Re: [PATCH v16 2/6] ring-buffer: Introducing ring-buffer mapping functions
From: Steven Rostedt
Date: Sun Feb 11 2024 - 17:18:53 EST
On Fri, 9 Feb 2024 16:34:44 +0000
Vincent Donnefort <vdonnefort@xxxxxxxxxx> wrote:
I have some comment updates, but I also notice a need to change the
code slightly. Nothing major, but enough to perhaps have a v17.
>
> 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..182e05a3004a
> --- /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.
Number of subbufs?
And does that include the reader page? Should probably add that in the comment.
> + * @reader.lost_events: Number of events lost at the time of the reader swap.
> + * @reader.id: subbuf ID of the current reader. From 0 to @nr_subbufs - 1
I'm guessing @nr_subbufs is not the reader page.
> + * @reader.read: Number of bytes read on the reader subbuf.
> + * @flags: Placeholder for now, no defined values.
Should explicitly state flags will be zero until new features are
supported. In other words, there is a defined value, that's zero ;-)
> + * @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: Reserved for future use.
> + * @Reserved2: Reserved for future use.
> + */
> +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 ca796675c0a1..4543fc51567d 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>
> @@ -338,6 +339,7 @@ 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 +486,12 @@ 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 addr */
/* ID to subbuf addr */
Just to be a little more clear.
> + 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 */
> @@ -1548,6 +1556,7 @@ 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));
> @@ -1738,8 +1747,6 @@ 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;
> @@ -5160,6 +5167,22 @@ 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)
> {
> @@ -5204,6 +5227,9 @@ 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))
Isn't the buffer_mutex held when we modify mapped? I believe it's held
here. I don't think we need a READ_ONCE() here. Is there a reason for it?
Hmm, looking down, it looks like you take the buffer->mutex after
setting mapped, is that necessary? If we take the buffer->mutex we can
sync the reset with mapping.
> + rb_update_meta_page(cpu_buffer);
> +
> rb_head_page_activate(cpu_buffer);
> cpu_buffer->pages_removed = 0;
> }
> @@ -5418,6 +5444,12 @@ 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;
> @@ -5682,7 +5714,8 @@ 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)) {
Here the buffer_mutex isn't held, but still, what's the purpose of the READ_ONCE?
I wonder if we just grab the reader_lock when setting the mapped
variable if that would be better than using READ/WRITE_ONCE, which I'm
not sure is helpful.
> struct buffer_data_page *rpage = cpu_buffer->reader_page->page;
> unsigned int rpos = read;
> unsigned int pos = 0;
> @@ -5901,6 +5934,11 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
>
> cpu_buffer = buffer->buffers[cpu];
>
> + if (READ_ONCE(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);
> @@ -6002,6 +6040,304 @@ 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 | __GFP_ZERO));
The above is the main reason I'm looking for a v17. If that
alloc_page() fails, what exactly is going to be returned here?
#define page_to_virt(x) __va(PFN_PHYS(page_to_pfn(x)))
#define __va(x) ((void *)((unsigned long)(x)+PAGE_OFFSET))
I don't think that will be the result you expect. We need to do the
alloc_page(), test the result of that, and then call page_to_virt() on a
page that is not NULL.
> + 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;
> +
I'm thinking for extra safety we could add:
if (!addr)
return;
So this function could be called twice without issue.
> + 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;
> + }
if (WARN_ON(id >= nr_subbufs))
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;
> +
> + rb_update_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);
Is there a reason not to take the buffer->mutex before setting mapped?
> +
> + if (cpu_buffer->mapped) {
> + if (cpu_buffer->mapped == UINT_MAX)
> + err = -EBUSY;
> + else
> + WRITE_ONCE(cpu_buffer->mapped, cpu_buffer->mapped + 1);
As mentioned before, it may be better to take the reader_lock when
setting mapped and that should add the protection we want with mapped
set and other readers.
> + mutex_unlock(&cpu_buffer->mapping_lock);
> + return err;
> + }
> +
> + /* prevent another thread from changing buffer/sub-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);
Instead use:
subbuf_ids = kcalloc(cpu_buffer->nr_pages + 1, sizeof(*subbuf_ids), 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);
> + cpu_buffer->mapped = 1;
> +
> + 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;
> + unsigned long flags;
> + 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 out;
> + } else if (cpu_buffer->mapped > 1) {
> + WRITE_ONCE(cpu_buffer->mapped, cpu_buffer->mapped - 1);
> + goto out;
> + }
> +
> + mutex_lock(&buffer->mutex);
> + raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
> +
> + cpu_buffer->mapped = 0;
> +
> + raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
> +
> + rb_free_subbuf_ids(cpu_buffer);
> + rb_free_meta_page(cpu_buffer);
> + atomic_dec(&cpu_buffer->resize_disabled);
> +
> + mutex_unlock(&buffer->mutex);
> +out:
> + 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:
> + rb_update_meta_page(cpu_buffer);
> + raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
> + rb_put_mapped_buffer(cpu_buffer);
> +
Hmm, there's no protection here. If this task gets preempted for a long
time, it is possible that the cpu_buffer->reader_page can become NULL,
causing a NULL kernel dereference.
-- Steve
> + /* Some archs do not have data cache coherency between kernel and user-space */
> + flush_dcache_folio(virt_to_folio(cpu_buffer->reader_page->page));
> +
> + 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