[PATCH v21 2/5] ring-buffer: Introducing ring-buffer mapping functions

[PATCH v21 2/5] ring-buffer: Introducing ring-buffer mapping functions

[Vincent Donnefort]

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..84f8744fa110 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 +6096,367 @@ 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;
+
+	if (vma->vm_flags & VM_WRITE || vma->vm_flags & VM_EXEC ||
+	    !(vma->vm_flags & VM_MAYSHARE))
+		return -EPERM;
+
+	vm_flags_mod(vma,
+		     VM_MIXEDMAP | VM_PFNMAP |
+		     VM_DONTCOPY | VM_DONTDUMP | VM_DONTEXPAND | VM_IO,
+		     VM_MAYWRITE);
+
+	lockdep_assert_held(&cpu_buffer->mapping_lock);
+
+	subbuf_order = cpu_buffer->buffer->subbuf_order;
+	subbuf_pages = 1 << subbuf_order;
+
+	nr_subbufs = cpu_buffer->nr_pages + 1; /* + reader-subbuf */
+	nr_pages = ((nr_subbufs) << subbuf_order) - pgoff + 1; /* + meta-page */
+
+	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) {
+		pages[p++] = virt_to_page(cpu_buffer->meta_page);
+
+		/*
+		 * TODO: Align sub-buffers on their size, once
+		 * vm_insert_pages() supports the zero-page.
+		 */
+	} else {
+		/* Skip the meta-page */
+		pgoff--;
+
+		if (pgoff % subbuf_pages) {
+			err = -EINVAL;
+			goto out;
+		}
+
+		s += pgoff / subbuf_pages;
+	}
+
+	while (s < nr_subbufs && p < nr_pages) {
+		struct page *page = virt_to_page(cpu_buffer->subbuf_ids[s]);
+		int off = 0;
+
+		for (; off < (1 << (subbuf_order)); off++, page++) {
+			if (p >= nr_pages)
+				break;
+
+			pages[p++] = page;
+		}
+		s++;
+	}
+
+	err = vm_insert_pages(vma, vma->vm_start, pages, &nr_pages);
+
+out:
+	kfree(pages);
+
+	return err;
+}
+#else
+static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
+			struct vm_area_struct *vma)
+{
+	return -EOPNOTSUPP;
+}
+#endif
+
+int ring_buffer_map(struct trace_buffer *buffer, int cpu,
+		    struct vm_area_struct *vma)
+{
+	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) {
+		err = __rb_map_vma(cpu_buffer, vma);
+		if (!err)
+			err = __rb_inc_dec_mapped(cpu_buffer, true);
+		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 = 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);
+	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+
+	err = __rb_map_vma(cpu_buffer, vma);
+	if (!err) {
+		raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+		cpu_buffer->mapped = 1;
+		raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+	} else {
+		kfree(cpu_buffer->subbuf_ids);
+		cpu_buffer->subbuf_ids = NULL;
+		rb_free_meta_page(cpu_buffer);
+	}
+
+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) {
+		__rb_inc_dec_mapped(cpu_buffer, false);
+		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);
+
+	kfree(cpu_buffer->subbuf_ids);
+	cpu_buffer->subbuf_ids = NULL;
+	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;
+}
+
+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:
+	/* Some archs do not have data cache coherency between kernel and user-space */
+	flush_dcache_folio(virt_to_folio(cpu_buffer->reader_page->page));
+
+	rb_update_meta_page(cpu_buffer);
+
+	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+	rb_put_mapped_buffer(cpu_buffer);
+
+	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
-- 
2.44.0.769.g3c40516874-goog

Re: [PATCH v21 2/5] ring-buffer: Introducing ring-buffer mapping functions

[David Hildenbrand]

I gave it some more thought, and I think we are still missing something 
(I wish PFNMAP/MIXEDMAP wouldn't be that hard).

> +
> +/*
> + *   +--------------+  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;
> +

I'd add some comments here like

/* Refuse any MAP_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 pages (VM_DONTCOPY | VM_DONTDUMP) and tell
  * GUP to leave them alone as well (VM_IO).
  */
> +	vm_flags_mod(vma,
> +		     VM_MIXEDMAP | VM_PFNMAP |
> +		     VM_DONTCOPY | VM_DONTDUMP | VM_DONTEXPAND | VM_IO,
> +		     VM_MAYWRITE);

I am still really unsure about VM_PFNMAP ... it's not a PFNMAP at all 
and, as stated, vm_insert_pages() even complains quite a lot when it 
would have to set VM_MIXEDMAP and VM_PFNMAP is already set, likely for a 
very good reason.

