Commit 117c3920 authored by Vincent Donnefort's avatar Vincent Donnefort Committed by Steven Rostedt (Google)
Browse files

ring-buffer: Introducing ring-buffer mapping functions 

In preparation for allowing the user-space to map a ring-buffer, add
a set of mapping functions:

  ring_buffer_{map,unmap}()

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.

Link: https://lore.kernel.org/linux-trace-kernel/20240510140435.3550353-3-vdonnefort@google.com



CC: <linux-mm@kvack.org>
Signed-off-by: default avatarVincent Donnefort <vdonnefort@google.com>
Acked-by: default avatarDavid Hildenbrand <david@redhat.com>
Signed-off-by: default avatarSteven Rostedt (Google) <rostedt@goodmis.org>
parent c09d4167

include/linux/ring_buffer.h

+6 −0
Original line number Diff line number Diff line
@@ -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;
@@ -223,4 +225,8 @@ 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,

		    struct vm_area_struct *vma);

int ring_buffer_unmap(struct trace_buffer *buffer, int cpu);

int ring_buffer_map_get_reader(struct trace_buffer *buffer, int cpu);

#endif /* _LINUX_RING_BUFFER_H */

include/uapi/linux/trace_mmap.h

0 → 100644
+46 −0
Original line number Diff line number Diff line 
/* 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 sub-buffer.

 * @nr_subbufs:		Number of subbfs in the ring-buffer, including the reader.

 * @reader.lost_events:	Number of events lost at the time of the reader swap.

 * @reader.id:		subbuf ID of the current reader. ID range [0 : @nr_subbufs - 1]

 * @reader.read:	Number of bytes read on the reader subbuf.

 * @flags:		Placeholder for now, 0 until new features are supported.

 * @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.

 * @Reserved1:		Internal use only.

 * @Reserved2:		Internal use only.

 */

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_ */

kernel/trace/ring_buffer.c

+411 −3
Original line number Diff line number Diff line
@@ -9,6 +9,7 @@ 
#include <linux/ring_buffer.h>

#include <linux/trace_clock.h>

#include <linux/sched/clock.h>

#include <linux/cacheflush.h>

#include <linux/trace_seq.h>

#include <linux/spinlock.h>

#include <linux/irq_work.h>
@@ -26,6 +27,7 @@ 
#include <linux/list.h>

#include <linux/cpu.h>

#include <linux/oom.h>

#include <linux/mm.h>



#include <asm/local64.h>

#include <asm/local.h>
@@ -338,6 +340,7 @@ 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 +487,12 @@ struct ring_buffer_per_cpu { 
	u64				read_stamp;

	/* pages removed since last reset */

	unsigned long			pages_removed;



	unsigned int			mapped;

	struct mutex			mapping_lock;

	unsigned long			*subbuf_ids;	/* ID to subbuf VA */

	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 */
@@ -1599,6 +1608,7 @@ 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));
@@ -1789,8 +1799,6 @@ bool ring_buffer_time_stamp_abs(struct trace_buffer *buffer) 
	return buffer->time_stamp_abs;

}



static void rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer);



static inline unsigned long rb_page_entries(struct buffer_page *bpage)

{

	return local_read(&bpage->entries) & RB_WRITE_MASK;
@@ -5211,6 +5219,22 @@ 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;



	meta->reader.read = cpu_buffer->reader_page->read;

	meta->reader.id = cpu_buffer->reader_page->id;

	meta->reader.lost_events = cpu_buffer->lost_events;



	meta->entries = local_read(&cpu_buffer->entries);

	meta->overrun = local_read(&cpu_buffer->overrun);

	meta->read = cpu_buffer->read;



	/* Some archs do not have data cache coherency between kernel and user-space */

	flush_dcache_folio(virt_to_folio(cpu_buffer->meta_page));

}



static void

rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)

{
@@ -5255,6 +5279,9 @@ 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;

}
@@ -5469,6 +5496,12 @@ int ring_buffer_swap_cpu(struct trace_buffer *buffer_a, 
	cpu_buffer_a = buffer_a->buffers[cpu];

	cpu_buffer_b = buffer_b->buffers[cpu];



	/* It's up to the callers to not try to swap mapped buffers */

	if (WARN_ON_ONCE(cpu_buffer_a->mapped || 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;
@@ -5733,7 +5766,8 @@ 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;
@@ -5956,6 +5990,11 @@ 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);
@@ -6057,6 +6096,375 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order) 
}

EXPORT_SYMBOL_GPL(ring_buffer_subbuf_order_set);



static int rb_alloc_meta_page(struct ring_buffer_per_cpu *cpu_buffer)

{

	struct page *page;



	if (cpu_buffer->meta_page)

		return 0;



	page = alloc_page(GFP_USER | __GFP_ZERO);

	if (!page)

		return -ENOMEM;



	cpu_buffer->meta_page = page_to_virt(page);



	return 0;

}



static void rb_free_meta_page(struct ring_buffer_per_cpu *cpu_buffer)

{

	unsigned long addr = (unsigned long)cpu_buffer->meta_page;



	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;

	cpu_buffer->reader_page->id = id++;



	first_subbuf = subbuf = rb_set_head_page(cpu_buffer);

	do {

		if (WARN_ON(id >= nr_subbufs))

			break;



		subbuf_ids[id] = (unsigned long)subbuf->page;

		subbuf->id = id;



		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 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 void rb_put_mapped_buffer(struct ring_buffer_per_cpu *cpu_buffer)

{

	mutex_unlock(&cpu_buffer->mapping_lock);

}



/*

 * Fast-path for rb_buffer_(un)map(). Called whenever the meta-page doesn't need

 * to be set-up or torn-down.

