[PATCH v10 0/2] Introducing trace buffer mapping by user-space

[PATCH v10 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 userspace direct access
to the ring-buffer pages via a mapping. An application can now become a
consumer of the ring-buffer, in a similar fashion to what trace_pipe
offers.

Support for this new feature in libtracefs can be found here:

  https://lore.kernel.org/all/20231228201100.78aae259@rorschach.local.home

Vincent

v9 -> v10:
  * Refactor rb_update_meta_page()
  * In-loop declaration for foreach_subbuf_page()
  * Check for cpu_buffer->mapped overflow

v8 -> v9:
  * Fix the unlock path in ring_buffer_map()
  * Fix cpu_buffer cast with rb_work_rq->is_cpu_buffer
  * Rebase on linux-trace/for-next (3cb3091138ca0921c4569bcf7ffa062519639b6a)

v7 -> v8:
  * Drop the subbufs renaming into bpages
  * Use subbuf as a name when relevant

v6 -> v7:
  * Rebase onto lore.kernel.org/lkml/20231215175502.106587604@goodmis.org/
  * Support for subbufs
  * Rename subbufs into bpages

v5 -> v6:
  * Rebase on next-20230802.
  * (unsigned long) -> (void *) cast for virt_to_page().
  * Add a wait for the GET_READER_PAGE ioctl.
  * Move writer fields update (overrun/pages_lost/entries/pages_touched)
    in the irq_work.
  * Rearrange id in struct buffer_page.
  * Rearrange the meta-page.
  * ring_buffer_meta_page -> trace_buffer_meta_page.
  * Add meta_struct_len into the meta-page.

v4 -> v5:
  * Trivial rebase onto 6.5-rc3 (previously 6.4-rc3)

v3 -> v4:
  * Add to the meta-page:
       - pages_lost / pages_read (allow to compute how full is the
	 ring-buffer)
       - read (allow to compute how many entries can be read)
       - A reader_page struct.
  * Rename ring_buffer_meta_header -> ring_buffer_meta
  * Rename ring_buffer_get_reader_page -> ring_buffer_map_get_reader_page
  * Properly consume events on ring_buffer_map_get_reader_page() with
    rb_advance_reader().

v2 -> v3:
  * Remove data page list (for non-consuming read)
    ** Implies removing order > 0 meta-page
  * Add a new meta page field ->read
  * Rename ring_buffer_meta_page_header into ring_buffer_meta_header

v1 -> v2:
  * Hide data_pages from the userspace struct
  * Fix META_PAGE_MAX_PAGES
  * Support for order > 0 meta-page
  * Add missing page->mapping.

---

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

 include/linux/ring_buffer.h     |   7 +
 include/uapi/linux/trace_mmap.h |  31 +++
 kernel/trace/ring_buffer.c      | 384 +++++++++++++++++++++++++++++++-
 kernel/trace/trace.c            |  79 ++++++-
 4 files changed, 497 insertions(+), 4 deletions(-)
 create mode 100644 include/uapi/linux/trace_mmap.h


base-commit: 3cb3091138ca0921c4569bcf7ffa062519639b6a
-- 
2.43.0.472.g3155946c3a-goog

[PATCH v10 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_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..f950648b0ba9
--- /dev/null
+++ b/include/uapi/linux/trace_mmap.h
@@ -0,0 +1,29 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef _UAPI_TRACE_MMAP_H_
+#define _UAPI_TRACE_MMAP_H_
+
+#include <linux/types.h>
+
+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;	/* Events lost at the time of the reader swap */
+		__u32		id;		/* Reader subbuf ID from 0 to nr_subbufs - 1 */
+		__u32		read;		/* Number of bytes read on the reader subbuf */
+	} reader;
+
+	__u32		subbuf_size;		/* Size of each subbuf including the header */
+	__u32		nr_subbufs;		/* Number of subbufs in the ring-buffer */
+
+	__u32		meta_page_size;		/* Size of the meta-page */
+	__u32		meta_struct_len;	/* Len of this struct */
+};
+
+#endif /* _UAPI_TRACE_MMAP_H_ */
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 173d2595ce2d..63426e525781 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 */
 };
 
@@ -388,6 +389,7 @@ struct rb_irq_work {
 	bool				waiters_pending;
 	bool				full_waiters_pending;
 	bool				wakeup_full;
+	bool				is_cpu_buffer;
 };
 
 /*
@@ -484,6 +486,12 @@ 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 */
@@ -739,6 +747,33 @@ static __always_inline bool full_hit(struct trace_buffer *buffer, int cpu, int f
 	return (dirty * 100) > (full * nr_pages);
 }
 
+static void rb_update_meta_page(struct rb_irq_work *rbwork)
+{
+	struct ring_buffer_per_cpu *cpu_buffer;
+
+	if (!rbwork->is_cpu_buffer)
+		return;
+	/*
+	 * If the waiter is a cpu_buffer, this might be due to a
+	 * userspace mapping. Let's update the meta-page.
+	 */
+	cpu_buffer = container_of(rbwork, struct ring_buffer_per_cpu,
+				  irq_work);
+
+	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));
+		WRITE_ONCE(cpu_buffer->meta_page->overrun,
+			   local_read(&cpu_buffer->overrun));
+		WRITE_ONCE(cpu_buffer->meta_page->subbufs_touched,
+			   local_read(&cpu_buffer->pages_touched));
+		WRITE_ONCE(cpu_buffer->meta_page->subbufs_lost,
+			   local_read(&cpu_buffer->pages_lost));
+	}
+}
+
 /*
  * rb_wake_up_waiters - wake up tasks waiting for ring buffer input
  *
@@ -749,6 +784,8 @@ static void rb_wake_up_waiters(struct irq_work *work)
 {
 	struct rb_irq_work *rbwork = container_of(work, struct rb_irq_work, work);
 
