[PATCH v2 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.

Signed-off-by: Vincent Donnefort <vdonnefort@xxxxxxxxxx>

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..7794314a80e9
--- /dev/null
+++ b/include/uapi/linux/trace_mmap.h
@@ -0,0 +1,25 @@
+/* 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_header {
+#if __BITS_PER_LONG == 64
+	__u64	entries;
+	__u64	overrun;
+#else
+	__u32	entries;
+	__u32	overrun;
+#endif
+	__u32	pages_touched;
+	__u32	meta_page_size;
+	__u32	reader_page;	/* page ID for the reader page */
+	__u32	nr_data_pages;	/* doesn't take into account the reader_page */
+	__u32	data_page_head;	/* ring-buffer head as an offset from data_start */
+	__u32	data_start;	/* offset within the meta page */
+};
+
+#endif /* _UAPI_TRACE_MMAP_H_ */
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index af50d931b020..8b9e773fef2e 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 */
 };
 
 /*
@@ -489,6 +490,11 @@ typedef struct rb_time_struct rb_time_t;
 
 #define MAX_NEST	5
 
+struct ring_buffer_meta_page {
+	struct ring_buffer_meta_page_header	hdr;
+	__u32					data_pages[];
+};
+
 /*
  * head_page == tail_page && head == tail then buffer is empty.
  */
@@ -529,6 +535,13 @@ struct ring_buffer_per_cpu {
 	rb_time_t			before_stamp;
 	u64				event_stamp[MAX_NEST];
 	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;
+
 	/* 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 +1465,38 @@ 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)
+{
+	struct ring_buffer_meta_page *meta = cpu_buffer->meta_page;
+	unsigned long head_id;
+
+	if (!READ_ONCE(cpu_buffer->mapped))
+		return;
+
+	head_id = meta->hdr.data_page_head;
+	meta->hdr.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->hdr.reader_page = cpu_buffer->head_page->id;
+	meta->data_pages[meta->hdr.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 +1518,12 @@ 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 +1609,13 @@ 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->hdr.pages_touched,
+			   local_read(&cpu_buffer->pages_touched));
+	}
 }
 
 static int rb_check_bpage(struct ring_buffer_per_cpu *cpu_buffer,
@@ -1735,6 +1784,7 @@ 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 +2223,6 @@ 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 +2572,13 @@ 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->hdr.overrun,
+				   local_read(&cpu_buffer->overrun));
+		}
+
 		/*
 		 * The entries will be zeroed out when we move the
 		 * tail page.
@@ -3179,6 +3235,14 @@ 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->hdr.entries,
+			   local_read(&cpu_buffer->entries));
+	}
+
 	rb_end_commit(cpu_buffer);
 }
 
@@ -3482,7 +3546,7 @@ 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 +4707,9 @@ 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 +5351,12 @@ 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->hdr.entries, 0);
+		WRITE_ONCE(cpu_buffer->meta_page->hdr.pages_touched, 0);
+		WRITE_ONCE(cpu_buffer->meta_page->hdr.overrun, 0);
+	}
+
 	rb_head_page_activate(cpu_buffer);
 }
 
@@ -5489,6 +5561,11 @@ 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 +5799,8 @@ 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 +5917,306 @@ 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_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) {
+		/* Calculate 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)
+{
+	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;
+}
+
+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 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 > data_page_end) {
+			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->hdr.entries = 0;
+	meta->hdr.overrun = 0;
+	meta->hdr.pages_touched = 0;
+	meta->hdr.reader_page = cpu_buffer->reader_page->id;
+	meta->hdr.nr_data_pages = cpu_buffer->nr_pages;
+	meta->hdr.meta_page_size = 1 << (cpu_buffer->meta_order + PAGE_SHIFT);
+	meta->hdr.data_page_head = 0;
+	meta->hdr.data_start = offsetof(struct ring_buffer_meta_page, data_pages);
+}
+
+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->hdr.entries, 0,
+		local_read(&cpu_buffer->entries));
+	cmpxchg(&cpu_buffer->meta_page->hdr.overrun, 0,
+		local_read(&cpu_buffer->overrun));
+	cmpxchg(&cpu_buffer->meta_page->hdr.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
+ *   |     ...      |
+ *   |              |  pgoff=(1<<cpu_buffer->meta_order - 1)
+ *   +--------------+
+ *   |  data page1  |  page_ids=0
+ *   +--------------+
+ *   |  data page2  |  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 < (1 << cpu_buffer->meta_order))
+		return virt_to_page((void *)cpu_buffer->meta_page + (pgoff << PAGE_SHIFT));
+
+	pgoff -= (1 << cpu_buffer->meta_order);
+
+	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