Re: [PATCH 1/2] ring-buffer: Introducing ring-buffer mapping functions

Re: [PATCH 1/2] ring-buffer: Introducing ring-buffer mapping functions

[kernel test robot]

BCC: lkp@intel.com
CC: oe-kbuild-all@lists.linux.dev
In-Reply-To: <20230317143310.1604700-2-vdonnefort@google.com>
References: <20230317143310.1604700-2-vdonnefort@google.com>
TO: Vincent Donnefort <vdonnefort@google.com>
TO: rostedt@goodmis.org
TO: mhiramat@kernel.org
TO: linux-kernel@vger.kernel.org
TO: linux-trace-kernel@vger.kernel.org
CC: kernel-team@android.com
CC: Vincent Donnefort <vdonnefort@google.com>

Hi Vincent,

Thank you for the patch! Perhaps something to improve:

[auto build test WARNING on linus/master]
[also build test WARNING on rostedt-trace/for-next v6.3-rc2 next-20230317]
[cannot apply to rostedt-trace/for-next-urgent]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Vincent-Donnefort/ring-buffer-Introducing-ring-buffer-mapping-functions/20230317-223437
patch link:    https://lore.kernel.org/r/20230317143310.1604700-2-vdonnefort%40google.com
patch subject: [PATCH 1/2] ring-buffer: Introducing ring-buffer mapping functions
:::::: branch date: 12 hours ago
:::::: commit date: 12 hours ago
config: i386-randconfig-m021 (https://download.01.org/0day-ci/archive/20230318/202303181001.bu7VCBhF-lkp@intel.com/config)
compiler: gcc-11 (Debian 11.3.0-8) 11.3.0

If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <lkp@intel.com>
| Reported-by: Dan Carpenter <error27@gmail.com>
| Link: https://lore.kernel.org/r/202303181001.bu7VCBhF-lkp@intel.com/

smatch warnings:
kernel/trace/ring_buffer.c:6137 ring_buffer_get_reader_page() warn: passing a valid pointer to 'PTR_ERR'
kernel/trace/ring_buffer.c:5996 rb_get_mapped_buffer() warn: inconsistent returns '&cpu_buffer->mapping_lock'.
kernel/trace/ring_buffer.c:6158 ring_buffer_update_meta_page() warn: passing a valid pointer to 'PTR_ERR'

vim +/PTR_ERR +6137 kernel/trace/ring_buffer.c

e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5978  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5979  static inline struct ring_buffer_per_cpu *
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5980  rb_get_mapped_buffer(struct trace_buffer *buffer, int cpu)
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5981  {
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5982  	struct ring_buffer_per_cpu *cpu_buffer;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5983  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5984  	if (!cpumask_test_cpu(cpu, buffer->cpumask))
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5985  		return ERR_PTR(-EINVAL);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5986  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5987  	cpu_buffer = buffer->buffers[cpu];
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5988  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5989  	mutex_lock(&cpu_buffer->mapping_lock);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5990  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5991  	if (!cpu_buffer->mapped) {
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5992  		mutex_unlock(&cpu_buffer->mapping_lock);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5993  		return ERR_PTR(-ENODEV);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5994  	}
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5995  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17 @5996  	return cpu_buffer;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5997  }
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5998  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  5999  static inline void rb_put_mapped_buffer(struct ring_buffer_per_cpu *cpu_buffer)
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6000  {
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6001  	mutex_unlock(&cpu_buffer->mapping_lock);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6002  }
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6003  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6004  int ring_buffer_map(struct trace_buffer *buffer, int cpu)
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6005  {
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6006  	struct ring_buffer_per_cpu *cpu_buffer;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6007  	unsigned long flags, *page_ids;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6008  	int err = 0;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6009  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6010  	if (!cpumask_test_cpu(cpu, buffer->cpumask))
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6011  		return -EINVAL;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6012  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6013  	cpu_buffer = buffer->buffers[cpu];
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6014  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6015  	mutex_lock(&cpu_buffer->mapping_lock);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6016  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6017  	if (cpu_buffer->mapped) {
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6018  		WRITE_ONCE(cpu_buffer->mapped, cpu_buffer->mapped + 1);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6019  		goto unlock;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6020  	}
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6021  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6022  	/* prevent another thread from changing buffer sizes */
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6023  	mutex_lock(&buffer->mutex);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6024  	atomic_inc(&cpu_buffer->resize_disabled);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6025  	mutex_unlock(&buffer->mutex);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6026  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6027  	err = rb_alloc_meta_page(cpu_buffer);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6028  	if (err) {
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6029  		atomic_dec(&cpu_buffer->resize_disabled);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6030  		goto unlock;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6031  	}
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6032  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6033  	/* page_ids include the reader page while nr_pages does not */
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6034  	page_ids = kzalloc(sizeof(*page_ids) * (cpu_buffer->nr_pages + 1),
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6035  			   GFP_KERNEL);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6036  	if (!page_ids) {
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6037  		rb_free_meta_page(cpu_buffer);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6038  		atomic_dec(&cpu_buffer->resize_disabled);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6039  		err = -ENOMEM;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6040  		goto unlock;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6041  	}
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6042  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6043  	/*
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6044  	 * Lock all readers to block any page swap until the page IDs are
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6045  	 * assigned.
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6046  	 */
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6047  	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6048  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6049  	rb_setup_ids_meta_page(cpu_buffer, page_ids);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6050  	/*
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6051  	 * Ensure the writer will observe the meta-page before
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6052  	 * cpu_buffer->mapped.
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6053  	 */
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6054  	smp_wmb();
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6055  	WRITE_ONCE(cpu_buffer->mapped, 1);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6056  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6057  	/* Init meta_page values unless the writer did it already */
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6058  	cmpxchg(&cpu_buffer->meta_page->entries, 0,
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6059  		local_read(&cpu_buffer->entries));
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6060  	cmpxchg(&cpu_buffer->meta_page->overrun, 0,
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6061  		local_read(&cpu_buffer->overrun));
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6062  	cmpxchg(&cpu_buffer->meta_page->pages_touched, 0,
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6063  		local_read(&cpu_buffer->pages_touched));
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6064  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6065  	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6066  unlock:
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6067  	mutex_unlock(&cpu_buffer->mapping_lock);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6068  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6069  	return err;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6070  }
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6071  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6072  int ring_buffer_unmap(struct trace_buffer *buffer, int cpu)
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6073  {
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6074  	struct ring_buffer_per_cpu *cpu_buffer;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6075  	int err = 0;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6076  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6077  	if (!cpumask_test_cpu(cpu, buffer->cpumask))
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6078  		return -EINVAL;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6079  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6080  	cpu_buffer = buffer->buffers[cpu];
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6081  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6082  	mutex_lock(&cpu_buffer->mapping_lock);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6083  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6084  	if (!cpu_buffer->mapped) {
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6085  		err = -ENODEV;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6086  		goto unlock;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6087  	}
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6088  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6089  	WRITE_ONCE(cpu_buffer->mapped, cpu_buffer->mapped - 1);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6090  	if (!cpu_buffer->mapped) {
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6091  		/* Wait the writer and readers to observe !mapped */
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6092  		synchronize_rcu();
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6093  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6094  		rb_free_page_ids(cpu_buffer);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6095  		rb_free_meta_page(cpu_buffer);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6096  		atomic_dec(&cpu_buffer->resize_disabled);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6097  	}
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6098  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6099  unlock:
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6100  	mutex_unlock(&cpu_buffer->mapping_lock);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6101  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6102  	return err;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6103  }
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6104  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6105  /*
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6106   *   +--------------+
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6107   *   |   meta page  |  pgoff=0
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6108   *   +--------------+
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6109   *   |  data page1  |  pgoff=1 page_ids=0
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6110   *   +--------------+
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6111   *   |  data page2  |  pgoff=2 page_ids=1
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6112   *         ...
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6113   */
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6114  struct page *ring_buffer_map_fault(struct trace_buffer *buffer, int cpu,
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6115  				   unsigned long pgoff)
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6116  {
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6117  	struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu];
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6118  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6119  	if (!pgoff)
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6120  		return virt_to_page(cpu_buffer->meta_page);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6121  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6122  	pgoff--;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6123  	if (pgoff > cpu_buffer->nr_pages)
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6124  		return NULL;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6125  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6126  	return virt_to_page(cpu_buffer->page_ids[pgoff]);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6127  }
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6128  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6129  int ring_buffer_get_reader_page(struct trace_buffer *buffer, int cpu)
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6130  {
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6131  	struct ring_buffer_per_cpu *cpu_buffer;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6132  	struct buffer_page *reader;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6133  	unsigned long flags;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6134  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6135  	cpu_buffer = rb_get_mapped_buffer(buffer, cpu);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6136  	if (IS_ERR(cpu_buffer))
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17 @6137  		return (int)PTR_ERR(cpu_buffer);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6138  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6139  	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6140  	reader = cpu_buffer->reader_page;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6141  	reader->read = rb_page_size(reader);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6142  	if (!rb_per_cpu_empty(cpu_buffer))
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6143  		WARN_ON(!rb_get_reader_page(cpu_buffer));
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6144  	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6145  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6146  	rb_put_mapped_buffer(cpu_buffer);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6147  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6148  	return 0;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6149  }
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6150  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6151  int ring_buffer_update_meta_page(struct trace_buffer *buffer, int cpu)
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6152  {
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6153  	struct ring_buffer_per_cpu *cpu_buffer;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6154  	unsigned long flags;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6155  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6156  	cpu_buffer = rb_get_mapped_buffer(buffer, cpu);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6157  	if (IS_ERR(cpu_buffer))
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17 @6158  		return PTR_ERR(cpu_buffer);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6159  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6160  	/* Update the head page if the writer moved it */
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6161  	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6162  	rb_set_head_page(cpu_buffer);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6163  	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6164  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6165  	rb_put_mapped_buffer(cpu_buffer);
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6166  
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6167  	return 0;
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6168  }
e6f6ebfdb93b0a Vincent Donnefort 2023-03-17  6169  