+	rb_update_meta_page(rbwork);
+
 	wake_up_all(&rbwork->waiters);
 	if (rbwork->full_waiters_pending || rbwork->wakeup_full) {
 		rbwork->wakeup_full = false;
@@ -1541,6 +1578,8 @@ 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);
+	cpu_buffer->irq_work.is_cpu_buffer = true;
 
 	bpage = kzalloc_node(ALIGN(sizeof(*bpage), cache_line_size()),
 			    GFP_KERNEL, cpu_to_node(cpu));
@@ -4544,6 +4583,14 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
 		cpu_buffer->last_overrun = overwrite;
 	}
 
+	if (cpu_buffer->mapped) {
+		WRITE_ONCE(cpu_buffer->meta_page->reader.read, 0);
+		WRITE_ONCE(cpu_buffer->meta_page->reader.id, reader->id);
+		WRITE_ONCE(cpu_buffer->meta_page->reader.lost_events, cpu_buffer->lost_events);
+		WRITE_ONCE(cpu_buffer->meta_page->subbufs_read,
+			   local_read(&cpu_buffer->pages_read));
+	}
+
 	goto again;
 
  out:
@@ -4611,6 +4658,12 @@ static void rb_advance_reader(struct ring_buffer_per_cpu *cpu_buffer)
 	length = rb_event_length(event);
 	cpu_buffer->reader_page->read += length;
 	cpu_buffer->read_bytes += length;
+	if (cpu_buffer->mapped) {
+		WRITE_ONCE(cpu_buffer->meta_page->reader.read,
+			   cpu_buffer->reader_page->read);
+		WRITE_ONCE(cpu_buffer->meta_page->read,
+			   cpu_buffer->read);
+	}
 }
 
 static void rb_advance_iter(struct ring_buffer_iter *iter)
@@ -5158,6 +5211,19 @@ static void rb_clear_buffer_page(struct buffer_page *page)
 	page->read = 0;
 }
 
+static void rb_reset_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
+{
+	struct trace_buffer_meta *meta = cpu_buffer->meta_page;
+
+	WRITE_ONCE(meta->entries, 0);
+	WRITE_ONCE(meta->overrun, 0);
+	WRITE_ONCE(meta->read, cpu_buffer->read);
+	WRITE_ONCE(meta->subbufs_touched, 0);
+	WRITE_ONCE(meta->subbufs_lost, 0);
+	WRITE_ONCE(meta->subbufs_read, local_read(&cpu_buffer->pages_read));
+	WRITE_ONCE(meta->reader.read, cpu_buffer->reader_page->read);
+}
+
 static void
 rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
 {
@@ -5202,6 +5268,9 @@ 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_reset_meta_page(cpu_buffer);
+
 	rb_head_page_activate(cpu_buffer);
 	cpu_buffer->pages_removed = 0;
 }
@@ -5416,6 +5485,11 @@ 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;
@@ -5679,7 +5753,8 @@ 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;
@@ -5898,6 +5973,11 @@ 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);
@@ -5999,6 +6079,308 @@ 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));
+	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;
+	meta->reader.id = cpu_buffer->reader_page->id;
+	rb_reset_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);
+	/*
+	 * Ensure rb_update_meta() 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->subbufs_touched, 0,
+		local_read(&cpu_buffer->pages_touched));
+	cmpxchg(&cpu_buffer->meta_page->subbufs_lost, 0,
+		local_read(&cpu_buffer->pages_lost));
+
+	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();
+
+		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:
+	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
-- 
2.43.0.472.g3155946c3a-goog

Re: [PATCH v10 1/2] ring-buffer: Introducing ring-buffer mapping functions

[Masami Hiramatsu]

On Fri,  5 Jan 2024 09:47:28 +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..f950648b0ba9
> --- /dev/null
> +++ b/include/uapi/linux/trace_mmap.h
> @@ -0,0 +1,29 @@
> +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> +#ifndef _UAPI_TRACE_MMAP_H_
> +#define _UAPI_TRACE_MMAP_H_
> +
> +#include <linux/types.h>
> +
> +struct trace_buffer_meta {

It is better to add a description about this data structure in
kerneldoc style, instead of adding a comment on each field,
because this structure will be exposed to user space.

> +	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;	/* Events lost at the time of the reader swap */
> +		__u32		id;		/* Reader subbuf ID from 0 to nr_subbufs - 1 */
> +		__u32		read;		/* Number of bytes read on the reader subbuf */
> +	} reader;
> +
> +	__u32		subbuf_size;		/* Size of each subbuf including the header */
> +	__u32		nr_subbufs;		/* Number of subbufs in the ring-buffer */
> +
> +	__u32		meta_page_size;		/* Size of the meta-page */
> +	__u32		meta_struct_len;	/* Len of this struct */
> +};
> +
> +#endif /* _UAPI_TRACE_MMAP_H_ */
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 173d2595ce2d..63426e525781 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 */
>  };
>  
> @@ -388,6 +389,7 @@ struct rb_irq_work {
>  	bool				waiters_pending;
>  	bool				full_waiters_pending;
>  	bool				wakeup_full;
> +	bool				is_cpu_buffer;

I think 'is_cpu_buffer' is a bit unclear (or generic),
what about 'meta_page_update'?

