...gle.com
Cc: rostedt@...dmis.org, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
	kernel-team@...roid.com
Subject: Re: [PATCH v13 2/6] ring-buffer: Introducing ring-buffer mapping
 functions

On Tue, Jan 30, 2024 at 11:55:10PM +0900, Masami Hiramatsu wrote:
> Hi Vincent,
> 
> Thanks for update the code.
> 
> On Mon, 29 Jan 2024 14:27:58 +0000
> Vincent Donnefort <vdonnefort@...gle.com> wrote:
> 
> > In preparation for allowing the user-space to map a ring-buffer, add
> > a set of mapping functions:
> > 
> >   ring_buffer_{map,unmap}()
> >   ring_buffer_map_fault()
> > 
> > And controls on the ring-buffer:
> > 
> >   ring_buffer_map_get_reader()  /* swap reader and head */
> > 
> > Mapping the ring-buffer also involves:
> > 
> >   A unique ID for each subbuf of the ring-buffer, currently they are
> >   only identified through their in-kernel VA.
> > 
> >   A meta-page, where are stored ring-buffer statistics and a
> >   description for the current reader
> > 
> > The linear mapping exposes the meta-page, and each subbuf of the
> > ring-buffer, ordered following their unique ID, assigned during the
> > first mapping.
> > 
> > Once mapped, no subbuf can get in or out of the ring-buffer: the buffer
> > size will remain unmodified and the splice enabling functions will in
> > reality simply memcpy the data instead of swapping subbufs.
> > 
> > Signed-off-by: Vincent Donnefort <vdonnefort@...gle.com>
> > 
> > diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
> > index fa802db216f9..0841ba8bab14 100644
> > --- a/include/linux/ring_buffer.h
> > +++ b/include/linux/ring_buffer.h
> > @@ -6,6 +6,8 @@
> >  #include <linux/seq_file.h>
> >  #include <linux/poll.h>
> >  
> > +#include <uapi/linux/trace_mmap.h>
> > +
> >  struct trace_buffer;
> >  struct ring_buffer_iter;
> >  
> > @@ -221,4 +223,9 @@ int trace_rb_cpu_prepare(unsigned int cpu, struct hlist_node *node);
> >  #define trace_rb_cpu_prepare	NULL
> >  #endif
> >  
> > +int ring_buffer_map(struct trace_buffer *buffer, int cpu);
> > +int ring_buffer_unmap(struct trace_buffer *buffer, int cpu);
> > +struct page *ring_buffer_map_fault(struct trace_buffer *buffer, int cpu,
> > +				   unsigned long pgoff);
> > +int ring_buffer_map_get_reader(struct trace_buffer *buffer, int cpu);
> >  #endif /* _LINUX_RING_BUFFER_H */
> > diff --git a/include/uapi/linux/trace_mmap.h b/include/uapi/linux/trace_mmap.h
> > new file mode 100644
> > index 000000000000..d4bb67430719
> > --- /dev/null
> > +++ b/include/uapi/linux/trace_mmap.h
> > @@ -0,0 +1,43 @@
> > +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> > +#ifndef _TRACE_MMAP_H_
> > +#define _TRACE_MMAP_H_
> > +
> > +#include <linux/types.h>
> > +
> > +/**
> > + * struct trace_buffer_meta - Ring-buffer Meta-page description
> > + * @meta_page_size:	Size of this meta-page.
> > + * @meta_struct_len:	Size of this structure.
> > + * @subbuf_size:	Size of each subbuf, including the header.
> > + * @nr_subbufs:		Number of subbfs in the ring-buffer.
> > + * @reader.lost_events:	Number of events lost at the time of the reader swap.
> > + * @reader.id:		subbuf ID of the current reader. From 0 to @nr_subbufs - 1