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests

[PATCH 0/2] Introducing trace buffer mapping by user-space

[Vincent Donnefort]

The tracing ring-buffers can be stored on disk or sent to network without any
copy via splice. However the later doesn't allow real time processing of the
traces. A solution is to give access to userspace to the ring-buffer pages
directly via a mapping. A piece of software can now become a reader of the
ring-buffer, and drive a consuming or non-consuming read in a similar fashion to
what trace and trace_pipe offer.

Attached to this cover letter an example of consuming read for a ring-buffer,
using libtracefs.

Vincent

--

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdarg.h>
#include <signal.h>
#include <errno.h>
#include <unistd.h>
#include <tracefs.h>
#include <kbuffer.h>
#include <event-parse.h>

#include <asm/types.h>
#include <sys/mman.h>
#include <sys/ioctl.h>

#define TRACE_MMAP_IOCTL_GET_READER_PAGE	_IO('T', 0x1)

struct ring_buffer_meta_page {
	__u64		entries;
	__u64		overrun;
	__u32		pages_touched;
	__u32		reader_page;
	__u32		nr_data_pages;
	__u32		data_page_head;
	__u32		data_pages[];
};

/* Need to access private struct to save counters */
struct kbuffer {
	unsigned long long 	timestamp;
	long long		lost_events;
	unsigned long		flags;
	void			*subbuffer;
	void			*data;
	unsigned int		index;
	unsigned int		curr;
	unsigned int		next;
	unsigned int		size;
	unsigned int		start;
	unsigned int		first;

	unsigned int (*read_4)(void *ptr);
	unsigned long long (*read_8)(void *ptr);
	unsigned long long (*read_long)(struct kbuffer *kbuf, void *ptr);
	int (*next_event)(struct kbuffer *kbuf);
};

static char *argv0;
static bool need_exit;

static char *get_this_name(void)
{
	static char *this_name;
	char *arg;
	char *p;

	if (this_name)
		return this_name;

	arg = argv0;
	p = arg+strlen(arg);

	while (p >= arg && *p != '/')
		p--;
	p++;

	this_name = p;
	return p;
}

static void __vdie(const char *fmt, va_list ap, int err)
{
	int ret = errno;
	char *p = get_this_name();

	if (err && errno)
		perror(p);
	else
		ret = -1;

	fprintf(stderr, "  ");
	vfprintf(stderr, fmt, ap);

	fprintf(stderr, "\n");
	exit(ret);
}

void pdie(const char *fmt, ...)
{
	va_list ap;

	va_start(ap, fmt);
	__vdie(fmt, ap, 1);
	va_end(ap);
}

static void read_page(struct tep_handle *tep, struct kbuffer *kbuf,
		      void *data, int page)
{
	static struct trace_seq seq;
	struct tep_record record;

	if (seq.buffer)
		trace_seq_reset(&seq);
	else
		trace_seq_init(&seq);

	while ((record.data = kbuffer_read_event(kbuf, &record.ts))) {
		kbuffer_next_event(kbuf, NULL);
		tep_print_event(tep, &seq, &record,
				"%s-%d %9d\t%s\n", TEP_PRINT_COMM,
				TEP_PRINT_PID, TEP_PRINT_TIME, TEP_PRINT_NAME);
		trace_seq_do_printf(&seq);
		trace_seq_reset(&seq);
	}
}

static int next_reader_page(int fd, struct ring_buffer_meta_page *meta,
			    struct kbuffer *kbuf)
{
	int prev_reader_page = meta->reader_page;

	if (ioctl(fd, TRACE_MMAP_IOCTL_GET_READER_PAGE) < 0)
		pdie("ioctl");

	return meta->reader_page;
}

static void signal_handler(int unused)
{
	printf("Exit!\n");
	need_exit = true;
}

int main(int argc, char **argv)
{
	int page_size, data_len, page, fd, start = -1;
	struct ring_buffer_meta_page *map;
	struct kbuffer *kbuf, prev_kbuf;
	struct tep_handle *tep;
	__u64 prev_entries;
	void *meta, *data;
	char *buf, path[32];
	int cpu;

	argv0 = argv[0];
	cpu = atoi(argv[1]);
	snprintf(path, 32, "per_cpu/cpu%d/trace_pipe_raw", cpu);

	tep = tracefs_local_events(NULL);
	kbuf = tep_kbuffer(tep);
	page_size = getpagesize();

	fd = tracefs_instance_file_open(NULL, path, O_RDONLY);
	if (fd < 0)
		pdie("raw");