>  };
>  
>  /*
> @@ -484,6 +486,12 @@ 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 */
> @@ -739,6 +747,33 @@ static __always_inline bool full_hit(struct trace_buffer *buffer, int cpu, int f
>  	return (dirty * 100) > (full * nr_pages);
>  }
>  
> +static void rb_update_meta_page(struct rb_irq_work *rbwork)
> +{
> +	struct ring_buffer_per_cpu *cpu_buffer;
> +
> +	if (!rbwork->is_cpu_buffer)
> +		return;
> +	/*
> +	 * If the waiter is a cpu_buffer, this might be due to a
> +	 * userspace mapping. Let's update the meta-page.
> +	 */
> +	cpu_buffer = container_of(rbwork, struct ring_buffer_per_cpu,
> +				  irq_work);
> +
> +	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));
> +		WRITE_ONCE(cpu_buffer->meta_page->overrun,
> +			   local_read(&cpu_buffer->overrun));
> +		WRITE_ONCE(cpu_buffer->meta_page->subbufs_touched,
> +			   local_read(&cpu_buffer->pages_touched));
> +		WRITE_ONCE(cpu_buffer->meta_page->subbufs_lost,
> +			   local_read(&cpu_buffer->pages_lost));
> +	}
> +}
> +
>  /*
>   * rb_wake_up_waiters - wake up tasks waiting for ring buffer input
>   *
> @@ -749,6 +784,8 @@ static void rb_wake_up_waiters(struct irq_work *work)
>  {
>  	struct rb_irq_work *rbwork = container_of(work, struct rb_irq_work, work);
>  
> +	rb_update_meta_page(rbwork);
> +
>  	wake_up_all(&rbwork->waiters);
>  	if (rbwork->full_waiters_pending || rbwork->wakeup_full) {
>  		rbwork->wakeup_full = false;
> @@ -1541,6 +1578,8 @@ 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);
> +	cpu_buffer->irq_work.is_cpu_buffer = true;
>  
>  	bpage = kzalloc_node(ALIGN(sizeof(*bpage), cache_line_size()),
>  			    GFP_KERNEL, cpu_to_node(cpu));
> @@ -4544,6 +4583,14 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
>  		cpu_buffer->last_overrun = overwrite;
>  	}
>  
> +	if (cpu_buffer->mapped) {
> +		WRITE_ONCE(cpu_buffer->meta_page->reader.read, 0);
> +		WRITE_ONCE(cpu_buffer->meta_page->reader.id, reader->id);
> +		WRITE_ONCE(cpu_buffer->meta_page->reader.lost_events, cpu_buffer->lost_events);
> +		WRITE_ONCE(cpu_buffer->meta_page->subbufs_read,
> +			   local_read(&cpu_buffer->pages_read));
> +	}
> +
>  	goto again;
>  
>   out:
> @@ -4611,6 +4658,12 @@ static void rb_advance_reader(struct ring_buffer_per_cpu *cpu_buffer)
>  	length = rb_event_length(event);
>  	cpu_buffer->reader_page->read += length;
>  	cpu_buffer->read_bytes += length;
> +	if (cpu_buffer->mapped) {
> +		WRITE_ONCE(cpu_buffer->meta_page->reader.read,
> +			   cpu_buffer->reader_page->read);
> +		WRITE_ONCE(cpu_buffer->meta_page->read,
> +			   cpu_buffer->read);
> +	}
>  }
>  
>  static void rb_advance_iter(struct ring_buffer_iter *iter)
> @@ -5158,6 +5211,19 @@ static void rb_clear_buffer_page(struct buffer_page *page)
>  	page->read = 0;
>  }
>  
> +static void rb_reset_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
> +{
> +	struct trace_buffer_meta *meta = cpu_buffer->meta_page;
> +
> +	WRITE_ONCE(meta->entries, 0);
> +	WRITE_ONCE(meta->overrun, 0);
> +	WRITE_ONCE(meta->read, cpu_buffer->read);
> +	WRITE_ONCE(meta->subbufs_touched, 0);
> +	WRITE_ONCE(meta->subbufs_lost, 0);
> +	WRITE_ONCE(meta->subbufs_read, local_read(&cpu_buffer->pages_read));
> +	WRITE_ONCE(meta->reader.read, cpu_buffer->reader_page->read);
> +}
> +
>  static void
>  rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
>  {
> @@ -5202,6 +5268,9 @@ 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_reset_meta_page(cpu_buffer);
> +
>  	rb_head_page_activate(cpu_buffer);
>  	cpu_buffer->pages_removed = 0;
>  }
> @@ -5416,6 +5485,11 @@ 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;
> @@ -5679,7 +5753,8 @@ 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;
> @@ -5898,6 +5973,11 @@ 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);
> @@ -5999,6 +6079,308 @@ 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));
> +	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;
> +	meta->reader.id = cpu_buffer->reader_page->id;
> +	rb_reset_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);
> +	/*
> +	 * Ensure rb_update_meta() will observe the meta-page before
                 ^rb_update_meta_page()

> +	 * 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->subbufs_touched, 0,
> +		local_read(&cpu_buffer->pages_touched));
> +	cmpxchg(&cpu_buffer->meta_page->subbufs_lost, 0,
> +		local_read(&cpu_buffer->pages_lost));

Instead of using these cmpxchg, can we move this initialization before
smp_wmb()? Thus we can avoid conflict with rb_update_meta_page()

Thank you,

> +
> +	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();
> +
> +		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:
> +	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
> -- 
> 2.43.0.472.g3155946c3a-goog
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>

Re: [PATCH v10 1/2] ring-buffer: Introducing ring-buffer mapping functions

[Vincent Donnefort]

On Tue, Jan 09, 2024 at 11:42:30PM +0900, Masami Hiramatsu wrote:
> On Fri,  5 Jan 2024 09:47:28 +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..f950648b0ba9
> > --- /dev/null
> > +++ b/include/uapi/linux/trace_mmap.h
> > @@ -0,0 +1,29 @@
> > +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> > +#ifndef _UAPI_TRACE_MMAP_H_
> > +#define _UAPI_TRACE_MMAP_H_
> > +
> > +#include <linux/types.h>
> > +
> > +struct trace_buffer_meta {
> 
> It is better to add a description about this data structure in
> kerneldoc style, instead of adding a comment on each field,
> because this structure will be exposed to user space.