Can't we limit ourselves to VM_IO?

But then, I wonder if it really helps much regarding GUP: yes, it blocks 
ordinary GUP (see check_vma_flags()) but as 
insert_page_into_pte_locked() does *not* set pte_special(), GUP-fast 
(gup_fast_pte_range()) will not reject it.

Really, if you want GUP-fast to reject it, remap_pfn_range() and friends 
are the way to go, that will set pte_special() such that also GUP-fast 
will leave it alone, just like vm_normal_page() would.

So ... I know Linus recommended VM_PFNMAP/VM_IO to stop GUP, but it 
alone won't stop all of GUP. We really have to mark the PTE as special, 
which vm_insert_page() must not do (because it is refcounted!).

Which means: do we really have to stop GUP from grabbing that page?

Using vm_insert_page() only with VM_MIXEDMAP (and without 
VM_PFNMAP|VM_IO) would be better.

If we want to stop all of GUP, remap_pfn_range() currently seems 
unavoidable :(


> +
> +	lockdep_assert_held(&cpu_buffer->mapping_lock);
> +
> +	subbuf_order = cpu_buffer->buffer->subbuf_order;
> +	subbuf_pages = 1 << subbuf_order;
> +
> +	nr_subbufs = cpu_buffer->nr_pages + 1; /* + reader-subbuf */
> +	nr_pages = ((nr_subbufs) << subbuf_order) - pgoff + 1; /* + meta-page */
> +
> +	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) {
> +		pages[p++] = virt_to_page(cpu_buffer->meta_page);
> +
> +		/*
> +		 * TODO: Align sub-buffers on their size, once
> +		 * vm_insert_pages() supports the zero-page.
> +		 */
> +	} else {
> +		/* Skip the meta-page */
> +		pgoff--;
> +
> +		if (pgoff % subbuf_pages) {
> +			err = -EINVAL;
> +			goto out;
> +		}
> +
> +		s += pgoff / subbuf_pages;
> +	}
> +
> +	while (s < nr_subbufs && p < nr_pages) {
> +		struct page *page = virt_to_page(cpu_buffer->subbuf_ids[s]);
> +		int off = 0;
> +
> +		for (; off < (1 << (subbuf_order)); off++, page++) {
> +			if (p >= nr_pages)
> +				break;
> +
> +			pages[p++] = page;
> +		}
> +		s++;
> +	}
> +
> +	err = vm_insert_pages(vma, vma->vm_start, pages, &nr_pages);

vm_insert_pages() documents: "In case of error, we may have mapped a 
subset of the provided pages. It is the caller's responsibility to 
account for this case."

Which could for example happen, when allocating a page table fails.

Would we able to deal with that here?


Again, I wish it would all be easier ...

-- 
Cheers,

David / dhildenb

Re: [PATCH v21 2/5] ring-buffer: Introducing ring-buffer mapping functions

[Vincent Donnefort]

Hi David,

Thanks for your quick response.

On Wed, Apr 24, 2024 at 05:26:39PM +0200, David Hildenbrand wrote:
> 
> I gave it some more thought, and I think we are still missing something (I
> wish PFNMAP/MIXEDMAP wouldn't be that hard).
> 
> > +
> > +/*
> > + *   +--------------+  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;
> > +
> 
> I'd add some comments here like
> 
> /* Refuse any MAP_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 pages (VM_DONTCOPY | VM_DONTDUMP) and tell
>  * GUP to leave them alone as well (VM_IO).
>  */
> > +	vm_flags_mod(vma,
> > +		     VM_MIXEDMAP | VM_PFNMAP |
> > +		     VM_DONTCOPY | VM_DONTDUMP | VM_DONTEXPAND | VM_IO,
> > +		     VM_MAYWRITE);
> 
> I am still really unsure about VM_PFNMAP ... it's not a PFNMAP at all and,
> as stated, vm_insert_pages() even complains quite a lot when it would have
> to set VM_MIXEDMAP and VM_PFNMAP is already set, likely for a very good
> reason.
> 
> Can't we limit ourselves to VM_IO?
> 
> But then, I wonder if it really helps much regarding GUP: yes, it blocks
> ordinary GUP (see check_vma_flags()) but as insert_page_into_pte_locked()
> does *not* set pte_special(), GUP-fast (gup_fast_pte_range()) will not
> reject it.
> 
> Really, if you want GUP-fast to reject it, remap_pfn_range() and friends are
> the way to go, that will set pte_special() such that also GUP-fast will
> leave it alone, just like vm_normal_page() would.
> 
> So ... I know Linus recommended VM_PFNMAP/VM_IO to stop GUP, but it alone
> won't stop all of GUP. We really have to mark the PTE as special, which
> vm_insert_page() must not do (because it is refcounted!).

