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

Masami Hiramatsu
2024-01-09T14:42:35Z

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>