| Message ID | 20240209163448.944970-3-vdonnefort@google.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | Introducing trace buffer mapping by user-space | expand |
On Fri, 9 Feb 2024 16:34:44 +0000 Vincent Donnefort <vdonnefort@google.com> 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
[...] > > +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. The idea was to not take any of the buffer mutex, reader lock if the refcount is simply inc/dec. Locks are only used if the meta-page is installed/uninstalled. The WRITE_ONCE/READ_ONCE is there only to make sure no compiler optimisation could lead a reader to wrongly interpret that refcount while it is inc/dec. That is probably not necessary and I'm happy to either drop it completely or replace it by taking buffer mutex and reader lock whenever the refcount is inc/dec. > > > + 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. ditto above. > > > > 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. Ouch, you're right! > > > + 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. Ack. > > > + 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. Right, this should be behind the reader lock as well :-\ > > -- 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 >
On Mon, 12 Feb 2024 10:44:26 +0000 Vincent Donnefort <vdonnefort@google.com> wrote: > > > 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. > > The idea was to not take any of the buffer mutex, reader lock if the refcount is > simply inc/dec. Locks are only used if the meta-page is > installed/uninstalled. Does it matter? The inc/dec is only done at initial mapping, right? That's a very slow path. > > The WRITE_ONCE/READ_ONCE is there only to make sure no compiler optimisation > could lead a reader to wrongly interpret that refcount while it is inc/dec. That > is probably not necessary and I'm happy to either drop it completely or replace > it by taking buffer mutex and reader lock whenever the refcount is inc/dec. Yeah, probably best to drop it and keep updates within the mutex. -- Steve
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. + * @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 + * @reader.read: Number of bytes read on the reader subbuf. + * @flags: Placeholder for now, no defined values. + * @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 */ + 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)) + 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)) { 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)); + 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; + + 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); + + if (cpu_buffer->mapped) { + if (cpu_buffer->mapped == UINT_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/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); + 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); + + /* 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
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>