Hum, apologies, I am not sure to follow the connection here. Why do you think
the recommendation was to prevent GUP?

> 
> Which means: do we really have to stop GUP from grabbing that page?
> 
> Using vm_insert_page() only with VM_MIXEDMAP (and without VM_PFNMAP|VM_IO)
> would be better.

Under the assumption we do not want to stop all GUP, why not using VM_IO over
VM_MIXEDMAP which is I believe more restrictive?

> 
> If we want to stop all of GUP, remap_pfn_range() currently seems unavoidable
> :(
> 
> 
> > +
> > +	lockdep_assert_held(&cpu_buffer->mapping_lock);
> > +
> > +	subbuf_order = cpu_buffer->buffer->subbuf_order;
> > +	subbuf_pages = 1 << subbuf_order;
> > +
> > +	nr_subbufs = cpu_buffer->nr_pages + 1; /* + reader-subbuf */
> > +	nr_pages = ((nr_subbufs) << subbuf_order) - pgoff + 1; /* + meta-page */
> > +
> > +	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) {
> > +		pages[p++] = virt_to_page(cpu_buffer->meta_page);
> > +
> > +		/*
> > +		 * TODO: Align sub-buffers on their size, once
> > +		 * vm_insert_pages() supports the zero-page.
> > +		 */
> > +	} else {
> > +		/* Skip the meta-page */
> > +		pgoff--;
> > +
> > +		if (pgoff % subbuf_pages) {
> > +			err = -EINVAL;
> > +			goto out;
> > +		}
> > +
> > +		s += pgoff / subbuf_pages;
> > +	}
> > +
> > +	while (s < nr_subbufs && p < nr_pages) {
> > +		struct page *page = virt_to_page(cpu_buffer->subbuf_ids[s]);
> > +		int off = 0;
> > +
> > +		for (; off < (1 << (subbuf_order)); off++, page++) {
> > +			if (p >= nr_pages)
> > +				break;
> > +
> > +			pages[p++] = page;
> > +		}
> > +		s++;
> > +	}
> > +
> > +	err = vm_insert_pages(vma, vma->vm_start, pages, &nr_pages);
> 
> vm_insert_pages() documents: "In case of error, we may have mapped a subset
> of the provided pages. It is the caller's responsibility to account for this
> case."
> 
> Which could for example happen, when allocating a page table fails.
> 
> Would we able to deal with that here?

As we are in the mmap path, on an error, I would expect the vma to be destroyed
and those pages whom insertion succeeded to be unmapped?

But perhaps shall we proactively zap_page_range_single()?

> 
> 
> Again, I wish it would all be easier ...
> 
> -- 
> Cheers,
> 
> David / dhildenb
> 
> To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@android.com.
> 

Re: [PATCH v21 2/5] ring-buffer: Introducing ring-buffer mapping functions

[David Hildenbrand]

On 24.04.24 22:31, Vincent Donnefort wrote:
> Hi David,
> 
> Thanks for your quick response.
> 
> On Wed, Apr 24, 2024 at 05:26:39PM +0200, David Hildenbrand wrote:
>>
>> I gave it some more thought, and I think we are still missing something (I
>> wish PFNMAP/MIXEDMAP wouldn't be that hard).
>>
>>> +
>>> +/*
>>> + *   +--------------+  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;
>>> +
>>
>> I'd add some comments here like
>>
>> /* Refuse any MAP_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 pages (VM_DONTCOPY | VM_DONTDUMP) and tell
>>   * GUP to leave them alone as well (VM_IO).
>>   */
>>> +	vm_flags_mod(vma,
>>> +		     VM_MIXEDMAP | VM_PFNMAP |
>>> +		     VM_DONTCOPY | VM_DONTDUMP | VM_DONTEXPAND | VM_IO,
>>> +		     VM_MAYWRITE);
>>
>> I am still really unsure about VM_PFNMAP ... it's not a PFNMAP at all and,
>> as stated, vm_insert_pages() even complains quite a lot when it would have
>> to set VM_MIXEDMAP and VM_PFNMAP is already set, likely for a very good
>> reason.
>>
>> Can't we limit ourselves to VM_IO?
>>
>> But then, I wonder if it really helps much regarding GUP: yes, it blocks
>> ordinary GUP (see check_vma_flags()) but as insert_page_into_pte_locked()
>> does *not* set pte_special(), GUP-fast (gup_fast_pte_range()) will not
>> reject it.
>>
>> Really, if you want GUP-fast to reject it, remap_pfn_range() and friends are
>> the way to go, that will set pte_special() such that also GUP-fast will
>> leave it alone, just like vm_normal_page() would.
>>
>> So ... I know Linus recommended VM_PFNMAP/VM_IO to stop GUP, but it alone
>> won't stop all of GUP. We really have to mark the PTE as special, which
>> vm_insert_page() must not do (because it is refcounted!).
> 
> Hum, apologies, I am not sure to follow the connection here. Why do you think
> the recommendation was to prevent GUP?