> > + * @reader.read:	Number of bytes read on the reader subbuf.
> > + * @entries:		Number of entries in the ring-buffer.
> > + * @overrun:		Number of entries lost in the ring-buffer.
> > + * @read:		Number of entries that have been read.
> > + */
> > +struct trace_buffer_meta {
> > +	__u32		meta_page_size;
> > +	__u32		meta_struct_len;
> > +
> > +	__u32		subbuf_size;
> > +	__u32		nr_subbufs;
> > +
> > +	struct {
> > +		__u64	lost_events;
> > +		__u32	id;
> > +		__u32	read;
> > +	} reader;
> > +
> > +	__u64	flags;
> > +
> > +	__u64	entries;
> > +	__u64	overrun;
> > +	__u64	read;
> > +
> > +	__u64	Reserved1;
> > +	__u64	Reserved2;
> > +};
> > +
> > +#endif /* _TRACE_MMAP_H_ */
> > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> > index 8179e0a8984e..081065e76d4a 100644
> > --- a/kernel/trace/ring_buffer.c
> > +++ b/kernel/trace/ring_buffer.c
> > @@ -338,6 +338,7 @@ struct buffer_page {
> >  	local_t		 entries;	/* entries on this page */
> >  	unsigned long	 real_end;	/* real end of data */
> >  	unsigned	 order;		/* order of the page */
> > +	u32		 id;		/* ID for external mapping */
> >  	struct buffer_data_page *page;	/* Actual data page */
> >  };
> >  
> > @@ -484,6 +485,12 @@ struct ring_buffer_per_cpu {
> >  	u64				read_stamp;
> >  	/* pages removed since last reset */
> >  	unsigned long			pages_removed;
> > +
> > +	int				mapped;
> > +	struct mutex			mapping_lock;
> > +	unsigned long			*subbuf_ids;	/* ID to addr */
> > +	struct trace_buffer_meta	*meta_page;
> > +
> >  	/* ring buffer pages to update, > 0 to add, < 0 to remove */
> >  	long				nr_pages_to_update;
> >  	struct list_head		new_pages; /* new pages to add */
> > @@ -1548,6 +1555,7 @@ rb_allocate_cpu_buffer(struct trace_buffer *buffer, long nr_pages, int cpu)
> >  	init_irq_work(&cpu_buffer->irq_work.work, rb_wake_up_waiters);
> >  	init_waitqueue_head(&cpu_buffer->irq_work.waiters);
> >  	init_waitqueue_head(&cpu_buffer->irq_work.full_waiters);
> > +	mutex_init(&cpu_buffer->mapping_lock);
> >  
> >  	bpage = kzalloc_node(ALIGN(sizeof(*bpage), cache_line_size()),
> >  			    GFP_KERNEL, cpu_to_node(cpu));
> > @@ -5160,6 +5168,19 @@ static void rb_clear_buffer_page(struct buffer_page *page)
> >  	page->read = 0;
> >  }
> >  
> > +static void rb_update_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
> > +{
> > +	struct trace_buffer_meta *meta = cpu_buffer->meta_page;
> > +
> > +	WRITE_ONCE(meta->reader.read, cpu_buffer->reader_page->read);
> > +	WRITE_ONCE(meta->reader.id, cpu_buffer->reader_page->id);
> > +	WRITE_ONCE(meta->reader.lost_events, cpu_buffer->lost_events);
> > +
> > +	WRITE_ONCE(meta->entries, local_read(&cpu_buffer->entries));
> > +	WRITE_ONCE(meta->overrun, local_read(&cpu_buffer->overrun));
> > +	WRITE_ONCE(meta->read, cpu_buffer->read);
> > +}
> > +
> >  static void
> >  rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
> >  {
> > @@ -5204,6 +5225,9 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
> >  	cpu_buffer->lost_events = 0;
> >  	cpu_buffer->last_overrun = 0;
> >  
> > +	if (cpu_buffer->mapped)
> 
> There are some cpu_buffer->mapped are accessed via WRITE_ONCE/READ_ONCE()
> but others are not. What makes those different?