Ack.

> 
> > +	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;	/* Events lost at the time of the reader swap */
> > +		__u32		id;		/* Reader subbuf ID from 0 to nr_subbufs - 1 */
> > +		__u32		read;		/* Number of bytes read on the reader subbuf */
> > +	} reader;
> > +
> > +	__u32		subbuf_size;		/* Size of each subbuf including the header */
> > +	__u32		nr_subbufs;		/* Number of subbufs in the ring-buffer */
> > +
> > +	__u32		meta_page_size;		/* Size of the meta-page */
> > +	__u32		meta_struct_len;	/* Len of this struct */
> > +};
> > +
> > +#endif /* _UAPI_TRACE_MMAP_H_ */
> > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> > index 173d2595ce2d..63426e525781 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 */
> >  };
> >  
> > @@ -388,6 +389,7 @@ struct rb_irq_work {
> >  	bool				waiters_pending;
> >  	bool				full_waiters_pending;
> >  	bool				wakeup_full;
> > +	bool				is_cpu_buffer;
> 
> I think 'is_cpu_buffer' is a bit unclear (or generic),
> what about 'meta_page_update'?

Hum not sure about that change. This was really to identify if parent of
rb_irq_work is a cpu_buffer or a trace_buffer. It can be a cpu_buffer regardless
of the need to update the meta-page.

> 
> >  };
> >  
> >  /*
> > @@ -484,6 +486,12 @@ 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 */
> > @@ -739,6 +747,33 @@ static __always_inline bool full_hit(struct trace_buffer *buffer, int cpu, int f
> >  	return (dirty * 100) > (full * nr_pages);
> >  }
> >  
> > +static void rb_update_meta_page(struct rb_irq_work *rbwork)
> > +{
> > +	struct ring_buffer_per_cpu *cpu_buffer;
> > +
> > +	if (!rbwork->is_cpu_buffer)
> > +		return;
> > +	/*
> > +	 * If the waiter is a cpu_buffer, this might be due to a
> > +	 * userspace mapping. Let's update the meta-page.
> > +	 */
> > +	cpu_buffer = container_of(rbwork, struct ring_buffer_per_cpu,
> > +				  irq_work);
> > +
> > +	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));
> > +		WRITE_ONCE(cpu_buffer->meta_page->overrun,
> > +			   local_read(&cpu_buffer->overrun));
> > +		WRITE_ONCE(cpu_buffer->meta_page->subbufs_touched,
> > +			   local_read(&cpu_buffer->pages_touched));
> > +		WRITE_ONCE(cpu_buffer->meta_page->subbufs_lost,
> > +			   local_read(&cpu_buffer->pages_lost));
> > +	}
> > +}
> > +
> >  /*
> >   * rb_wake_up_waiters - wake up tasks waiting for ring buffer input
> >   *
> > @@ -749,6 +784,8 @@ static void rb_wake_up_waiters(struct irq_work *work)
> >  {
> >  	struct rb_irq_work *rbwork = container_of(work, struct rb_irq_work, work);
> >  
> > +	rb_update_meta_page(rbwork);
> > +
> >  	wake_up_all(&rbwork->waiters);
> >  	if (rbwork->full_waiters_pending || rbwork->wakeup_full) {
> >  		rbwork->wakeup_full = false;
> > @@ -1541,6 +1578,8 @@ 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);
> > +	cpu_buffer->irq_work.is_cpu_buffer = true;
> >  
> >  	bpage = kzalloc_node(ALIGN(sizeof(*bpage), cache_line_size()),
> >  			    GFP_KERNEL, cpu_to_node(cpu));
> > @@ -4544,6 +4583,14 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
> >  		cpu_buffer->last_overrun = overwrite;
> >  	}
> >  
> > +	if (cpu_buffer->mapped) {
> > +		WRITE_ONCE(cpu_buffer->meta_page->reader.read, 0);
> > +		WRITE_ONCE(cpu_buffer->meta_page->reader.id, reader->id);
> > +		WRITE_ONCE(cpu_buffer->meta_page->reader.lost_events, cpu_buffer->lost_events);
> > +		WRITE_ONCE(cpu_buffer->meta_page->subbufs_read,
> > +			   local_read(&cpu_buffer->pages_read));
> > +	}
> > +
> >  	goto again;
> >  
> >   out:
> > @@ -4611,6 +4658,12 @@ static void rb_advance_reader(struct ring_buffer_per_cpu *cpu_buffer)
> >  	length = rb_event_length(event);
> >  	cpu_buffer->reader_page->read += length;
> >  	cpu_buffer->read_bytes += length;
> > +	if (cpu_buffer->mapped) {
> > +		WRITE_ONCE(cpu_buffer->meta_page->reader.read,
> > +			   cpu_buffer->reader_page->read);
> > +		WRITE_ONCE(cpu_buffer->meta_page->read,
> > +			   cpu_buffer->read);
> > +	}
> >  }
> >  
> >  static void rb_advance_iter(struct ring_buffer_iter *iter)
> > @@ -5158,6 +5211,19 @@ static void rb_clear_buffer_page(struct buffer_page *page)
> >  	page->read = 0;
> >  }
> >  
> > +static void rb_reset_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
> > +{
> > +	struct trace_buffer_meta *meta = cpu_buffer->meta_page;
> > +
> > +	WRITE_ONCE(meta->entries, 0);
> > +	WRITE_ONCE(meta->overrun, 0);
> > +	WRITE_ONCE(meta->read, cpu_buffer->read);
> > +	WRITE_ONCE(meta->subbufs_touched, 0);
> > +	WRITE_ONCE(meta->subbufs_lost, 0);
> > +	WRITE_ONCE(meta->subbufs_read, local_read(&cpu_buffer->pages_read));
> > +	WRITE_ONCE(meta->reader.read, cpu_buffer->reader_page->read);
> > +}
> > +
> >  static void
> >  rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
> >  {
> > @@ -5202,6 +5268,9 @@ 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_reset_meta_page(cpu_buffer);
> > +
> >  	rb_head_page_activate(cpu_buffer);
> >  	cpu_buffer->pages_removed = 0;
> >  }
> > @@ -5416,6 +5485,11 @@ 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;
> > @@ -5679,7 +5753,8 @@ 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;
> > @@ -5898,6 +5973,11 @@ 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);
> > @@ -5999,6 +6079,308 @@ 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));
> > +	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;
> > +	meta->reader.id = cpu_buffer->reader_page->id;
> > +	rb_reset_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);
> > +	/*
> > +	 * Ensure rb_update_meta() will observe the meta-page before
>                  ^rb_update_meta_page()
> 
> > +	 * 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->subbufs_touched, 0,
> > +		local_read(&cpu_buffer->pages_touched));
> > +	cmpxchg(&cpu_buffer->meta_page->subbufs_lost, 0,
> > +		local_read(&cpu_buffer->pages_lost));
> 
> Instead of using these cmpxchg, can we move this initialization before
> smp_wmb()? Thus we can avoid conflict with rb_update_meta_page()