Ah, I'm hallucinating! :) "not let people play games with the mapping" to me
implied "make sure nobody touches it". If GUP is acceptable that makes stuff
a lot easier. VM_IO will block some GUP, but not all of it.

> 
>>
>> Which means: do we really have to stop GUP from grabbing that page?
>>
>> Using vm_insert_page() only with VM_MIXEDMAP (and without VM_PFNMAP|VM_IO)
>> would be better.
> 
> Under the assumption we do not want to stop all GUP, why not using VM_IO over
> VM_MIXEDMAP which is I believe more restrictive?

VM_MIXEDMAP will be implicitly set by vm_insert_page(). There is a lengthy comment
for vm_normal_page() that explains all this madness. VM_MIXEDMAP is primarily
relevant for COW mappings, which you just forbid completely.

remap_pfn_range_notrack() documents the semantics of some of the other flags:

	 *   VM_IO tells people not to look at these pages
	 *	(accesses can have side effects).
	 *   VM_PFNMAP tells the core MM that the base pages are just
	 *	raw PFN mappings, and do not have a "struct page" associated
	 *	with them.
	 *   VM_DONTEXPAND
	 *      Disable vma merging and expanding with mremap().
	 *   VM_DONTDUMP
	 *      Omit vma from core dump, even when VM_IO turned off.

VM_PFNMAP is very likely really not what we want, unless we really perform raw
PFN mappings ... VM_IO we can set without doing much harm.

So I would suggest dropping VM_PFNMAP when using vm_insert_pages(), using only VM_IO
and likely just letting vm_insert_pages() set VM_MIXEDMAP for you.

[...]

>>
>> vm_insert_pages() documents: "In case of error, we may have mapped a subset
>> of the provided pages. It is the caller's responsibility to account for this
>> case."
>>
>> Which could for example happen, when allocating a page table fails.
>>
>> Would we able to deal with that here?
> 
> As we are in the mmap path, on an error, I would expect the vma to be destroyed
> and those pages whom insertion succeeded to be unmapped?
> 

Ah, we simply fail ->mmap().

In mmap_region(), if call_mmap() failed, we "goto unmap_and_free_vma" where we have

/* Undo any partial mapping done by a device driver. */
unmap_region(mm, &vmi.mas, vma, prev, next, vma->vm_start, vma->vm_end, vma->vm_end, true);


> But perhaps shall we proactively zap_page_range_single()?

No mmap_region() should indeed be handling it correctly already!

-- 
Cheers,

David / dhildenb

Re: [PATCH v21 2/5] ring-buffer: Introducing ring-buffer mapping functions

[Vincent Donnefort]