	meta = mmap(NULL, page_size, PROT_READ, MAP_SHARED, fd, 0);
	if (meta == MAP_FAILED)
		pdie("mmap");
	map = meta;

	printf("entries:	%llu\n", map->entries);
	printf("overrun:	%llu\n", map->overrun);
	printf("pages_touched:	%u\n", map->pages_touched);
	printf("reader_page:	%u\n", map->reader_page);
	printf("nr_data_pages:	%u\n\n", map->nr_data_pages);

	data_len = page_size * (map->nr_data_pages + 1);

	data = mmap(NULL, data_len, PROT_READ, MAP_SHARED, fd, page_size);
	if (data == MAP_FAILED)
		pdie("mmap data");

	signal(SIGINT, signal_handler);

	page = ((struct ring_buffer_meta_page*)meta)->reader_page;
again:
	do {
		kbuffer_load_subbuffer(kbuf, data + page_size * page);

		if (page != start) {
			printf("READER PAGE: %d\n", map->reader_page);
		} else {
			kbuf->curr = prev_kbuf.curr;
			kbuf->index = prev_kbuf.index;
			kbuf->next = prev_kbuf.next;
			kbuf->timestamp = prev_kbuf.timestamp;
			kbuffer_next_event(kbuf, NULL);
		}

		prev_entries = map->entries;
		start = page;

		read_page(tep, kbuf, data, page);
	} while ((page = next_reader_page(fd, meta, kbuf)) != start);

	prev_kbuf.curr = kbuf->curr;
	prev_kbuf.index = kbuf->index;
	prev_kbuf.next = kbuf->next;
	prev_kbuf.timestamp = kbuf->timestamp;

	while (prev_entries == *(volatile __u64 *)&map->entries && !need_exit)
		usleep(100000);

	if (!need_exit)
		goto again;

	munmap(data, data_len);
	munmap(meta, page_size);
	close(fd);

	return 0;
}

Vincent Donnefort (2):
  ring-buffer: Introducing ring-buffer mapping functions
  tracing: Allow user-space mapping of the ring-buffer

 include/linux/ring_buffer.h     |   8 +
 include/uapi/linux/trace_mmap.h |  27 +++
 kernel/trace/ring_buffer.c      | 334 +++++++++++++++++++++++++++++++-
 kernel/trace/trace.c            |  73 ++++++-
 4 files changed, 436 insertions(+), 6 deletions(-)
 create mode 100644 include/uapi/linux/trace_mmap.h

-- 
2.40.0.rc1.284.g88254d51c5-goog

[PATCH 1/2] ring-buffer: Introducing ring-buffer mapping functions

[Vincent Donnefort]

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_get_reader_page()  /* swap reader and head */
  ring_buffer_update_meta_page()

Mapping the ring-buffer also involves:

  A unique ID for each page of the ring-buffer, as currently the pages
  are only identified through their in-kernel VA.

  A meta-page, where are stored statistics about the ring-buffer and
  a page IDs list, ordered. A field gives what page is the reader
  one and one to gives where the ring-buffer starts in the list of data
  pages.

The linear mapping exposes the meta-page, and each page of the
ring-buffer, ordered following their unique ID, assigned during the
first mapping.

Once mapped, no page 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 pages.

Also, the meta-page being... a single page, this limits at the moment the
number of pages in the ring-buffer that can be mapped: ~3MB on a 4K pages
system.

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

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index 782e14f62201..4897e17ebdde 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;
 
@@ -211,4 +213,10 @@ 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_get_reader_page(struct trace_buffer *buffer, int cpu);
+int ring_buffer_update_meta_page(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..b5caed17a066
--- /dev/null
+++ b/include/uapi/linux/trace_mmap.h
@@ -0,0 +1,24 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef _UAPI_TRACE_MMAP_H_
+#define _UAPI_TRACE_MMAP_H_
+
+#include <asm/bitsperlong.h>
+
+#include <linux/types.h>
+
+struct ring_buffer_meta_page {
+#if __BITS_PER_LONG == 64
+	__u64	entries;
+	__u64	overrun;
+#else
+	__u32	entries;
+	__u32	overrun;
+#endif
+	__u32	pages_touched;
+	__u32	reader_page;
+	__u32	nr_data_pages;	/* doesn't take into account the reader_page */
+	__u32	data_page_head;	/* index of data_pages[] */
+	__u32	data_pages[];
+};
+
+#endif /* _UAPI_TRACE_MMAP_H_ */
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index af50d931b020..08765310380b 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -332,6 +332,7 @@ struct buffer_page {
 	local_t		 entries;	/* entries on this page */
 	unsigned long	 real_end;	/* real end of data */
 	struct buffer_data_page *page;	/* Actual data page */
+	u32		 id;		/* ID for external mapping */
 };
 
 /*
@@ -529,6 +530,12 @@ struct ring_buffer_per_cpu {
 	rb_time_t			before_stamp;
 	u64				event_stamp[MAX_NEST];
 	u64				read_stamp;
+
+	int				mapped;
+	struct mutex			mapping_lock;
+	unsigned long			*page_ids;	/* ID to addr */
+	struct ring_buffer_meta_page	*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 */
@@ -1452,12 +1459,37 @@ static inline void rb_inc_page(struct buffer_page **bpage)
 	*bpage = list_entry(p, struct buffer_page, list);
 }
 
+static inline void
+rb_meta_page_head_move(struct ring_buffer_per_cpu *cpu_buffer, unsigned long num)
+{
+	unsigned long head_id;
+
+	if (!READ_ONCE(cpu_buffer->mapped))
+		return;
+
+	head_id = cpu_buffer->meta_page->data_page_head;
+	cpu_buffer->meta_page->data_page_head = (head_id + num) % cpu_buffer->nr_pages;
+}
+
+static inline void
+rb_meta_page_head_swap(struct ring_buffer_per_cpu *cpu_buffer)
+{
+	struct ring_buffer_meta_page *meta = cpu_buffer->meta_page;
+
+	if (!READ_ONCE(cpu_buffer->mapped))
+		return;
+
+	meta->reader_page = cpu_buffer->head_page->id;
+	meta->data_pages[meta->data_page_head] = cpu_buffer->reader_page->id;
+}
+
 static struct buffer_page *
 rb_set_head_page(struct ring_buffer_per_cpu *cpu_buffer)
 {
 	struct buffer_page *head;
 	struct buffer_page *page;
 	struct list_head *list;
+	unsigned long cnt = 0;
 	int i;
 
 	if (RB_WARN_ON(cpu_buffer, !cpu_buffer->head_page))
@@ -1479,9 +1511,12 @@ rb_set_head_page(struct ring_buffer_per_cpu *cpu_buffer)
 		do {
 			if (rb_is_head_page(page, page->list.prev)) {
 				cpu_buffer->head_page = page;
+				rb_meta_page_head_move(cpu_buffer, cnt);
+
 				return page;
 			}
 			rb_inc_page(&page);
+			cnt++;
 		} while (page != head);
 	}
 