 */

static int __rb_inc_dec_mapped(struct ring_buffer_per_cpu *cpu_buffer,

			       bool inc)

{

	unsigned long flags;



	lockdep_assert_held(&cpu_buffer->mapping_lock);



	if (inc && cpu_buffer->mapped == UINT_MAX)

		return -EBUSY;



	if (WARN_ON(!inc && cpu_buffer->mapped == 0))

		return -EINVAL;



	mutex_lock(&cpu_buffer->buffer->mutex);

	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);



	if (inc)

		cpu_buffer->mapped++;

	else

		cpu_buffer->mapped--;



	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);

	mutex_unlock(&cpu_buffer->buffer->mutex);



	return 0;

}



/*

 *   +--------------+  pgoff == 0

 *   |   meta page  |

 *   +--------------+  pgoff == 1

 *   | subbuffer 0  |

 *   |              |

 *   +--------------+  pgoff == (1 + (1 << subbuf_order))

 *   | subbuffer 1  |

 *   |              |

 *         ...

 */

#ifdef CONFIG_MMU

static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,

			struct vm_area_struct *vma)

{

	unsigned long nr_subbufs, nr_pages, vma_pages, pgoff = vma->vm_pgoff;

	unsigned int subbuf_pages, subbuf_order;

	struct page **pages;

	int p = 0, s = 0;

	int err;



	/* Refuse MP_PRIVATE or writable mappings */

	if (vma->vm_flags & VM_WRITE || vma->vm_flags & VM_EXEC ||

	    !(vma->vm_flags & VM_MAYSHARE))

		return -EPERM;



	/*

	 * Make sure the mapping cannot become writable later. Also tell the VM

	 * to not touch these pages (VM_DONTCOPY | VM_DONTEXPAND).

	 */

	vm_flags_mod(vma, VM_DONTCOPY | VM_DONTEXPAND | VM_DONTDUMP,

		     VM_MAYWRITE);



	lockdep_assert_held(&cpu_buffer->mapping_lock);



	subbuf_order = cpu_buffer->buffer->subbuf_order;

	subbuf_pages = 1 << subbuf_order;



	nr_subbufs = cpu_buffer->nr_pages + 1; /* + reader-subbuf */

	nr_pages = ((nr_subbufs) << subbuf_order) - pgoff + 1; /* + meta-page */



	vma_pages = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;

	if (!vma_pages || vma_pages > nr_pages)

		return -EINVAL;



	nr_pages = vma_pages;



	pages = kcalloc(nr_pages, sizeof(*pages), GFP_KERNEL);

	if (!pages)

		return -ENOMEM;



	if (!pgoff) {

		pages[p++] = virt_to_page(cpu_buffer->meta_page);



		/*

		 * TODO: Align sub-buffers on their size, once

		 * vm_insert_pages() supports the zero-page.

		 */

	} else {

		/* Skip the meta-page */

		pgoff--;



		if (pgoff % subbuf_pages) {

			err = -EINVAL;

			goto out;

		}



		s += pgoff / subbuf_pages;

	}



	while (p < nr_pages) {

		struct page *page = virt_to_page(cpu_buffer->subbuf_ids[s]);

		int off = 0;



		if (WARN_ON_ONCE(s >= nr_subbufs)) {

			err = -EINVAL;

			goto out;

		}



		for (; off < (1 << (subbuf_order)); off++, page++) {

			if (p >= nr_pages)

				break;



			pages[p++] = page;

		}

		s++;

	}



	err = vm_insert_pages(vma, vma->vm_start, pages, &nr_pages);



out:

	kfree(pages);



	return err;

}

#else

static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,

			struct vm_area_struct *vma)

{

	return -EOPNOTSUPP;

}

#endif



int ring_buffer_map(struct trace_buffer *buffer, int cpu,

		    struct vm_area_struct *vma)

{

	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) {

		err = __rb_map_vma(cpu_buffer, vma);

		if (!err)

			err = __rb_inc_dec_mapped(cpu_buffer, true);

		mutex_unlock(&cpu_buffer->mapping_lock);

		return err;

	}



	/* prevent another thread from changing buffer/sub-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 = kcalloc(cpu_buffer->nr_pages + 1, sizeof(*subbuf_ids), 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);

	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);



	err = __rb_map_vma(cpu_buffer, vma);

	if (!err) {

		raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);

		cpu_buffer->mapped = 1;

		raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);

	} else {

		kfree(cpu_buffer->subbuf_ids);

		cpu_buffer->subbuf_ids = NULL;

		rb_free_meta_page(cpu_buffer);

	}



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;

	unsigned long flags;

	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 out;

	} else if (cpu_buffer->mapped > 1) {

		__rb_inc_dec_mapped(cpu_buffer, false);

		goto out;

	}



	mutex_lock(&buffer->mutex);

	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);



	cpu_buffer->mapped = 0;



	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);



	kfree(cpu_buffer->subbuf_ids);

	cpu_buffer->subbuf_ids = NULL;

	rb_free_meta_page(cpu_buffer);

	atomic_dec(&cpu_buffer->resize_disabled);



	mutex_unlock(&buffer->mutex);



out:

	mutex_unlock(&cpu_buffer->mapping_lock);



	return err;

}



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:

	/* Some archs do not have data cache coherency between kernel and user-space */

	flush_dcache_folio(virt_to_folio(cpu_buffer->reader_page->page));



	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