Good point, Not sure why I made this more complicated than it should be.

> 
> Thank you,
>

[...]

Re: [PATCH v10 1/2] ring-buffer: Introducing ring-buffer mapping functions

[Steven Rostedt]

On Tue, 9 Jan 2024 15:13:51 +0000
Vincent Donnefort <vdonnefort@google.com> wrote:

> > > @@ -388,6 +389,7 @@ struct rb_irq_work {
> > >  	bool				waiters_pending;
> > >  	bool				full_waiters_pending;
> > >  	bool				wakeup_full;
> > > +	bool				is_cpu_buffer;  
> > 
> > I think 'is_cpu_buffer' is a bit unclear (or generic),
> > what about 'meta_page_update'?  
> 
> Hum not sure about that change. This was really to identify if parent of
> rb_irq_work is a cpu_buffer or a trace_buffer. It can be a cpu_buffer regardless
> of the need to update the meta-page.

Yeah, this was added because the irq_work is called with the rb_work for
both the cpu_buffer and the global struct tracing_buffer object. The
meta_page is only available to the cpu_buffer and does not exist on the
struct trace_buffer, so this is checked before doing a "container_of()" on
the wrong structure. Both the cpu_buffer and the global buffer call the
same function with the rb_work structure.

So "is_cpu_buffer" is the right terminology as it's unrelated to the meta page:

struct trace_buffer {
	[..]
	struct rb_irq_work	irq_work;
	[..]
};

	struct trace_buffer *buffer;
	buffer->irq_work.is_cpu_buffer = false;

struct ring_buffer_per_cpu {
	[..]
	struct rb_irq_work	irq_work;
	[..]
}

	struct ring_buffer_per_cpu *cpu_buffer;
	cpu_buffer->irq_work.is_cpu_buffer = true;


[..]
	init_irq_work(&buffer->irq_work.work, rb_wake_up_waiters);
[..]
	init_irq_work(&cpu_buffer->irq_work.work, rb_wake_up_waiters);

// both the buffer and cpu_buffer call rb_wake_up_waiters()

static void rb_wake_up_waiters(struct irq_work *work)
{
	struct rb_irq_work *rbwork = container_of(work, struct rb_irq_work, work);

// This container_of() gets rbwork which is the rb_irq_work structure that
// both buffer and cpu_buffer have.

	if (rbwork->is_cpu_buffer) {
		struct ring_buffer_per_cpu *cpu_buffer;

		cpu_buffer = container_of(rbwork, struct ring_buffer_per_cpu,
					  irq_work);

// The above crashes if done by the buffer and not the cpu_buffer.
// The "is_cpu_buffer" is there to differentiate the two rb_work entities.
// It is a way to say this is safe to do the above "container_of()".

		/*
		 * If the waiter is a cpu_buffer, this might be due to a
		 * userspace mapping. Let's update the meta-page.
		 */
		rb_update_meta_page(cpu_buffer);
	}


-- Steve

Re: [PATCH v10 1/2] ring-buffer: Introducing ring-buffer mapping functions

[Masami Hiramatsu]

On Tue, 9 Jan 2024 15:13:51 +0000
Vincent Donnefort <vdonnefort@google.com> wrote:

> > > @@ -388,6 +389,7 @@ struct rb_irq_work {
> > >  	bool				waiters_pending;
> > >  	bool				full_waiters_pending;
> > >  	bool				wakeup_full;
> > > +	bool				is_cpu_buffer;
> > 
> > I think 'is_cpu_buffer' is a bit unclear (or generic),
> > what about 'meta_page_update'?
> 
> Hum not sure about that change. This was really to identify if parent of
> rb_irq_work is a cpu_buffer or a trace_buffer. It can be a cpu_buffer regardless
> of the need to update the meta-page.