@@ -1567,6 +1602,13 @@ static void rb_tail_page_update(struct ring_buffer_per_cpu *cpu_buffer,
 		/* Again, either we update tail_page or an interrupt does */
 		(void)cmpxchg(&cpu_buffer->tail_page, tail_page, next_page);
 	}
+
+	if (READ_ONCE(cpu_buffer->mapped)) {
+		/* Ensure the meta_page is ready */
+		smp_rmb();
+		WRITE_ONCE(cpu_buffer->meta_page->pages_touched,
+			   local_read(&cpu_buffer->pages_touched));
+	}
 }
 
 static int rb_check_bpage(struct ring_buffer_per_cpu *cpu_buffer,
@@ -1735,6 +1777,7 @@ 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));
@@ -2173,7 +2216,6 @@ int ring_buffer_resize(struct trace_buffer *buffer, unsigned long size,
 	/* prevent another thread from changing buffer sizes */
 	mutex_lock(&buffer->mutex);
 
-
 	if (cpu_id == RING_BUFFER_ALL_CPUS) {
 		/*
 		 * Don't succeed if resizing is disabled, as a reader might be
@@ -2523,6 +2565,13 @@ rb_handle_head_page(struct ring_buffer_per_cpu *cpu_buffer,
 		local_sub(BUF_PAGE_SIZE, &cpu_buffer->entries_bytes);
 		local_inc(&cpu_buffer->pages_lost);
 
+		if (READ_ONCE(cpu_buffer->mapped)) {
+			/* Ensure the meta_page is ready */
+			smp_rmb();
+			WRITE_ONCE(cpu_buffer->meta_page->overrun,
+				   local_read(&cpu_buffer->overrun));
+		}
+
 		/*
 		 * The entries will be zeroed out when we move the
 		 * tail page.
@@ -3179,6 +3228,14 @@ static inline void rb_event_discard(struct ring_buffer_event *event)
 static void rb_commit(struct ring_buffer_per_cpu *cpu_buffer)
 {
 	local_inc(&cpu_buffer->entries);
+
+	if (READ_ONCE(cpu_buffer->mapped)) {
+		/* Ensure the meta_page is ready */
+		smp_rmb();
+		WRITE_ONCE(cpu_buffer->meta_page->entries,
+			   local_read(&cpu_buffer->entries));
+	}
+
 	rb_end_commit(cpu_buffer);
 }
 
@@ -3482,7 +3539,7 @@ static void check_buffer(struct ring_buffer_per_cpu *cpu_buffer,
 		return;
 
 	/*
-	 * If this interrupted another event, 
+	 * If this interrupted another event,
 	 */
 	if (atomic_inc_return(this_cpu_ptr(&checking)) != 1)
 		goto out;
@@ -4643,7 +4700,9 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
 	 * Now make the new head point back to the reader page.
 	 */
 	rb_list_head(reader->list.next)->prev = &cpu_buffer->reader_page->list;
+	rb_meta_page_head_swap(cpu_buffer);
 	rb_inc_page(&cpu_buffer->head_page);
+	rb_meta_page_head_move(cpu_buffer, 1);
 
 	local_inc(&cpu_buffer->pages_read);
 
@@ -5285,6 +5344,12 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
 	cpu_buffer->lost_events = 0;
 	cpu_buffer->last_overrun = 0;
 
+	if (READ_ONCE(cpu_buffer->mapped)) {
+		WRITE_ONCE(cpu_buffer->meta_page->entries, 0);
+		WRITE_ONCE(cpu_buffer->meta_page->pages_touched, 0);
+		WRITE_ONCE(cpu_buffer->meta_page->overrun, 0);
+	}
+
 	rb_head_page_activate(cpu_buffer);
 }
 
@@ -5489,6 +5554,11 @@ 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;
@@ -5722,7 +5792,8 @@ 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 ||
+	    READ_ONCE(cpu_buffer->mapped)) {
 		struct buffer_data_page *rpage = cpu_buffer->reader_page->page;
 		unsigned int rpos = read;
 		unsigned int pos = 0;
@@ -5839,6 +5910,263 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
 }
 EXPORT_SYMBOL_GPL(ring_buffer_read_page);
 
+#define META_PAGE_MAX_PAGES \
+	((PAGE_SIZE - (offsetof(struct ring_buffer_meta_page, data_page_head))) >> 2)
+
+static void rb_free_page_ids(struct ring_buffer_per_cpu *cpu_buffer)
+{
+	kfree(cpu_buffer->page_ids);
+	cpu_buffer->page_ids = NULL;
+}
+
+static int rb_alloc_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
+{
+	if (cpu_buffer->meta_page)
+		return 0;
+
+	if (cpu_buffer->nr_pages > META_PAGE_MAX_PAGES)
+		return -E2BIG;
+
+	cpu_buffer->meta_page = page_to_virt(alloc_page(GFP_USER));
+	if (!cpu_buffer->meta_page)
+		return -ENOMEM;
+
+	return 0;
+}
+
+static void rb_free_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
+{
+	free_page((unsigned long)cpu_buffer->meta_page);
+	cpu_buffer->meta_page = NULL;
+}
+
+static void rb_setup_ids_meta_page(struct ring_buffer_per_cpu *cpu_buffer,
+				   unsigned long *page_ids)
+{
+	struct ring_buffer_meta_page *meta = cpu_buffer->meta_page;
+	struct buffer_page *first_page, *bpage;
+	int id = 0;
+
+	page_ids[id] = (unsigned long)cpu_buffer->reader_page->page;
+	cpu_buffer->reader_page->id = id++;
+
+	first_page = bpage = rb_set_head_page(cpu_buffer);
+	do {
+		if (id > META_PAGE_MAX_PAGES) {
+			WARN_ON(1);
+			break;
+		}
+
+		page_ids[id] = (unsigned long)bpage->page;
+		bpage->id = id;
+		meta->data_pages[id - 1] = id;
+
+		rb_inc_page(&bpage);
+		id++;
+	} while (bpage != first_page);
+
+	/* install page ID to kern VA translation */
+	cpu_buffer->page_ids = page_ids;
+
+	meta->entries = 0;
+	meta->overrun = 0;
+	meta->pages_touched = 0;
+	meta->reader_page = cpu_buffer->reader_page->id;
+	meta->nr_data_pages = cpu_buffer->nr_pages;
+	meta->data_page_head = 0;
+}
+
+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, *page_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) {
+		WRITE_ONCE(cpu_buffer->mapped, cpu_buffer->mapped + 1);
+		goto unlock;
+	}
+
+	/* prevent another thread from changing buffer sizes */
+	mutex_lock(&buffer->mutex);
+	atomic_inc(&cpu_buffer->resize_disabled);
+	mutex_unlock(&buffer->mutex);
+
+	err = rb_alloc_meta_page(cpu_buffer);
+	if (err) {
+		atomic_dec(&cpu_buffer->resize_disabled);
+		goto unlock;
+	}
+
+	/* page_ids include the reader page while nr_pages does not */
+	page_ids = kzalloc(sizeof(*page_ids) * (cpu_buffer->nr_pages + 1),
+			   GFP_KERNEL);
+	if (!page_ids) {
+		rb_free_meta_page(cpu_buffer);
+		atomic_dec(&cpu_buffer->resize_disabled);
+		err = -ENOMEM;
+		goto unlock;
+	}
+
+	/*
+	 * Lock all readers to block any page swap until the page IDs are
+	 * assigned.
+	 */
+	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+
+	rb_setup_ids_meta_page(cpu_buffer, page_ids);
+	/*
+	 * Ensure the writer will observe the meta-page before
+	 * cpu_buffer->mapped.
+	 */
+	smp_wmb();
+	WRITE_ONCE(cpu_buffer->mapped, 1);
+
+	/* Init meta_page values unless the writer did it already */
+	cmpxchg(&cpu_buffer->meta_page->entries, 0,
+		local_read(&cpu_buffer->entries));
+	cmpxchg(&cpu_buffer->meta_page->overrun, 0,
+		local_read(&cpu_buffer->overrun));
+	cmpxchg(&cpu_buffer->meta_page->pages_touched, 0,
+		local_read(&cpu_buffer->pages_touched));
+
+	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+unlock:
+	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 the writer and readers to observe !mapped */
+		synchronize_rcu();
+
+		rb_free_page_ids(cpu_buffer);
+		rb_free_meta_page(cpu_buffer);
+		atomic_dec(&cpu_buffer->resize_disabled);
+	}
+
+unlock:
+	mutex_unlock(&cpu_buffer->mapping_lock);
+
+	return err;
+}
+
+/*
+ *   +--------------+
+ *   |   meta page  |  pgoff=0
+ *   +--------------+
+ *   |  data page1  |  pgoff=1 page_ids=0
+ *   +--------------+
+ *   |  data page2  |  pgoff=2 page_ids=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];
+
+	if (!pgoff)
+		return virt_to_page(cpu_buffer->meta_page);
+
+	pgoff--;
+	if (pgoff > cpu_buffer->nr_pages)
+		return NULL;
+
+	return virt_to_page(cpu_buffer->page_ids[pgoff]);
+}
+
+int ring_buffer_get_reader_page(struct trace_buffer *buffer, int cpu)
+{
+	struct ring_buffer_per_cpu *cpu_buffer;
+	struct buffer_page *reader;
+	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);
+	reader = cpu_buffer->reader_page;
+	reader->read = rb_page_size(reader);
+	if (!rb_per_cpu_empty(cpu_buffer))
+		WARN_ON(!rb_get_reader_page(cpu_buffer));
+	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+
+	rb_put_mapped_buffer(cpu_buffer);
+
+	return 0;
+}
+
+int ring_buffer_update_meta_page(struct trace_buffer *buffer, int cpu)
+{
+	struct ring_buffer_per_cpu *cpu_buffer;
+	unsigned long flags;
+
+	cpu_buffer = rb_get_mapped_buffer(buffer, cpu);
+	if (IS_ERR(cpu_buffer))
+		return PTR_ERR(cpu_buffer);
+
+	/* Update the head page if the writer moved it */
+	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+	rb_set_head_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
-- 
2.40.0.rc1.284.g88254d51c5-goog

