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 buffer page 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 bpage of the
ring-buffer, ordered following their unique ID, assigned during the
first mapping.
Once mapped, no bpage 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 the buffer pages.
Signed-off-by: Vincent Donnefort <vdonnef...@google.com>
diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index 929ed54dd651..e77a2685fe52 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..9536f0b7c094
--- /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 bpages_touched;
+ unsigned long bpages_lost;
+ unsigned long bpages_read;
+
+ struct {
+ unsigned long lost_events; /* Events lost at the time of
the reader swap */
+ __u32 id; /* Reader bpage ID from 0 to
nr_bpages - 1 */
+ __u32 read; /* Number of bytes read on the
reader bpage */
+ } reader;
+
+ __u32 bpage_size; /* Size of each buffer page
including the header */
+ __u32 nr_bpages; /* Number of buffer pages 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 f95ad0f5be1b..2a6307af9c6c 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -335,6 +335,7 @@ struct buffer_page {
local_t write; /* index for next write */
unsigned read; /* index for next read */
local_t entries; /* entries on this page */
+ u32 id; /* ID for external mapping */
unsigned long real_end; /* real end of data */
unsigned order; /* order of the page */
struct buffer_data_page *page; /* Actual data page */
@@ -483,6 +484,12 @@ 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 *bpage_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 */
@@ -760,6 +767,22 @@ 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 ring_buffer_per_cpu *cpu_buffer)
+{
+ if (unlikely(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->bpages_touched,
+ local_read(&cpu_buffer->pages_touched));
+ WRITE_ONCE(cpu_buffer->meta_page->bpages_lost,
+ local_read(&cpu_buffer->pages_lost));
+ }
+}
+
/*
* rb_wake_up_waiters - wake up tasks waiting for ring buffer input
*
@@ -769,6 +792,10 @@ static __always_inline bool full_hit(struct trace_buffer
*buffer, int cpu, int f
static void rb_wake_up_waiters(struct irq_work *work)
{
struct rb_irq_work *rbwork = container_of(work, struct rb_irq_work,
work);
+ struct ring_buffer_per_cpu *cpu_buffer =
+ container_of(rbwork, struct ring_buffer_per_cpu, irq_work);
+
+ rb_update_meta_page(cpu_buffer);
wake_up_all(&rbwork->waiters);
if (rbwork->full_waiters_pending || rbwork->wakeup_full) {
@@ -1562,6 +1589,7 @@ 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));
@@ -4474,6 +4502,14 @@ 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->bpages_read,
+ local_read(&cpu_buffer->pages_read));
+ }
+
goto again;
out:
@@ -4541,6 +4577,12 @@ 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)
@@ -5088,6 +5130,19 @@ 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->bpages_touched, 0);
+ WRITE_ONCE(meta->bpages_lost, 0);
+ WRITE_ONCE(meta->bpages_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)
{
@@ -5132,6 +5187,9 @@ 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;
}
@@ -5346,6 +5404,11 @@ 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;
@@ -5609,7 +5672,8 @@ 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;
@@ -5828,6 +5892,11 @@ int ring_buffer_page_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->bpage_size);
@@ -5929,6 +5998,308 @@ int ring_buffer_page_order_set(struct trace_buffer
*buffer, int order)
}
EXPORT_SYMBOL_GPL(ring_buffer_page_order_set);
+#define bpage_subpage(sub_off, start) \
+ virt_to_page((void *)(start + (sub_off << PAGE_SHIFT)))
+
+#define foreach_bpage_subpage(sub_off, bpage_order, start, page) \
+ for (sub_off = 0, page = bpage_subpage(0, start); \
+ sub_off < (1 << bpage_order); \
+ sub_off++, page = bpage_subpage(sub_off, start))
+
+static inline void bpage_map_prepare(unsigned long start, int order)
+{
+ struct page *page;
+ int subpage_off;
+
+ /*
+ * When allocating order > 0 pages, only the first struct page has a
+ * refcount > 1. Increasing the refcount here ensures the none of the
+ * struct page composing the bpage is freeed when the mapping is closed.
+ */
+ foreach_bpage_subpage(subpage_off, order, start, page)
+ page_ref_inc(page);
+}
+
+static inline void bpage_unmap(unsigned long start, int order)
+{
+ struct page *page;
+ int subpage_off;
+
+ foreach_bpage_subpage(subpage_off, order, start, page) {
+ page_ref_dec(page);
+ page->mapping = NULL;
+ }
+}
+
+static void rb_free_bpage_ids(struct ring_buffer_per_cpu *cpu_buffer)
+{
+ int sub_id;
+
+ for (sub_id = 0; sub_id < cpu_buffer->nr_pages + 1; sub_id++)
+ bpage_unmap(cpu_buffer->bpage_ids[sub_id],
+ cpu_buffer->buffer->bpage_order);
+
+ kfree(cpu_buffer->bpage_ids);
+ cpu_buffer->bpage_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 *bpage_ids)
+{
+ struct trace_buffer_meta *meta = cpu_buffer->meta_page;
+ unsigned int nr_bpages = cpu_buffer->nr_pages + 1;
+ struct buffer_page *first_page, *bpage;
+ int id = 0;
+
+ bpage_ids[id] = (unsigned long)cpu_buffer->reader_page->page;
+ bpage_map_prepare(bpage_ids[id], cpu_buffer->buffer->bpage_order);
+ cpu_buffer->reader_page->id = id++;
+
+ first_page = bpage = rb_set_head_page(cpu_buffer);
+ do {
+
+ if (id >= nr_bpages) {
+ WARN_ON(1);
+ break;
+ }
+
+ bpage_ids[id] = (unsigned long)bpage->page;
+ bpage->id = id;
+ bpage_map_prepare(bpage_ids[id],
cpu_buffer->buffer->bpage_order);
+
+ rb_inc_page(&bpage);
+ id++;
+ } while (bpage != first_page);
+
+ /* install page ID to kern VA translation */
+ cpu_buffer->bpage_ids = bpage_ids;
+
+ meta->meta_page_size = PAGE_SIZE;
+ meta->meta_struct_len = sizeof(*meta);
+ meta->nr_bpages = nr_bpages;
+ meta->bpage_size = cpu_buffer->buffer->bpage_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, *bpage_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);
+
+ err = rb_alloc_meta_page(cpu_buffer);
+ if (err)
+ goto unlock;
+
+ /* bpage_ids include the reader while nr_pages does not */
+ bpage_ids = kzalloc(sizeof(*bpage_ids) * (cpu_buffer->nr_pages + 1),
+ GFP_KERNEL);
+ if (!bpage_ids) {
+ rb_free_meta_page(cpu_buffer);
+ err = -ENOMEM;
+ goto unlock;
+ }
+
+ atomic_inc(&cpu_buffer->resize_disabled);
+
+ /*
+ * 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, bpage_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->bpages_touched, 0,
+ local_read(&cpu_buffer->pages_touched));
+ cmpxchg(&cpu_buffer->meta_page->bpages_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_bpage_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
+ * | bpage 0 |
+ * +--------------+ pgoff == 1 + (1 << bpage_order)
+ * | bpage 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 bpage_id, bpage_offset, addr;
+ struct page *page;
+
+ if (!pgoff)
+ return virt_to_page((void *)cpu_buffer->meta_page);
+
+ pgoff--;
+
+ bpage_id = pgoff >> buffer->bpage_order;
+ if (bpage_id > cpu_buffer->nr_pages)
+ return NULL;
+
+ bpage_offset = pgoff & ((1UL << buffer->bpage_order) - 1);
+ addr = cpu_buffer->bpage_ids[bpage_id] + (bpage_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