Yeah, I just meant that is "for_cpu_buffer", not "rb_irq_work is_cpu_buffer".
So when reading the code, I just felt uncomfortable.

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>

Re: [PATCH v10 1/2] ring-buffer: Introducing ring-buffer mapping functions

[Steven Rostedt]

On Wed, 10 Jan 2024 08:42:05 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> On Tue, 9 Jan 2024 15:13:51 +0000
> Vincent Donnefort <vdonnefort@google.com> wrote:
> 
> > > > @@ -388,6 +389,7 @@ struct rb_irq_work {
> > > >  	bool				waiters_pending;
> > > >  	bool				full_waiters_pending;
> > > >  	bool				wakeup_full;
> > > > +	bool				is_cpu_buffer;  
> > > 
> > > I think 'is_cpu_buffer' is a bit unclear (or generic),
> > > what about 'meta_page_update'?  
> > 
> > Hum not sure about that change. This was really to identify if parent of
> > rb_irq_work is a cpu_buffer or a trace_buffer. It can be a cpu_buffer regardless
> > of the need to update the meta-page.  
> 
> Yeah, I just meant that is "for_cpu_buffer", not "rb_irq_work is_cpu_buffer".
> So when reading the code, I just felt uncomfortable.
> 

How about "in_cpu_buffer" as that is what it is.

struct ring_buffer_per_cpu {
	struct rb_irq_work {
		bool	in_cpu_buffer;
	}
}

Would that make you feel more comfortable? ;-)

-- Steve

Re: [PATCH v10 1/2] ring-buffer: Introducing ring-buffer mapping functions

[Vincent Donnefort]

On Tue, Jan 09, 2024 at 06:58:13PM -0500, Steven Rostedt wrote:
> On Wed, 10 Jan 2024 08:42:05 +0900
> Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
> 
> > On Tue, 9 Jan 2024 15:13:51 +0000
> > Vincent Donnefort <vdonnefort@google.com> wrote:
> > 
> > > > > @@ -388,6 +389,7 @@ struct rb_irq_work {
> > > > >  	bool				waiters_pending;
> > > > >  	bool				full_waiters_pending;
> > > > >  	bool				wakeup_full;
> > > > > +	bool				is_cpu_buffer;  
> > > > 
> > > > I think 'is_cpu_buffer' is a bit unclear (or generic),
> > > > what about 'meta_page_update'?  
> > > 
> > > Hum not sure about that change. This was really to identify if parent of
> > > rb_irq_work is a cpu_buffer or a trace_buffer. It can be a cpu_buffer regardless
> > > of the need to update the meta-page.  
> > 
> > Yeah, I just meant that is "for_cpu_buffer", not "rb_irq_work is_cpu_buffer".
> > So when reading the code, I just felt uncomfortable.
> > 
> 
> How about "in_cpu_buffer" as that is what it is.
> 
> struct ring_buffer_per_cpu {
> 	struct rb_irq_work {
> 		bool	in_cpu_buffer;
> 	}
> }
> 
> Would that make you feel more comfortable? ;-)
> 
> -- Steve

I'll actually solve that by moving that update from the rb_irq_work to
ring_buffer_map_get_reader().

Reason is the rb_irq_work is only triggered when !O_NONBLOCK is set.

> 
> -- 
> To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@android.com.
> 

Re: [PATCH v10 1/2] ring-buffer: Introducing ring-buffer mapping functions

[Vincent Donnefort]

[...]

> > > +	 */
> > > +	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->subbufs_touched, 0,
> > > +		local_read(&cpu_buffer->pages_touched));
> > > +	cmpxchg(&cpu_buffer->meta_page->subbufs_lost, 0,
> > > +		local_read(&cpu_buffer->pages_lost));
> > 
> > Instead of using these cmpxchg, can we move this initialization before
> > smp_wmb()? Thus we can avoid conflict with rb_update_meta_page()
> 
> Good point, Not sure why I made this more complicated than it should be.

Ha, I know, that was before v6, where the writer was updating all of that. From
v6 the update has been moved to the irq_rb_work.

> 
> > 
> > Thank you,
> >
> 
> [...]

[PATCH v10 2/2] tracing: Allow user-space mapping of the ring-buffer

[Vincent Donnefort]

Currently, user-space extracts data from the ring-buffer via splice,
which is handy for storage or network sharing. However, due to splice
limitations, it is imposible to do real-time analysis without a copy.

A solution for that problem is to let the user-space map the ring-buffer
directly.

The mapping is exposed via the per-CPU file trace_pipe_raw. The first
element of the mapping is the meta-page. It is followed by each
subbuffer constituting the ring-buffer, ordered by their unique page ID:

  * Meta-page -- include/uapi/linux/trace_mmap.h for a description
  * Subbuf ID 0
  * Subbuf ID 1
     ...

It is therefore easy to translate a subbuf ID into an offset in the
mapping:

  reader_id = meta->reader->id;
  reader_offset = meta->meta_page_size + reader_id * meta->subbuf_size;

When new data is available, the mapper must call a newly introduced ioctl:
TRACE_MMAP_IOCTL_GET_READER. This will update the Meta-page reader ID to
point to the next reader containing unread data.

