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Message-ID: <1e98eecf-617e-49d5-a1c1-43895f050d9b@redhat.com>
Date: Fri, 10 May 2024 16:06:08 +0200
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Subject: Re: [PATCH v23 2/5] ring-buffer: Introducing ring-buffer mapping
 functions
To: Vincent Donnefort <vdonnefort@google.com>, rostedt@goodmis.org,
 mhiramat@kernel.org, linux-kernel@vger.kernel.org,
 linux-trace-kernel@vger.kernel.org
Cc: mathieu.desnoyers@efficios.com, kernel-team@android.com,
 rdunlap@infradead.org, rppt@kernel.org, linux-mm@kvack.org
References: <20240510140435.3550353-1-vdonnefort@google.com>
 <20240510140435.3550353-3-vdonnefort@google.com>
From: David Hildenbrand <david@redhat.com>
Content-Language: en-US
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Organization: Red Hat
In-Reply-To: <20240510140435.3550353-3-vdonnefort@google.com>
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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