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 <vdonnef...@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 9b95297339b6..ed788721e3c0 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -337,6 +337,7 @@ 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 */
 };
 
@@ -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                   *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 */
@@ -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->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
  *
@@ -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->subbufs_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->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)
 {
@@ -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_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);
@@ -5929,6 +5998,306 @@ 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(off, sub_order, start, page)       \
+       for (off = 0, page = subbuf_page(0, start);             \
+            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;
+       int subbuf_off;
+
+       /*
+        * 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(subbuf_off, order, subbuf_start, page)
+               page_ref_inc(page);
+}
+
+static inline void subbuf_unmap(unsigned long subbuf_start, int order)
+{
+       struct page *page;
+       int subbuf_off;
+
+       foreach_subbuf_page(subbuf_off, 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) {
+               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;
+
+       /* 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


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