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

diff --git a/include/uapi/linux/trace_mmap.h b/include/uapi/linux/trace_mmap.h
index f950648b0ba9..8c49489c5867 100644
--- a/include/uapi/linux/trace_mmap.h
+++ b/include/uapi/linux/trace_mmap.h
@@ -26,4 +26,6 @@ struct trace_buffer_meta {
 	__u32		meta_struct_len;	/* Len of this struct */
 };
 
+#define TRACE_MMAP_IOCTL_GET_READER		_IO('T', 0x1)
+
 #endif /* _UAPI_TRACE_MMAP_H_ */
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 46dbe22121e9..cfeaf2cd204e 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -8583,15 +8583,31 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
 	return ret;
 }
 
-/* An ioctl call with cmd 0 to the ring buffer file will wake up all waiters */
 static long tracing_buffers_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 {
 	struct ftrace_buffer_info *info = file->private_data;
 	struct trace_iterator *iter = &info->iter;
+	int err;
 
-	if (cmd)
-		return -ENOIOCTLCMD;
+	if (cmd == TRACE_MMAP_IOCTL_GET_READER) {
+		if (!(file->f_flags & O_NONBLOCK)) {
+			err = ring_buffer_wait(iter->array_buffer->buffer,
+					       iter->cpu_file,
+					       iter->tr->buffer_percent);
+			if (err)
+				return err;
+		}
 
+		return ring_buffer_map_get_reader(iter->array_buffer->buffer,
+						  iter->cpu_file);
+	} else if (cmd) {
+		return -ENOTTY;
+	}
+
+	/*
+	 * An ioctl call with cmd 0 to the ring buffer file will wake up all
+	 * waiters
+	 */
 	mutex_lock(&trace_types_lock);
 
 	iter->wait_index++;
@@ -8604,6 +8620,62 @@ static long tracing_buffers_ioctl(struct file *file, unsigned int cmd, unsigned
 	return 0;
 }
 
+static vm_fault_t tracing_buffers_mmap_fault(struct vm_fault *vmf)
+{
+	struct ftrace_buffer_info *info = vmf->vma->vm_file->private_data;
+	struct trace_iterator *iter = &info->iter;
+	vm_fault_t ret = VM_FAULT_SIGBUS;
+	struct page *page;
+
+	page = ring_buffer_map_fault(iter->array_buffer->buffer, iter->cpu_file,
+				     vmf->pgoff);
+	if (!page)
+		return ret;
+
+	get_page(page);
+	vmf->page = page;
+	vmf->page->mapping = vmf->vma->vm_file->f_mapping;
+	vmf->page->index = vmf->pgoff;
+
+	return 0;
+}
+
+static void tracing_buffers_mmap_close(struct vm_area_struct *vma)
+{
+	struct ftrace_buffer_info *info = vma->vm_file->private_data;
+	struct trace_iterator *iter = &info->iter;
+
+	ring_buffer_unmap(iter->array_buffer->buffer, iter->cpu_file);
+}
+
+static void tracing_buffers_mmap_open(struct vm_area_struct *vma)
+{
+	struct ftrace_buffer_info *info = vma->vm_file->private_data;
+	struct trace_iterator *iter = &info->iter;
+
+	WARN_ON(ring_buffer_map(iter->array_buffer->buffer, iter->cpu_file));
+}
+
+static const struct vm_operations_struct tracing_buffers_vmops = {
+	.open		= tracing_buffers_mmap_open,
+	.close		= tracing_buffers_mmap_close,
+	.fault		= tracing_buffers_mmap_fault,
+};
+
+static int tracing_buffers_mmap(struct file *filp, struct vm_area_struct *vma)
+{
+	struct ftrace_buffer_info *info = filp->private_data;
+	struct trace_iterator *iter = &info->iter;
+
+	if (vma->vm_flags & VM_WRITE)
+		return -EPERM;
+
+	vm_flags_mod(vma, VM_DONTCOPY | VM_DONTDUMP, VM_MAYWRITE);
+	vma->vm_ops = &tracing_buffers_vmops;
+
+	return ring_buffer_map(iter->array_buffer->buffer, iter->cpu_file);
+}
+
 static const struct file_operations tracing_buffers_fops = {
 	.open		= tracing_buffers_open,
 	.read		= tracing_buffers_read,
@@ -8612,6 +8684,7 @@ static const struct file_operations tracing_buffers_fops = {
 	.splice_read	= tracing_buffers_splice_read,
 	.unlocked_ioctl = tracing_buffers_ioctl,
 	.llseek		= no_llseek,
+	.mmap		= tracing_buffers_mmap,
 };
 
 static ssize_t
-- 
2.43.0.472.g3155946c3a-goog

Re: [PATCH v10 0/2] Introducing trace buffer mapping by user-space

[Masami Hiramatsu]

Hi Vincent,

On Fri,  5 Jan 2024 09:47:27 +0000
Vincent Donnefort <vdonnefort@google.com> wrote:

> 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 userspace direct access
> to the ring-buffer pages via a mapping. An application can now become a
> consumer of the ring-buffer, in a similar fashion to what trace_pipe
> offers.

I think this is very nice feature. But this series seems just a feature,
no document and no example code. Can you add 2 patches to add those?
I know libtracefs already provide a support code, but I think it is
better to have a test code under tools/testing/selftests/ring_buffer.

I also wonder what happen if other operation (e.g. taking snapshot) happens
while mmaping the ring buffer.