The cpu_buffer->mapped is READ_ONCE for the section where it is not protected
with a lock. That is (in this version) only ring_buffer_swap_cpu().

That said... 

a. This is not enough protection at this level, Ideally the _map should also
call synchronize_rcu() to make sure the _swap does see the ->mapped > 0.

b. With refcount for the snapshot in trace/trace.c, it is not possible for those
functions to race. trace_arm_snapshot() and tracing_buffers_mmap() are mutually
exclusive and already stabilized with the trace mutex.

So how about I completely remove those WRITE_ONCE/READ_ONCE and just rely on the
protection given in trace/trace.c instead of duplicating it in
trace/ring_buffer.c?

> 
> > +		rb_update_meta_page(cpu_buffer);
> > +
> >  	rb_head_page_activate(cpu_buffer);
> >  	cpu_buffer->pages_removed = 0;
> >  }
> > @@ -5418,6 +5442,11 @@ int ring_buffer_swap_cpu(struct trace_buffer *buffer_a,
> >  	cpu_buffer_a = buffer_a->buffers[cpu];
> >  	cpu_buffer_b = buffer_b->buffers[cpu];
> >  
> > +	if (READ_ONCE(cpu_buffer_a->mapped) || READ_ONCE(cpu_buffer_b->mapped)) {
> > +		ret = -EBUSY;
> > +		goto out;
> > +	}
> > +
> >  	/* At least make sure the two buffers are somewhat the same */
> >  	if (cpu_buffer_a->nr_pages != cpu_buffer_b->nr_pages)
> >  		goto out;
> > @@ -5682,7 +5711,8 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
> >  	 * Otherwise, we can simply swap the page with the one passed in.
> >  	 */
> >  	if (read || (len < (commit - read)) ||
> > -	    cpu_buffer->reader_page == cpu_buffer->commit_page) {
> > +	    cpu_buffer->reader_page == cpu_buffer->commit_page ||
> > +	    cpu_buffer->mapped) {
> >  		struct buffer_data_page *rpage = cpu_buffer->reader_page->page;
> >  		unsigned int rpos = read;
> >  		unsigned int pos = 0;
> > @@ -5901,6 +5931,11 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
> >  
> >  		cpu_buffer = buffer->buffers[cpu];
> >  
> > +		if (cpu_buffer->mapped) {
> > +			err = -EBUSY;
> > +			goto error;
> > +		}
> > +
> >  		/* Update the number of pages to match the new size */
> >  		nr_pages = old_size * buffer->buffers[cpu]->nr_pages;
> >  		nr_pages = DIV_ROUND_UP(nr_pages, buffer->subbuf_size);
> > @@ -6002,6 +6037,295 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
> >  }
> >  EXPORT_SYMBOL_GPL(ring_buffer_subbuf_order_set);
> >  
> > +#define subbuf_page(off, start) \
> > +	virt_to_page((void *)(start + (off << PAGE_SHIFT)))
> > +
> > +#define foreach_subbuf_page(sub_order, start, page)		\
> > +	page = subbuf_page(0, (start));				\
> > +	for (int __off = 0; __off < (1 << (sub_order));		\
> > +	     __off++, page = subbuf_page(__off, (start)))
> > +
> > +static inline void subbuf_map_prepare(unsigned long subbuf_start, int order)
> > +{
> > +	struct page *page;
> > +
> > +	/*
> > +	 * When allocating order > 0 pages, only the first struct page has a
> > +	 * refcount > 1. Increasing the refcount here ensures none of the struct
> > +	 * page composing the sub-buffer is freeed when the mapping is closed.
> > +	 */
> > +	foreach_subbuf_page(order, subbuf_start, page)
> > +		page_ref_inc(page);
> > +}
> > +
> > +static inline void subbuf_unmap(unsigned long subbuf_start, int order)
> > +{
> > +	struct page *page;
> > +
> > +	foreach_subbuf_page(order, subbuf_start, page) {
> > +		page_ref_dec(page);
> > +		page->mapping = NULL;
> > +	}
> > +}
> > +
> > +static void rb_free_subbuf_ids(struct ring_buffer_per_cpu *cpu_buffer)
> > +{
> > +	int sub_id;
> > +
> > +	for (sub_id = 0; sub_id < cpu_buffer->nr_pages + 1; sub_id++)
> > +		subbuf_unmap(cpu_buffer->subbuf_ids[sub_id],
> > +			     cpu_buffer->buffer->subbuf_order);
> > +
> > +	kfree(cpu_buffer->subbuf_ids);
> > +	cpu_buffer->subbuf_ids = NULL;
> > +}
> > +
> > +static int rb_alloc_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
> > +{
> > +	if (cpu_buffer->meta_page)
> > +		return 0;
> > +
> > +	cpu_buffer->meta_page = page_to_virt(alloc_page(GFP_USER | __GFP_ZERO));
> > +	if (!cpu_buffer->meta_page)
> > +		return -ENOMEM;
> > +
> > +	return 0;
> > +}
> > +
> > +static void rb_free_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
> > +{
> > +	unsigned long addr = (unsigned long)cpu_buffer->meta_page;
> > +
> > +	virt_to_page((void *)addr)->mapping = NULL;
> > +	free_page(addr);
> > +	cpu_buffer->meta_page = NULL;
