Date: Mon, 22 Apr 2024 11:27:11 +0200
From: David Hildenbrand <david@...hat.com>
To: Vincent Donnefort <vdonnefort@...gle.com>, rostedt@...dmis.org,
 mhiramat@...nel.org, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org
Cc: mathieu.desnoyers@...icios.com, kernel-team@...roid.com,
 rdunlap@...radead.org, linux-mm@...ck.org
Subject: Re: [PATCH v20 2/5] ring-buffer: Introducing ring-buffer mapping
 functions

On 19.04.24 20:25, David Hildenbrand wrote:
> On 06.04.24 19:36, 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@...ck.org>
>> Signed-off-by: Vincent Donnefort <vdonnefort@...gle.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..ffcd8dfcaa4f
>> --- /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:		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 cc9ebe593571..793ecc454039 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 +6096,358 @@ 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;
>> +
>> +	/* __rb_map_vma() pads the meta-page to align it with the sub-buffers */
>> +	meta->meta_page_size = PAGE_SIZE << cpu_buffer->buffer->subbuf_order;
>> +	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;
>> +}
>> +
>> +#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)))
>> +
>> +/*
>> + *   +--------------+  pgoff == 0
>> + *   |   meta page  |
>> + *   +--------------+  pgoff == 1
>> + *   |   000000000  |
>> + *   +--------------+  pgoff == (1 << subbuf_order)
>> + *   | subbuffer 0  |
>> + *   |              |
>> + *   +--------------+  pgoff == (2 * (1 << subbuf_order))
>> + *   | subbuffer 1  |
>> + *   |              |
>> + *         ...
>> + */
>> +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;
>> +
>> +	lockdep_assert_held(&cpu_buffer->mapping_lock);
>> +
>> +	subbuf_order = cpu_buffer->buffer->subbuf_order;
>> +	subbuf_pages = 1 << subbuf_order;
>> +
>> +	if (subbuf_order && pgoff % subbuf_pages)
>> +		return -EINVAL;
>> +
>> +	nr_subbufs = cpu_buffer->nr_pages + 1;
>> +	nr_pages = ((nr_subbufs + 1) << subbuf_order) - pgoff;
>> +
>> +	vma_pages = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
>> +	if (!vma_pages || vma_pages > nr_pages)
>> +		return -EINVAL;
>> +
>> +	nr_pages = vma_pages;
>> +
>> +	pages = kcalloc(nr_pages, sizeof(*pages), GFP_KERNEL);
>> +	if (!pages)
>> +		return -ENOMEM;
>> +
>> +	if (!pgoff) {
>> +		unsigned long meta_page_padding;
>> +
>> +		pages[p++] = virt_to_page(cpu_buffer->meta_page);
>> +
>> +		/*
>> +		 * Pad with the zero-page to align the meta-page with the
>> +		 * sub-buffers.
>> +		 */
>> +		meta_page_padding = subbuf_pages - 1;
>> +		while (meta_page_padding-- && p < nr_pages)
>> +			pages[p++] = ZERO_PAGE(0);
> 
> Using the shared zeropage in a MAP_SHARED mapping that is neither VM_IO
> nor VM_PFNMAP can be problematic. So we really need patch #3 logic to
> use VM_PFNMAP.
> 
> It would be cleaner/more obvious if these VMA flag setting would reside
> here, if possible.
> 
>> +	} else {
>> +		/* Skip the meta-page */
>> +		pgoff -= subbuf_pages;
>> +
>> +		s += pgoff / subbuf_pages;
>> +	}
>> +
>> +	while (s < nr_subbufs && p < nr_pages) {
>> +		struct page *page;
>> +
>> +		foreach_subbuf_page(subbuf_order, cpu_buffer->subbuf_ids[s], page) {
>> +			if (p >= nr_pages)
>> +				break;
>> +
>> +			pages[p++] = page;
>> +		}
>> +		s++;
>> +	}
>> +
>> +	err = vm_insert_pages(vma, vma->vm_start, pages, &nr_pages);
> 
> I think Linus suggested it ("avoid all the sub-page ref-counts entirely
> by using VM_PFNMAP, and use vm_insert_pages()"), but ...
> vm_insert_pages() will:
> * Mess with mapcounts
> * Mess with refcounts
> 
> See
> insert_pages()->insert_page_in_batch_locked()->insert_page_into_pte_locked().
> 
> So we'll mess with the mapcount and refcount of the shared zeropage ...
> hmmmm
> 
> If I am not wrong, vm_normal_page() would not return the shared zeropage
> in case we don't have CONFIG_ARCH_HAS_PTE_SPECIAL ... so
> unmap()->...->zap_present_ptes() would not decrement the refcount and we
> could overflow it. ... we also shouldn't ever mess with the mapcount of
> the shared zeropage in the first place.
> 
> vm_insert_page() is clearer on that: "This allows drivers to insert
> individual pages they've allocated into a user vma". You didn't allocate
> the shared zeropage.
> 
> I'm wondering if we even expect VM_MIXEDMAP and VM_PFNMAP to be set at
> the same time? vm_insert_pages() would BUG_ON in case VM_PFNMAP is
> already set and it would set VM_MIXEDMAP ... similarly
> vmf_insert_pfn_prot() would not be happy about that at all ...
> 
> BUG_ON((vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP)) ==
> (VM_PFNMAP|VM_MIXEDMAP));
> 
> ... remap_pfn_range() is used by io_uring_mmap for a similar purpose.
> But it only supports a single PFN range at a time and requires the
> caller to handle refcounting of pages.
> 
> It's getting late in Germany, so I might be missing something; but using
> the shared zeropage here might be a problem.
> 

I was just about to write code to cleanly support vm_insert_pages() to 
consume zeropages ... when I started to wonder about something else:


+	if (vma->vm_flags & VM_WRITE || vma->vm_flags & VM_EXEC ||
+	    !(vma->vm_flags & VM_MAYSHARE))
+		return -EPERM;
+

You disallow writable mappings. But what about using mprotect() 
afterwards to allow for write permissions?

Likely you'd want to remove VM_MAYWRITE from the flags, otherwise 
mprotect() might alter succeed.

-- 
Cheers,

David / dhildenb

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