Re: [PATCH 1/2] ring-buffer: Introducing ring-buffer mapping functions

[Steven Rostedt]

On Fri, 17 Mar 2023 14:33:09 +0000
Vincent Donnefort <vdonnefort@google.com> wrote:

> Also, the meta-page being... a single page, this limits at the moment the
> number of pages in the ring-buffer that can be mapped: ~3MB on a 4K pages
> system.

I hate this limitation, so I fixed it ;-)

I added a meta_page_size field to the meta page, and user space can do:

	meta = mmap(NULL, page_size, PROT_READ, MAP_SHARED, fd, 0);
	if (meta == MAP_FAILED)
		pdie("mmap");

	map = meta;
	meta_len = map->meta_page_size;

	if (meta_len > page_size) {
		munmap(meta, page_size);
		meta = mmap(NULL, meta_len, PROT_READ, MAP_SHARED, fd, 0);
		if (meta == MAP_FAILED)
			pdie("mmap");
		map = meta;
	}

This appears to work (but I'm still testing it).

-- Steve

diff --git a/include/uapi/linux/trace_mmap.h b/include/uapi/linux/trace_mmap.h
index 24bcec754a35..12f3f7ee33d9 100644
--- a/include/uapi/linux/trace_mmap.h
+++ b/include/uapi/linux/trace_mmap.h
@@ -18,6 +18,7 @@ struct ring_buffer_meta_page {
 	__u32	reader_page;
 	__u32	nr_data_pages;	/* doesn't take into account the reader_page */
 	__u32	data_page_head;	/* index of data_pages[] */
+	__u32	meta_page_size;	/* size of the meta page */
 	__u32	data_pages[];
 };
 
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 10a17e78cfe6..77c92e4a7adc 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -526,6 +526,7 @@ struct ring_buffer_per_cpu {
 	u64				read_stamp;
 
 	int				mapped;
+	int				meta_order;
 	struct mutex			mapping_lock;
 	unsigned long			*page_ids;	/* ID to addr */
 	struct ring_buffer_meta_page	*meta_page;
@@ -5898,7 +5899,7 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
 EXPORT_SYMBOL_GPL(ring_buffer_read_page);
 
 #define META_PAGE_MAX_PAGES \
-	((PAGE_SIZE - (offsetof(struct ring_buffer_meta_page, data_page_head))) >> 2)
+	((PAGE_SIZE - (offsetof(struct ring_buffer_meta_page, data_pages))) >> 2)
 
 static void rb_free_page_ids(struct ring_buffer_per_cpu *cpu_buffer)
 {
@@ -5908,22 +5909,34 @@ static void rb_free_page_ids(struct ring_buffer_per_cpu *cpu_buffer)
 
 static int rb_alloc_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
 {
+	struct page *meta_pages;
+	int pages;
+	int order = 0;
+
 	if (cpu_buffer->meta_page)
 		return 0;
 
-	if (cpu_buffer->nr_pages > META_PAGE_MAX_PAGES)
-		return -E2BIG;
-
-	cpu_buffer->meta_page = page_to_virt(alloc_page(GFP_USER));
-	if (!cpu_buffer->meta_page)
+	if (cpu_buffer->nr_pages > META_PAGE_MAX_PAGES) {
+		/* Calcualte how many more pages we need to hold indexes */
+		pages = DIV_ROUND_UP(cpu_buffer->nr_pages - META_PAGE_MAX_PAGES,
+				     PAGE_SIZE / sizeof(u32));
+		/* Add back the meta_page itself */
+		pages++;
+		order = fls(pages) - 1;
+	}
+	meta_pages = alloc_pages(GFP_USER, order);
+	if (!meta_pages)
 		return -ENOMEM;
 
+	cpu_buffer->meta_page = page_to_virt(meta_pages);
+	cpu_buffer->meta_order = order;
+
 	return 0;
 }
 
 static void rb_free_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
 {
-	free_page((unsigned long)cpu_buffer->meta_page);
+	free_pages((unsigned long)cpu_buffer->meta_page, cpu_buffer->meta_order);
 	cpu_buffer->meta_page = NULL;
 }
 
@@ -5932,14 +5945,20 @@ static void rb_setup_ids_meta_page(struct ring_buffer_per_cpu *cpu_buffer,
 {
 	struct ring_buffer_meta_page *meta = cpu_buffer->meta_page;
 	struct buffer_page *first_page, *bpage;
+	int data_page_end;
 	int id = 0;
 
 	page_ids[id] = (unsigned long)cpu_buffer->reader_page->page;
 	cpu_buffer->reader_page->id = id++;
 