> > +}
> > +
> > +static void rb_setup_ids_meta_page(struct ring_buffer_per_cpu *cpu_buffer,
> > +				   unsigned long *subbuf_ids)
> > +{
> > +	struct trace_buffer_meta *meta = cpu_buffer->meta_page;
> > +	unsigned int nr_subbufs = cpu_buffer->nr_pages + 1;
> > +	struct buffer_page *first_subbuf, *subbuf;
> > +	int id = 0;
> > +
> > +	subbuf_ids[id] = (unsigned long)cpu_buffer->reader_page->page;
> > +	subbuf_map_prepare(subbuf_ids[id], cpu_buffer->buffer->subbuf_order);
> > +	cpu_buffer->reader_page->id = id++;
> > +
> > +	first_subbuf = subbuf = rb_set_head_page(cpu_buffer);
> > +	do {
> > +		if (id >= nr_subbufs) {
> > +			WARN_ON(1);
> > +			break;
> > +		}
> > +
> > +		subbuf_ids[id] = (unsigned long)subbuf->page;
> > +		subbuf->id = id;
> > +		subbuf_map_prepare(subbuf_ids[id], cpu_buffer->buffer->subbuf_order);
> > +
> > +		rb_inc_page(&subbuf);
> > +		id++;
> > +	} while (subbuf != first_subbuf);
> > +
> > +	/* install subbuf ID to kern VA translation */
> > +	cpu_buffer->subbuf_ids = subbuf_ids;
> > +
> > +	meta->meta_page_size = PAGE_SIZE;
> > +	meta->meta_struct_len = sizeof(*meta);
> > +	meta->nr_subbufs = nr_subbufs;
> > +	meta->subbuf_size = cpu_buffer->buffer->subbuf_size + BUF_PAGE_HDR_SIZE;
> > +
> > +	rb_update_meta_page(cpu_buffer);
> > +}
> > +
> > +static inline struct ring_buffer_per_cpu *
> > +rb_get_mapped_buffer(struct trace_buffer *buffer, int cpu)
> > +{
> > +	struct ring_buffer_per_cpu *cpu_buffer;
> > +
> > +	if (!cpumask_test_cpu(cpu, buffer->cpumask))
> > +		return ERR_PTR(-EINVAL);
> > +
> > +	cpu_buffer = buffer->buffers[cpu];
> > +
> > +	mutex_lock(&cpu_buffer->mapping_lock);
> > +
> > +	if (!cpu_buffer->mapped) {
> > +		mutex_unlock(&cpu_buffer->mapping_lock);
> > +		return ERR_PTR(-ENODEV);
> > +	}
> > +
> > +	return cpu_buffer;
> > +}
> > +
> > +static inline void rb_put_mapped_buffer(struct ring_buffer_per_cpu *cpu_buffer)
> > +{
> > +	mutex_unlock(&cpu_buffer->mapping_lock);
> > +}
> > +
> > +int ring_buffer_map(struct trace_buffer *buffer, int cpu)
> > +{
> > +	struct ring_buffer_per_cpu *cpu_buffer;
> > +	unsigned long flags, *subbuf_ids;
> > +	int err = 0;
> > +
> > +	if (!cpumask_test_cpu(cpu, buffer->cpumask))
> > +		return -EINVAL;
> > +
> > +	cpu_buffer = buffer->buffers[cpu];
> > +
> > +	mutex_lock(&cpu_buffer->mapping_lock);
> > +
> > +	if (cpu_buffer->mapped) {
> > +		if (cpu_buffer->mapped == INT_MAX)
> > +			err = -EBUSY;
> > +		else
> > +			WRITE_ONCE(cpu_buffer->mapped, cpu_buffer->mapped + 1);
> > +		mutex_unlock(&cpu_buffer->mapping_lock);
> > +		return err;
> > +	}
> > +
> > +	/* prevent another thread from changing buffer sizes */
> > +	mutex_lock(&buffer->mutex);
> > +
> > +	err = rb_alloc_meta_page(cpu_buffer);
> > +	if (err)
> > +		goto unlock;
> > +
> > +	/* subbuf_ids include the reader while nr_pages does not */
> > +	subbuf_ids = kzalloc(sizeof(*subbuf_ids) * (cpu_buffer->nr_pages + 1),
> > +			   GFP_KERNEL);
> > +	if (!subbuf_ids) {
> > +		rb_free_meta_page(cpu_buffer);
> > +		err = -ENOMEM;
> > +		goto unlock;
> > +	}
> > +
> > +	atomic_inc(&cpu_buffer->resize_disabled);
> > +
> > +	/*
> > +	 * Lock all readers to block any subbuf swap until the subbuf IDs are
> > +	 * assigned.
> > +	 */
> > +	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
> > +
> > +	rb_setup_ids_meta_page(cpu_buffer, subbuf_ids);
> > +
> > +	WRITE_ONCE(cpu_buffer->mapped, 1);
> > +
> > +	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
> > +unlock:
> > +	mutex_unlock(&buffer->mutex);
> > +	mutex_unlock(&cpu_buffer->mapping_lock);
> > +
> > +	return err;
> > +}
> > +
> > +int ring_buffer_unmap(struct trace_buffer *buffer, int cpu)
> > +{
> > +	struct ring_buffer_per_cpu *cpu_buffer;
> > +	int err = 0;
> > +
> > +	if (!cpumask_test_cpu(cpu, buffer->cpumask))
> > +		return -EINVAL;
> > +
> > +	cpu_buffer = buffer->buffers[cpu];
> > +
> > +	mutex_lock(&cpu_buffer->mapping_lock);
> > +
> > +	if (!cpu_buffer->mapped) {
> > +		err = -ENODEV;
> > +		goto unlock;
> > +	}
> > +
> > +	WRITE_ONCE(cpu_buffer->mapped, cpu_buffer->mapped - 1);
> > +	if (!cpu_buffer->mapped) {
> > +		/* Wait for the writer and readers to observe !mapped */
> > +		synchronize_rcu();
> 
> How does this synchronize_rcu() work for ensuring to observe?
> (All of those writers/readers are in non-preemptive critical section?)

