Re: [PATCH v23 2/5] ring-buffer: Introducing ring-buffer mapping functions

David Hildenbrand
2024-05-10T14:13:03Z

On 10.05.24 16:04, Vincent Donnefort wrote:
> In preparation for allowing the user-space to map a ring-buffer, add
> a set of mapping functions:
>
> ring_buffer_{map,unmap}()
>
> 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.
>
> CC: <linux-mm@kvack.org>
> Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
>
> diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
> index dc5ae4e96aee..96d2140b471e 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;
>
> @@ -223,4 +225,8 @@ 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,
> + struct vm_area_struct *vma);
> +int ring_buffer_unmap(struct trace_buffer *buffer, int cpu);
> +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..b682e9925539
> --- /dev/null
> +++ b/include/uapi/linux/trace_mmap.h
> @@ -0,0 +1,46 @@
> +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> +#ifndef _TRACE_MMAP_H_
> +#define _TRACE_MMAP_H_
> +
> +#include <linux/types.h>
> +
> +/**
> + * struct trace_buffer_meta - Ring-buffer Meta-page description
> + * @meta_page_size: Size of this meta-page.
> + * @meta_struct_len: Size of this structure.
> + * @subbuf_size: Size of each sub-buffer.
> + * @nr_subbufs: Number of subbfs in the ring-buffer, including the reader.
> + * @reader.lost_events: Number of events lost at the time of the reader swap.
> + * @reader.id: subbuf ID of the current reader. ID range [0 : @nr_subbufs - 1]
> + * @reader.read: Number of bytes read on the reader subbuf.
> + * @flags: Placeholder for now, 0 until new features are supported.
> + * @entries: Number of entries in the ring-buffer.
> + * @overrun: Number of entries lost in the ring-buffer.
> + * @read: Number of entries that have been read.
> + * @Reserved1: Internal use only.
> + * @Reserved2: Internal use only.
> + */
> +struct trace_buffer_meta {
> + __u32 meta_page_size;
> + __u32 meta_struct_len;
> +
> + __u32 subbuf_size;
> + __u32 nr_subbufs;
> +
> + struct {
> + __u64 lost_events;
> + __u32 id;
> + __u32 read;
> + } reader;
> +
> + __u64 flags;
> +
> + __u64 entries;
> + __u64 overrun;
> + __u64 read;
> +
> + __u64 Reserved1;
> + __u64 Reserved2;
> +};
> +
> +#endif /* _TRACE_MMAP_H_ */
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index cc9ebe593571..849f6942bb96 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -9,6 +9,7 @@
> #include <linux/ring_buffer.h>
> #include <linux/trace_clock.h>
> #include <linux/sched/clock.h>
> +#include <linux/cacheflush.h>
> #include <linux/trace_seq.h>
> #include <linux/spinlock.h>
> #include <linux/irq_work.h>
> @@ -26,6 +27,7 @@
> #include <linux/list.h>
> #include <linux/cpu.h>
> #include <linux/oom.h>
> +#include <linux/mm.h>
>
> #include <asm/local64.h>
> #include <asm/local.h>
> @@ -338,6 +340,7 @@ 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 */
> };
>
> @@ -484,6 +487,12 @@ struct ring_buffer_per_cpu {
> u64 read_stamp;
> /* pages removed since last reset */
> unsigned long pages_removed;
> +
> + unsigned int mapped;
> + struct mutex mapping_lock;
> + unsigned long *subbuf_ids; /* ID to subbuf VA */
> + 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 */
> @@ -1599,6 +1608,7 @@ 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));
> @@ -1789,8 +1799,6 @@ bool ring_buffer_time_stamp_abs(struct trace_buffer *buffer)
> return buffer->time_stamp_abs;
> }
>
> -static void rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer);
> -
> static inline unsigned long rb_page_entries(struct buffer_page *bpage)
> {
> return local_read(&bpage->entries) & RB_WRITE_MASK;
> @@ -5211,6 +5219,22 @@ static void rb_clear_buffer_page(struct buffer_page *page)
> page->read = 0;
> }
>
> +static void rb_update_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
> +{
> + struct trace_buffer_meta *meta = cpu_buffer->meta_page;
> +
> + meta->reader.read = cpu_buffer->reader_page->read;
> + meta->reader.id = cpu_buffer->reader_page->id;