+	/* Calculate the last index of data_pages[] */
+	data_page_end = (1 << (cpu_buffer->meta_order + PAGE_SHIFT)) -
+		offsetof(struct ring_buffer_meta_page, data_pages);
+	data_page_end /= sizeof(u32);
+
 	first_page = bpage = rb_set_head_page(cpu_buffer);
 	do {
-		if (id > META_PAGE_MAX_PAGES) {
+		if (id > data_page_end) {
 			WARN_ON(1);
 			break;
 		}
@@ -5960,6 +5979,7 @@ static void rb_setup_ids_meta_page(struct ring_buffer_per_cpu *cpu_buffer,
 	meta->pages_touched = 0;
 	meta->reader_page = cpu_buffer->reader_page->id;
 	meta->nr_data_pages = cpu_buffer->nr_pages;
+	meta->meta_page_size = 1 << (cpu_buffer->meta_order + PAGE_SHIFT);
 	meta->data_page_head = 0;
 }
 
@@ -6092,10 +6112,12 @@ int ring_buffer_unmap(struct trace_buffer *buffer, int cpu)
 /*
  *   +--------------+
  *   |   meta page  |  pgoff=0
+ *   |     ...      |
+ *   |              |  pgoff=(1<<cpu_buffer->meta_order - 1)
  *   +--------------+
- *   |  data page1  |  pgoff=1 page_ids=0
+ *   |  data page1  |  page_ids=0
  *   +--------------+
- *   |  data page2  |  pgoff=2 page_ids=1
+ *   |  data page2  |  page_ids=1
  *         ...
  */
 struct page *ring_buffer_map_fault(struct trace_buffer *buffer, int cpu,
@@ -6103,10 +6125,11 @@ struct page *ring_buffer_map_fault(struct trace_buffer *buffer, int cpu,
 {
 	struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu];
 
-	if (!pgoff)
-		return virt_to_page(cpu_buffer->meta_page);
+	if (pgoff < (1 << cpu_buffer->meta_order) + 1)
+		return virt_to_page((void *)cpu_buffer->meta_page + (pgoff << PAGE_SHIFT));
+
+	pgoff -= (1 << cpu_buffer->meta_order);
 
-	pgoff--;
 	if (pgoff > cpu_buffer->nr_pages)
 		return NULL;
 

Re: [PATCH 1/2] ring-buffer: Introducing ring-buffer mapping functions

[Vincent Donnefort]

On Mon, Mar 20, 2023 at 09:45:16PM -0400, Steven Rostedt wrote:
> On Fri, 17 Mar 2023 14:33:09 +0000
> Vincent Donnefort <vdonnefort@google.com> wrote:
> 
> > Also, the meta-page being... a single page, this limits at the moment the
> > number of pages in the ring-buffer that can be mapped: ~3MB on a 4K pages
> > system.
> 
> I hate this limitation, so I fixed it ;-)

Thanks a lot for having a look. Do you mind if I fold this in my patch for a V2?

> 
> I added a meta_page_size field to the meta page, and user space can do:
> 
> 	meta = mmap(NULL, page_size, PROT_READ, MAP_SHARED, fd, 0);
> 	if (meta == MAP_FAILED)
> 		pdie("mmap");
> 
> 	map = meta;
> 	meta_len = map->meta_page_size;
> 
> 	if (meta_len > page_size) {
> 		munmap(meta, page_size);
> 		meta = mmap(NULL, meta_len, PROT_READ, MAP_SHARED, fd, 0);
> 		if (meta == MAP_FAILED)
> 			pdie("mmap");
> 		map = meta;
> 	}
> 
> This appears to work (but I'm still testing it).
> 
> -- Steve
> 
> diff --git a/include/uapi/linux/trace_mmap.h b/include/uapi/linux/trace_mmap.h
> index 24bcec754a35..12f3f7ee33d9 100644
> --- a/include/uapi/linux/trace_mmap.h
> +++ b/include/uapi/linux/trace_mmap.h
> @@ -18,6 +18,7 @@ struct ring_buffer_meta_page {
>  	__u32	reader_page;
>  	__u32	nr_data_pages;	/* doesn't take into account the reader_page */
>  	__u32	data_page_head;	/* index of data_pages[] */
> +	__u32	meta_page_size;	/* size of the meta page */

Do we want a specific field here? That could be deduced from nr_data_pages()
quite easily?


>  	__u32	data_pages[];
>  };
>  
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 10a17e78cfe6..77c92e4a7adc 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -526,6 +526,7 @@ struct ring_buffer_per_cpu {
>  	u64				read_stamp;
>  
>  	int				mapped;
> +	int				meta_order;
>  	struct mutex			mapping_lock;
>  	unsigned long			*page_ids;	/* ID to addr */
>  	struct ring_buffer_meta_page	*meta_page;
> @@ -5898,7 +5899,7 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
>  EXPORT_SYMBOL_GPL(ring_buffer_read_page);
>  
>  #define META_PAGE_MAX_PAGES \
> -	((PAGE_SIZE - (offsetof(struct ring_buffer_meta_page, data_page_head))) >> 2)
> +	((PAGE_SIZE - (offsetof(struct ring_buffer_meta_page, data_pages))) >> 2)
>

[...]

Re: [PATCH 1/2] ring-buffer: Introducing ring-buffer mapping functions

[Steven Rostedt]

On Tue, 21 Mar 2023 15:17:15 +0000
Vincent Donnefort <vdonnefort@google.com> wrote:

> On Mon, Mar 20, 2023 at 09:45:16PM -0400, Steven Rostedt wrote:
> > On Fri, 17 Mar 2023 14:33:09 +0000
> > Vincent Donnefort <vdonnefort@google.com> wrote:
> >   
> > > Also, the meta-page being... a single page, this limits at the moment the
> > > number of pages in the ring-buffer that can be mapped: ~3MB on a 4K pages
> > > system.  
> > 
> > I hate this limitation, so I fixed it ;-)  
> 
> Thanks a lot for having a look. Do you mind if I fold this in my patch for a V2?

Hold off, I found some bugs that I'm fixing ;-)

> 
> > 
> > I added a meta_page_size field to the meta page, and user space can do:
> > 
> > 	meta = mmap(NULL, page_size, PROT_READ, MAP_SHARED, fd, 0);
> > 	if (meta == MAP_FAILED)
> > 		pdie("mmap");
> > 
> > 	map = meta;
> > 	meta_len = map->meta_page_size;
> > 
> > 	if (meta_len > page_size) {
> > 		munmap(meta, page_size);
> > 		meta = mmap(NULL, meta_len, PROT_READ, MAP_SHARED, fd, 0);
> > 		if (meta == MAP_FAILED)
> > 			pdie("mmap");
> > 		map = meta;
> > 	}
> > 
> > This appears to work (but I'm still testing it).
> > 
> > -- Steve
> > 
> > diff --git a/include/uapi/linux/trace_mmap.h b/include/uapi/linux/trace_mmap.h
> > index 24bcec754a35..12f3f7ee33d9 100644
> > --- a/include/uapi/linux/trace_mmap.h
> > +++ b/include/uapi/linux/trace_mmap.h
> > @@ -18,6 +18,7 @@ struct ring_buffer_meta_page {
> >  	__u32	reader_page;
> >  	__u32	nr_data_pages;	/* doesn't take into account the reader_page */
> >  	__u32	data_page_head;	/* index of data_pages[] */
> > +	__u32	meta_page_size;	/* size of the meta page */  
> 
> Do we want a specific field here? That could be deduced from nr_data_pages()
> quite easily?