Thank you,

> 
> Support for this new feature in libtracefs can be found here:
> 
>   https://lore.kernel.org/all/20231228201100.78aae259@rorschach.local.home
> 
> Vincent
> 
> v9 -> v10:
>   * Refactor rb_update_meta_page()
>   * In-loop declaration for foreach_subbuf_page()
>   * Check for cpu_buffer->mapped overflow
> 
> v8 -> v9:
>   * Fix the unlock path in ring_buffer_map()
>   * Fix cpu_buffer cast with rb_work_rq->is_cpu_buffer
>   * Rebase on linux-trace/for-next (3cb3091138ca0921c4569bcf7ffa062519639b6a)
> 
> v7 -> v8:
>   * Drop the subbufs renaming into bpages
>   * Use subbuf as a name when relevant
> 
> v6 -> v7:
>   * Rebase onto lore.kernel.org/lkml/20231215175502.106587604@goodmis.org/
>   * Support for subbufs
>   * Rename subbufs into bpages
> 
> v5 -> v6:
>   * Rebase on next-20230802.
>   * (unsigned long) -> (void *) cast for virt_to_page().
>   * Add a wait for the GET_READER_PAGE ioctl.
>   * Move writer fields update (overrun/pages_lost/entries/pages_touched)
>     in the irq_work.
>   * Rearrange id in struct buffer_page.
>   * Rearrange the meta-page.
>   * ring_buffer_meta_page -> trace_buffer_meta_page.
>   * Add meta_struct_len into the meta-page.
> 
> v4 -> v5:
>   * Trivial rebase onto 6.5-rc3 (previously 6.4-rc3)
> 
> v3 -> v4:
>   * Add to the meta-page:
>        - pages_lost / pages_read (allow to compute how full is the
> 	 ring-buffer)
>        - read (allow to compute how many entries can be read)
>        - A reader_page struct.
>   * Rename ring_buffer_meta_header -> ring_buffer_meta
>   * Rename ring_buffer_get_reader_page -> ring_buffer_map_get_reader_page
>   * Properly consume events on ring_buffer_map_get_reader_page() with
>     rb_advance_reader().
> 
> v2 -> v3:
>   * Remove data page list (for non-consuming read)
>     ** Implies removing order > 0 meta-page
>   * Add a new meta page field ->read
>   * Rename ring_buffer_meta_page_header into ring_buffer_meta_header
> 
> v1 -> v2:
>   * Hide data_pages from the userspace struct
>   * Fix META_PAGE_MAX_PAGES
>   * Support for order > 0 meta-page
>   * Add missing page->mapping.
> 
> ---
> 
> Vincent Donnefort (2):
>   ring-buffer: Introducing ring-buffer mapping functions
>   tracing: Allow user-space mapping of the ring-buffer
> 
>  include/linux/ring_buffer.h     |   7 +
>  include/uapi/linux/trace_mmap.h |  31 +++
>  kernel/trace/ring_buffer.c      | 384 +++++++++++++++++++++++++++++++-
>  kernel/trace/trace.c            |  79 ++++++-
>  4 files changed, 497 insertions(+), 4 deletions(-)
>  create mode 100644 include/uapi/linux/trace_mmap.h
> 
> 
> base-commit: 3cb3091138ca0921c4569bcf7ffa062519639b6a
> -- 
> 2.43.0.472.g3155946c3a-goog
> 
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>

Re: [PATCH v10 0/2] Introducing trace buffer mapping by user-space

[Steven Rostedt]

Hi Masami, thanks for looking at this.

On Tue, 9 Jan 2024 22:04:45 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> > 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 userspace direct access
> > to the ring-buffer pages via a mapping. An application can now become a
> > consumer of the ring-buffer, in a similar fashion to what trace_pipe
> > offers.  
> 
> I think this is very nice feature. But this series seems just a feature,
> no document and no example code. Can you add 2 patches to add those?
> I know libtracefs already provide a support code, but I think it is
> better to have a test code under tools/testing/selftests/ring_buffer.

Yeah, we should have sample code and a test.

> 
> I also wonder what happen if other operation (e.g. taking snapshot) happens
> while mmaping the ring buffer.

Hmm, good point. We should disable snapshots when mapped, and also prevent
mapping with latency tracer if we are not already doing that.

-- Steve

Re: [PATCH v10 0/2] Introducing trace buffer mapping by user-space

[Vincent Donnefort]

On Tue, Jan 09, 2024 at 08:20:57AM -0500, Steven Rostedt wrote:
> 
> Hi Masami, thanks for looking at this.
> 
> On Tue, 9 Jan 2024 22:04:45 +0900
> Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
> 
> > > 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 userspace direct access
> > > to the ring-buffer pages via a mapping. An application can now become a
> > > consumer of the ring-buffer, in a similar fashion to what trace_pipe
> > > offers.  
> > 
> > I think this is very nice feature. But this series seems just a feature,
> > no document and no example code. Can you add 2 patches to add those?
> > I know libtracefs already provide a support code, but I think it is
> > better to have a test code under tools/testing/selftests/ring_buffer.
> 
> Yeah, we should have sample code and a test.

Ack. I will recycle what I had in the cover letter in a ring_buffer selftest.

> 
> > 
> > I also wonder what happen if other operation (e.g. taking snapshot) happens
> > while mmaping the ring buffer.
> 
> Hmm, good point. We should disable snapshots when mapped, and also prevent
> mapping with latency tracer if we are not already doing that.

ring_buffer_swap_cpu() is already disabled when mapped as well as
resize_disabled set. Is something else necessary?

> 
> -- Steve