> + meta->reader.lost_events = cpu_buffer->lost_events;
> +
> + meta->entries = local_read(&cpu_buffer->entries);
> + meta->overrun = local_read(&cpu_buffer->overrun);
> + meta->read = cpu_buffer->read;
> +
> + /* Some archs do not have data cache coherency between kernel and user-space */
> + flush_dcache_folio(virt_to_folio(cpu_buffer->meta_page));
> +}
> +
> static void
> rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
> {
> @@ -5255,6 +5279,9 @@ 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_update_meta_page(cpu_buffer);
> +
> rb_head_page_activate(cpu_buffer);
> cpu_buffer->pages_removed = 0;
> }
> @@ -5469,6 +5496,12 @@ int ring_buffer_swap_cpu(struct trace_buffer *buffer_a,
> cpu_buffer_a = buffer_a->buffers[cpu];
> cpu_buffer_b = buffer_b->buffers[cpu];
>
> + /* It's up to the callers to not try to swap mapped buffers */
> + if (WARN_ON_ONCE(cpu_buffer_a->mapped || 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;
> @@ -5733,7 +5766,8 @@ 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;
> @@ -5956,6 +5990,11 @@ 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);
> @@ -6057,6 +6096,375 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
> }
> EXPORT_SYMBOL_GPL(ring_buffer_subbuf_order_set);
>
> +static int rb_alloc_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
> +{
> + struct page *page;
> +
> + if (cpu_buffer->meta_page)
> + return 0;
> +
> + page = alloc_page(GFP_USER | __GFP_ZERO);
> + if (!page)
> + return -ENOMEM;
> +
> + cpu_buffer->meta_page = page_to_virt(page);
> +
> + return 0;
> +}
> +
> +static void rb_free_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
> +{
> + unsigned long addr = (unsigned long)cpu_buffer->meta_page;
> +
> + 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;
> + cpu_buffer->reader_page->id = id++;
> +
> + first_subbuf = subbuf = rb_set_head_page(cpu_buffer);
> + do {
> + if (WARN_ON(id >= nr_subbufs))
> + break;
> +
> + subbuf_ids[id] = (unsigned long)subbuf->page;
> + subbuf->id = id;
> +
> + 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;
> +
> + rb_update_meta_page(cpu_buffer);
> +}
> +
> +static 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 void rb_put_mapped_buffer(struct ring_buffer_per_cpu *cpu_buffer)
> +{
> + mutex_unlock(&cpu_buffer->mapping_lock);
> +}
> +
> +/*
> + * Fast-path for rb_buffer_(un)map(). Called whenever the meta-page doesn't need
> + * to be set-up or torn-down.
> + */
> +static int __rb_inc_dec_mapped(struct ring_buffer_per_cpu *cpu_buffer,
> + bool inc)
> +{
> + unsigned long flags;
> +
> + lockdep_assert_held(&cpu_buffer->mapping_lock);
> +
> + if (inc && cpu_buffer->mapped == UINT_MAX)
> + return -EBUSY;
> +
> + if (WARN_ON(!inc && cpu_buffer->mapped == 0))
> + return -EINVAL;
> +
> + mutex_lock(&cpu_buffer->buffer->mutex);
> + raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
> +
> + if (inc)
> + cpu_buffer->mapped++;
> + else
> + cpu_buffer->mapped--;
> +
> + raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
> + mutex_unlock(&cpu_buffer->buffer->mutex);
> +
> + return 0;
> +}
> +
> +/*
> + * +--------------+ pgoff == 0
> + * | meta page |
> + * +--------------+ pgoff == 1
> + * | subbuffer 0 |
> + * | |
> + * +--------------+ pgoff == (1 + (1 << subbuf_order))
> + * | subbuffer 1 |
> + * | |
> + * ...
> + */
> +#ifdef CONFIG_MMU
> +static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
> + struct vm_area_struct *vma)
> +{
> + unsigned long nr_subbufs, nr_pages, vma_pages, pgoff = vma->vm_pgoff;
> + unsigned int subbuf_pages, subbuf_order;
> + struct page **pages;
> + int p = 0, s = 0;
> + int err;
> +
> + /* Refuse MP_PRIVATE or writable mappings */
> + if (vma->vm_flags & VM_WRITE || vma->vm_flags & VM_EXEC ||
> + !(vma->vm_flags & VM_MAYSHARE))
> + return -EPERM;
> +
> + /*
> + * Make sure the mapping cannot become writable later. Also tell the VM
> + * to not touch these pages (VM_DONTCOPY | VM_DONTEXPAND).

Coment a bit outdated.

Acked-by: David Hildenbrand <david@redhat.com>

--
Cheers,

David / dhildenb