I rather not have too much implementation detail knowledge in user space.
It only removes a single entry, and it makes user space easier. In fact,
I'm thinking we should not include "__u32 data_pages[]" but instead add a:
"__u32 data_start" where user space does:

	__u32 *data_pages = (_u32 *)meta_page + meta_page->data_start;

That way we could extend the data provided by the meta_page in the future.

-- Steve


> 
> 
> >  	__u32	data_pages[];
> >  };
> >  
> > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> > index 10a17e78cfe6..77c92e4a7adc 100644
> > --- a/kernel/trace/ring_buffer.c
> > +++ b/kernel/trace/ring_buffer.c
> > @@ -526,6 +526,7 @@ struct ring_buffer_per_cpu {
> >  	u64				read_stamp;
> >  
> >  	int				mapped;
> > +	int				meta_order;
> >  	struct mutex			mapping_lock;
> >  	unsigned long			*page_ids;	/* ID to addr */
> >  	struct ring_buffer_meta_page	*meta_page;
> > @@ -5898,7 +5899,7 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
> >  EXPORT_SYMBOL_GPL(ring_buffer_read_page);
> >  
> >  #define META_PAGE_MAX_PAGES \
> > -	((PAGE_SIZE - (offsetof(struct ring_buffer_meta_page, data_page_head))) >> 2)
> > +	((PAGE_SIZE - (offsetof(struct ring_buffer_meta_page, data_pages))) >> 2)
> >  
> 
> [...]

Re: [PATCH 1/2] ring-buffer: Introducing ring-buffer mapping functions

[Vincent Donnefort]

On Tue, Mar 21, 2023 at 11:40:47AM -0400, Steven Rostedt wrote:
> On Tue, 21 Mar 2023 15:17:15 +0000
> Vincent Donnefort <vdonnefort@google.com> wrote:
> 
> > On Mon, Mar 20, 2023 at 09:45:16PM -0400, Steven Rostedt wrote:
> > > On Fri, 17 Mar 2023 14:33:09 +0000
> > > Vincent Donnefort <vdonnefort@google.com> wrote:
> > >   
> > > > Also, the meta-page being... a single page, this limits at the moment the
> > > > number of pages in the ring-buffer that can be mapped: ~3MB on a 4K pages
> > > > system.  
> > > 
> > > I hate this limitation, so I fixed it ;-)  
> > 
> > Thanks a lot for having a look. Do you mind if I fold this in my patch for a V2?
> 
> Hold off, I found some bugs that I'm fixing ;-)
> 
> > 
> > > 
> > > I added a meta_page_size field to the meta page, and user space can do:
> > > 
> > > 	meta = mmap(NULL, page_size, PROT_READ, MAP_SHARED, fd, 0);
> > > 	if (meta == MAP_FAILED)
> > > 		pdie("mmap");
> > > 
> > > 	map = meta;
> > > 	meta_len = map->meta_page_size;
> > > 
> > > 	if (meta_len > page_size) {
> > > 		munmap(meta, page_size);
> > > 		meta = mmap(NULL, meta_len, PROT_READ, MAP_SHARED, fd, 0);
> > > 		if (meta == MAP_FAILED)
> > > 			pdie("mmap");
> > > 		map = meta;
> > > 	}
> > > 
> > > This appears to work (but I'm still testing it).
> > > 
> > > -- Steve
> > > 
> > > diff --git a/include/uapi/linux/trace_mmap.h b/include/uapi/linux/trace_mmap.h
> > > index 24bcec754a35..12f3f7ee33d9 100644
> > > --- a/include/uapi/linux/trace_mmap.h
> > > +++ b/include/uapi/linux/trace_mmap.h
> > > @@ -18,6 +18,7 @@ struct ring_buffer_meta_page {
> > >  	__u32	reader_page;
> > >  	__u32	nr_data_pages;	/* doesn't take into account the reader_page */
> > >  	__u32	data_page_head;	/* index of data_pages[] */
> > > +	__u32	meta_page_size;	/* size of the meta page */  
> > 
> > Do we want a specific field here? That could be deduced from nr_data_pages()
> > quite easily?
> 
> I rather not have too much implementation detail knowledge in user space.
> It only removes a single entry, and it makes user space easier. In fact,

Ack.

> I'm thinking we should not include "__u32 data_pages[]" but instead add a:
> "__u32 data_start" where user space does:
> 
> 	__u32 *data_pages = (_u32 *)meta_page + meta_page->data_start;
> 
> That way we could extend the data provided by the meta_page in the future.

That'd be nice. Couldn't we keep both to simplify the code for the kernel side?

> 
> -- Steve
> 

Re: [PATCH 1/2] ring-buffer: Introducing ring-buffer mapping functions

[Steven Rostedt]

On Tue, 21 Mar 2023 16:20:42 +0000
Vincent Donnefort <vdonnefort@google.com> wrote:

> > > Do we want a specific field here? That could be deduced from nr_data_pages()
> > > quite easily?  
> > 
> > I rather not have too much implementation detail knowledge in user space.
> > It only removes a single entry, and it makes user space easier. In fact,  
> 
> Ack.
> 
> > I'm thinking we should not include "__u32 data_pages[]" but instead add a:
> > "__u32 data_start" where user space does:
> > 
> > 	__u32 *data_pages = (_u32 *)meta_page + meta_page->data_start;
> > 
> > That way we could extend the data provided by the meta_page in the future.  
> 
> That'd be nice. Couldn't we keep both to simplify the code for the kernel side?

I would not expose the data_pages[] to user space, because then they'll use
it, and that *will* become an API.

But we could expose it to the kernel side with;

include/uapi/linux/trace_mmap.h:

struct ring_buffer_meta_page {
#if __BITS_PER_LONG == 64
	__u64	entries;
	__u64	overrun;
#else
	__u32	entries;
	__u32	overrun;
#endif
	__u32	pages_touched;
	__u32	reader_page;
	__u32	nr_data_pages;	/* doesn't take into account the reader_page */
	__u32	data_page_head;	/* index of data_pages[] */
	__u32	meta_page_size;	/* size of the meta page */
	__u32	data_start;	/* offset to where data_pages are */
};

kernel/trace/ring_buffer.c:

struct ring_buffer_meta {
	struct ring_buffer_meta_page	meta;
	u32				data_pages[];
}

Then we can start each function with:

	struct ring_buffer_meta_page *meta = &cpu_buffer->meta_page.meta;
	u32 *data_pages = cpu_buffer->meta_page.data_pages;

-- Steve

Re: [PATCH 1/2] ring-buffer: Introducing ring-buffer mapping functions

[Steven Rostedt]

On Tue, 21 Mar 2023 11:40:47 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> > 
> > Thanks a lot for having a look. Do you mind if I fold this in my patch for a V2?  
> 
> Hold off, I found some bugs that I'm fixing ;-)

OK, you can fold this in. I also fixed an issue with your patch where it
was missing setting a page->mapping and also clearing it.

I haven't updated to replace "__u32 *data_pages[]" with an "__u32 data_start"
But I think that should still be done.