Apologies, that is something I should have reworked. In earlier versions of the
patch series, the writer could update themselves the meta-page.

This has been abandoned and now only rb_reset_cpu() and the get_reader ioctl are
updating the meta-page.

ring_buffer_map_get_reader() is already protected with the mapping mutex.
rb_reset_cpu() is behind the reader_lock, Taking the later would be way more
efficient than the synchronize_rcu()

> 
> Thank you,
> 
> > +
> > +		rb_free_subbuf_ids(cpu_buffer);
> > +		rb_free_meta_page(cpu_buffer);
> > +		atomic_dec(&cpu_buffer->resize_disabled);
> > +	}
> > +unlock:
> > +	mutex_unlock(&cpu_buffer->mapping_lock);
> > +
> > +	return err;
> > +}
> > +
> > +/*
> > + *   +--------------+  pgoff == 0
> > + *   |   meta page  |
> > + *   +--------------+  pgoff == 1
> > + *   | subbuffer 0  |
> > + *   +--------------+  pgoff == 1 + (1 << subbuf_order)
> > + *   | subbuffer 1  |
> > + *         ...
> > + */
> > +struct page *ring_buffer_map_fault(struct trace_buffer *buffer, int cpu,
> > +				   unsigned long pgoff)
> > +{
> > +	struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu];
> > +	unsigned long subbuf_id, subbuf_offset, addr;
> > +	struct page *page;
> > +
> > +	if (!pgoff)
> > +		return virt_to_page((void *)cpu_buffer->meta_page);
> > +
> > +	pgoff--;
> > +
> > +	subbuf_id = pgoff >> buffer->subbuf_order;
> > +	if (subbuf_id > cpu_buffer->nr_pages)
> > +		return NULL;
> > +
> > +	subbuf_offset = pgoff & ((1UL << buffer->subbuf_order) - 1);
> > +	addr = cpu_buffer->subbuf_ids[subbuf_id] + (subbuf_offset * PAGE_SIZE);
> > +	page = virt_to_page((void *)addr);
> > +
> > +	return page;
> > +}
> > +
> > +int ring_buffer_map_get_reader(struct trace_buffer *buffer, int cpu)
> > +{
> > +	struct ring_buffer_per_cpu *cpu_buffer;
> > +	unsigned long reader_size;
> > +	unsigned long flags;
> > +
> > +	cpu_buffer = rb_get_mapped_buffer(buffer, cpu);
> > +	if (IS_ERR(cpu_buffer))
> > +		return (int)PTR_ERR(cpu_buffer);
> > +
> > +	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
> > +consume:
> > +	if (rb_per_cpu_empty(cpu_buffer))
> > +		goto out;
> > +
> > +	reader_size = rb_page_size(cpu_buffer->reader_page);
> > +
> > +	/*
> > +	 * There are data to be read on the current reader page, we can
> > +	 * return to the caller. But before that, we assume the latter will read
> > +	 * everything. Let's update the kernel reader accordingly.
> > +	 */
> > +	if (cpu_buffer->reader_page->read < reader_size) {
> > +		while (cpu_buffer->reader_page->read < reader_size)
> > +			rb_advance_reader(cpu_buffer);
> > +		goto out;
> > +	}
> > +
> > +	if (WARN_ON(!rb_get_reader_page(cpu_buffer)))
> > +		goto out;
> > +
> > +	goto consume;
> > +out:
> > +	rb_update_meta_page(cpu_buffer);
> > +	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
> > +	rb_put_mapped_buffer(cpu_buffer);
> > +
> > +	return 0;
> > +}
> > +
> >  /*
> >   * We only allocate new buffers, never free them if the CPU goes down.
> >   * If we were to free the buffer, then the user would lose any trace that was in
> > -- 
> > 2.43.0.429.g432eaa2c6b-goog
> > 
> 
> 
> -- 
> Masami Hiramatsu (Google) <mhiramat@...nel.org>

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