Re: [PATCH v11 2/5] ring-buffer: Introducing ring-buffer mapping functions

[Vincent Donnefort <vdonnefort@google.com>]

On Mon, Jan 15, 2024 at 01:43:03PM +0900, Masami Hiramatsu wrote:
> On Thu, 11 Jan 2024 16:17:09 +0000
> Vincent Donnefort <vdonnefort@google.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@google.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..bde39a73ce65
> > --- /dev/null
> > +++ b/include/uapi/linux/trace_mmap.h
> > @@ -0,0 +1,45 @@
> > +/* 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
> > + * @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.
> > + * @subbufs_touched:	Number of subbufs that have been filled.
> > + * @subbufs_lost:	Number of subbufs lost to overrun.
> > + * @subbufs_read:	Number of subbufs that have been read.
> > + * @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.
> > + * @subbuf_size:	Size of each subbuf, including the header.
> > + * @nr_subbufs:		Number of subbfs in the ring-buffer.
> > + * @meta_page_size:	Size of this meta-page.
> > + * @meta_struct_len:	Size of this structure.
> > + */
> > +struct trace_buffer_meta {
> > +	unsigned long	entries;
> > +	unsigned long	overrun;
> > +	unsigned long	read;
> > +
> > +	unsigned long	subbufs_touched;
> > +	unsigned long	subbufs_lost;
> > +	unsigned long	subbufs_read;
> > +
> > +	struct {
> > +		unsigned long	lost_events;
> > +		__u32		id;
> > +		__u32		read;
> > +	} reader;
> > +
> > +	__u32		subbuf_size;
> > +	__u32		nr_subbufs;
> > +
> > +	__u32		meta_page_size;
> > +	__u32		meta_struct_len;
> > +};
> > +
> > +#endif /* _TRACE_MMAP_H_ */
> > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> > index db73e326fa04..e9ff1c95e896 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 */
> > @@ -1542,6 +1549,7 @@ 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 +5168,23 @@ 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->entries, local_read(&cpu_buffer->entries));
> > +	WRITE_ONCE(meta->overrun, local_read(&cpu_buffer->overrun));
> > +	WRITE_ONCE(meta->read, cpu_buffer->read);
> > +
> > +	WRITE_ONCE(meta->subbufs_touched, local_read(&cpu_buffer->pages_touched));
> > +	WRITE_ONCE(meta->subbufs_lost, local_read(&cpu_buffer->pages_lost));
> > +	WRITE_ONCE(meta->subbufs_read, local_read(&cpu_buffer->pages_read));
> > +
> > +	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);
> > +}
> > +
> >  static void
> >  rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
> >  {
> > @@ -5204,6 +5229,9 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
> >  	cpu_buffer->lost_events = 0;
> >  	cpu_buffer->last_overrun = 0;
> >  
> > +	if (cpu_buffer->mapped)
> > +		rb_update_meta_page(cpu_buffer);
> > +
> >  	rb_head_page_activate(cpu_buffer);
> >  	cpu_buffer->pages_removed = 0;
> >  }
> > @@ -5418,6 +5446,11 @@ 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;
> > +	}
> 
> Ah, this is not enough to stop snapshot. update_max_tr()@kernel/trace/trace.c
> is used for swapping all CPU buffer and it does not use this function.
> (but that should use this instead of accessing buffer directly...)
> 
> Maybe we need ring_buffer_swap() and it checks all cpu_buffer does not set
> mapping bits.
> 
> Thank you,

How about instead of having ring_buffer_swap_cpu() returning -EBUSY when mapped
we have two functions to block any mapping that trace.c could call?

 int rb_swap_prepare(struct ring_buffer *cpu_buffer)
 {
    mutex_lock(&cpu_buffer->mapping_lock);

    if (!cpu_buffer->mapped)
    	return 0;

    mutex_unlock(&cpu_buffer->mapping_lock);
 }

 int rb_swap_done(struct ring_buffer *cpu_buffer)
 {
    mutex_unlock(&cpu_buffer->mapping_lock);
 }

 int ring_buffer_swap_prepare(struct trace_buffer *buffer, int cpu)
 {
 ...
     
     if (cpu == RING_BUFFER_ALL_CPUS) {
    	 int cpu_i;

         for_each_buffer_cpu(buffer, cpu_i) {
	 	err = rb_swap_prepare(buffer->buffers[cpu_i]);
		if (err)
			break;
	 }

	 if (err)
	    /* Rollback ... */

	 return err;

     } 

     return rb_swap_prepare(buffer->buffers[cpu]);
 }

 void ring_buffer_swap_done(struct trace_buffer *buffer, int cpu)
 {
     if (cpu == RING_BUFFER_ALL_CPUS) {
    	 int cpu_i;

         for_each_buffer_cpu(buffer, cpu_i)
	 	rb_swap_done(buffer->buffers[cpu_i]);
	 return;
     }

     return rb_swap_done(buffer->buffers[cpu]);
 }

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