-- Steve

diff --git a/include/uapi/linux/trace_mmap.h b/include/uapi/linux/trace_mmap.h
index 24bcec754a35..12f3f7ee33d9 100644
--- a/include/uapi/linux/trace_mmap.h
+++ b/include/uapi/linux/trace_mmap.h
@@ -18,6 +18,7 @@ struct ring_buffer_meta_page {
 	__u32	reader_page;
 	__u32	nr_data_pages;	/* doesn't take into account the reader_page */
 	__u32	data_page_head;	/* index of data_pages[] */
+	__u32	meta_page_size;	/* size of the meta page */
 	__u32	data_pages[];
 };
 
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 10a17e78cfe6..d546fdd14fc3 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -526,6 +526,7 @@ struct ring_buffer_per_cpu {
 	u64				read_stamp;
 
 	int				mapped;
+	int				meta_order;
 	struct mutex			mapping_lock;
 	unsigned long			*page_ids;	/* ID to addr */
 	struct ring_buffer_meta_page	*meta_page;
@@ -5898,32 +5899,63 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
 EXPORT_SYMBOL_GPL(ring_buffer_read_page);
 
 #define META_PAGE_MAX_PAGES \
-	((PAGE_SIZE - (offsetof(struct ring_buffer_meta_page, data_page_head))) >> 2)
+	((PAGE_SIZE - (offsetof(struct ring_buffer_meta_page, data_pages))) >> 2)
+
+static void unmap_page(unsigned long addr)
+{
+	struct page *page = virt_to_page(addr);
+
+	page->mapping = NULL;
+}
 
 static void rb_free_page_ids(struct ring_buffer_per_cpu *cpu_buffer)
 {
+	int i;
+
+	for (i = 0; i < cpu_buffer->nr_pages; i++)
+		unmap_page(cpu_buffer->page_ids[i]);
+
 	kfree(cpu_buffer->page_ids);
 	cpu_buffer->page_ids = NULL;
 }
 
 static int rb_alloc_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
 {
+	struct page *meta_pages;
+	int pages;
+	int order = 0;
+
 	if (cpu_buffer->meta_page)
 		return 0;
 
-	if (cpu_buffer->nr_pages > META_PAGE_MAX_PAGES)
-		return -E2BIG;
-
-	cpu_buffer->meta_page = page_to_virt(alloc_page(GFP_USER));
-	if (!cpu_buffer->meta_page)
+	if (cpu_buffer->nr_pages > META_PAGE_MAX_PAGES) {
+		/* Calcualte how many more pages we need to hold indexes */
+		pages = DIV_ROUND_UP(cpu_buffer->nr_pages - META_PAGE_MAX_PAGES,
+				     PAGE_SIZE / sizeof(u32));
+		/* Add back the meta_page itself */
+		pages++;
+		order = fls(pages) - 1;
+	}
+	meta_pages = alloc_pages(GFP_USER, order);
+	if (!meta_pages)
 		return -ENOMEM;
 
+	cpu_buffer->meta_page = page_to_virt(meta_pages);
+	cpu_buffer->meta_order = order;
+
 	return 0;
 }
 
 static void rb_free_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
 {
-	free_page((unsigned long)cpu_buffer->meta_page);
+	unsigned long addr = (unsigned long)cpu_buffer->meta_page;
+	int i;
+
+	for (i = 0; i < (1 << cpu_buffer->meta_order); i++) {
+		unmap_page(addr);
+		addr += PAGE_SIZE;
+	}
+	free_pages((unsigned long)cpu_buffer->meta_page, cpu_buffer->meta_order);
 	cpu_buffer->meta_page = NULL;
 }
 
@@ -5932,14 +5964,20 @@ static void rb_setup_ids_meta_page(struct ring_buffer_per_cpu *cpu_buffer,
 {
 	struct ring_buffer_meta_page *meta = cpu_buffer->meta_page;
 	struct buffer_page *first_page, *bpage;
+	int data_page_end;
 	int id = 0;
 
 	page_ids[id] = (unsigned long)cpu_buffer->reader_page->page;
 	cpu_buffer->reader_page->id = id++;
 
+	/* Calculate the last index of data_pages[] */
+	data_page_end = (1 << (cpu_buffer->meta_order + PAGE_SHIFT)) -
+		offsetof(struct ring_buffer_meta_page, data_pages);
+	data_page_end /= sizeof(u32);
+
 	first_page = bpage = rb_set_head_page(cpu_buffer);
 	do {
-		if (id > META_PAGE_MAX_PAGES) {
+		if (id > data_page_end) {
 			WARN_ON(1);
 			break;
 		}
@@ -5960,6 +5998,7 @@ static void rb_setup_ids_meta_page(struct ring_buffer_per_cpu *cpu_buffer,
 	meta->pages_touched = 0;
 	meta->reader_page = cpu_buffer->reader_page->id;
 	meta->nr_data_pages = cpu_buffer->nr_pages;
+	meta->meta_page_size = 1 << (cpu_buffer->meta_order + PAGE_SHIFT);
 	meta->data_page_head = 0;
 }
 
@@ -6092,10 +6131,12 @@ int ring_buffer_unmap(struct trace_buffer *buffer, int cpu)
 /*
  *   +--------------+
  *   |   meta page  |  pgoff=0
+ *   |     ...      |
+ *   |              |  pgoff=(1<<cpu_buffer->meta_order - 1)
  *   +--------------+
- *   |  data page1  |  pgoff=1 page_ids=0
+ *   |  data page1  |  page_ids=0
  *   +--------------+
- *   |  data page2  |  pgoff=2 page_ids=1
+ *   |  data page2  |  page_ids=1
  *         ...
  */
 struct page *ring_buffer_map_fault(struct trace_buffer *buffer, int cpu,
@@ -6103,10 +6144,11 @@ struct page *ring_buffer_map_fault(struct trace_buffer *buffer, int cpu,
 {
 	struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu];
 
-	if (!pgoff)
-		return virt_to_page(cpu_buffer->meta_page);
+	if (pgoff < (1 << cpu_buffer->meta_order))
+		return virt_to_page((void *)cpu_buffer->meta_page + (pgoff << PAGE_SHIFT));
+
+	pgoff -= (1 << cpu_buffer->meta_order);
 
-	pgoff--;
 	if (pgoff > cpu_buffer->nr_pages)
 		return NULL;
 
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index ea48eabce7b7..2f43e4a842e7 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -8479,9 +8479,12 @@ static vm_fault_t tracing_buffers_mmap_fault(struct vm_fault *vmf)
 	if (!page)
 		return ret;
 
-	get_page(page);
 	vmf->page = page;
 
+	get_page(vmf->page);
+	vmf->page->mapping = vmf->vma->vm_file->f_mapping;
+	vmf->page->index   = vmf->pgoff;
+
 	return 0;
 }
 

Re: [PATCH 1/2] ring-buffer: Introducing ring-buffer mapping functions

[Vincent Donnefort]

On Tue, Mar 21, 2023 at 12:44:25PM -0400, Steven Rostedt wrote:
> On Tue, 21 Mar 2023 11:40:47 -0400
> Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > > 
> > > Thanks a lot for having a look. Do you mind if I fold this in my patch for a V2?  
> > 
> > Hold off, I found some bugs that I'm fixing ;-)
> 
> OK, you can fold this in. I also fixed an issue with your patch where it
> was missing setting a page->mapping and also clearing it.
> 
> I haven't updated to replace "__u32 *data_pages[]" with an "__u32 data_start"
> But I think that should still be done.
> 
> -- Steve
>

[...] 

Thanks! I'll prepare a v2 with all that!