On Wed, Apr 24, 2024 at 10:55:54PM +0200, David Hildenbrand wrote:
> On 24.04.24 22:31, Vincent Donnefort wrote:
> > Hi David,
> > 
> > Thanks for your quick response.
> > 
> > On Wed, Apr 24, 2024 at 05:26:39PM +0200, David Hildenbrand wrote:
> > > 
> > > I gave it some more thought, and I think we are still missing something (I
> > > wish PFNMAP/MIXEDMAP wouldn't be that hard).
> > > 
> > > > +
> > > > +/*
> > > > + *   +--------------+  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;
> > > > +
> > > 
> > > I'd add some comments here like
> > > 
> > > /* Refuse any MAP_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 pages (VM_DONTCOPY | VM_DONTDUMP) and tell
> > >   * GUP to leave them alone as well (VM_IO).
> > >   */
> > > > +	vm_flags_mod(vma,
> > > > +		     VM_MIXEDMAP | VM_PFNMAP |
> > > > +		     VM_DONTCOPY | VM_DONTDUMP | VM_DONTEXPAND | VM_IO,
> > > > +		     VM_MAYWRITE);
> > > 
> > > I am still really unsure about VM_PFNMAP ... it's not a PFNMAP at all and,
> > > as stated, vm_insert_pages() even complains quite a lot when it would have
> > > to set VM_MIXEDMAP and VM_PFNMAP is already set, likely for a very good
> > > reason.
> > > 
> > > Can't we limit ourselves to VM_IO?
> > > 
> > > But then, I wonder if it really helps much regarding GUP: yes, it blocks
> > > ordinary GUP (see check_vma_flags()) but as insert_page_into_pte_locked()
> > > does *not* set pte_special(), GUP-fast (gup_fast_pte_range()) will not
> > > reject it.
> > > 
> > > Really, if you want GUP-fast to reject it, remap_pfn_range() and friends are
> > > the way to go, that will set pte_special() such that also GUP-fast will
> > > leave it alone, just like vm_normal_page() would.
> > > 
> > > So ... I know Linus recommended VM_PFNMAP/VM_IO to stop GUP, but it alone
> > > won't stop all of GUP. We really have to mark the PTE as special, which
> > > vm_insert_page() must not do (because it is refcounted!).
> > 
> > Hum, apologies, I am not sure to follow the connection here. Why do you think
> > the recommendation was to prevent GUP?
> 
> Ah, I'm hallucinating! :) "not let people play games with the mapping" to me
> implied "make sure nobody touches it". If GUP is acceptable that makes stuff
> a lot easier. VM_IO will block some GUP, but not all of it.
> 
> > 
> > > 
> > > Which means: do we really have to stop GUP from grabbing that page?
> > > 
> > > Using vm_insert_page() only with VM_MIXEDMAP (and without VM_PFNMAP|VM_IO)
> > > would be better.
> > 
> > Under the assumption we do not want to stop all GUP, why not using VM_IO over
> > VM_MIXEDMAP which is I believe more restrictive?
> 
> VM_MIXEDMAP will be implicitly set by vm_insert_page(). There is a lengthy comment
> for vm_normal_page() that explains all this madness. VM_MIXEDMAP is primarily
> relevant for COW mappings, which you just forbid completely.
> 
> remap_pfn_range_notrack() documents the semantics of some of the other flags:
> 
> 	 *   VM_IO tells people not to look at these pages
> 	 *	(accesses can have side effects).
> 	 *   VM_PFNMAP tells the core MM that the base pages are just
> 	 *	raw PFN mappings, and do not have a "struct page" associated
> 	 *	with them.
> 	 *   VM_DONTEXPAND
> 	 *      Disable vma merging and expanding with mremap().
> 	 *   VM_DONTDUMP
> 	 *      Omit vma from core dump, even when VM_IO turned off.
> 
> VM_PFNMAP is very likely really not what we want, unless we really perform raw
> PFN mappings ... VM_IO we can set without doing much harm.
> 
> So I would suggest dropping VM_PFNMAP when using vm_insert_pages(), using only VM_IO
> and likely just letting vm_insert_pages() set VM_MIXEDMAP for you.

Sounds good, I will do that in v22.

> 
> [...]
> 
> > > 
> > > vm_insert_pages() documents: "In case of error, we may have mapped a subset
> > > of the provided pages. It is the caller's responsibility to account for this
> > > case."
> > > 
> > > Which could for example happen, when allocating a page table fails.
> > > 
> > > Would we able to deal with that here?
> > 
> > As we are in the mmap path, on an error, I would expect the vma to be destroyed
> > and those pages whom insertion succeeded to be unmapped?
> > 
> 
> Ah, we simply fail ->mmap().
> 
> In mmap_region(), if call_mmap() failed, we "goto unmap_and_free_vma" where we have
> 
> /* Undo any partial mapping done by a device driver. */
> unmap_region(mm, &vmi.mas, vma, prev, next, vma->vm_start, vma->vm_end, vma->vm_end, true);
> 
> 
> > But perhaps shall we proactively zap_page_range_single()?
> 
> No mmap_region() should indeed be handling it correctly already!

Ok, thanks for confirming!

> 
> -- 
> Cheers,
> 
> David / dhildenb
> 

[PATCH v21 3/5] tracing: Allow user-space mapping of the ring-buffer

[Vincent Donnefort]

Currently, user-space extracts data from the ring-buffer via splice,
which is handy for storage or network sharing. However, due to splice
limitations, it is imposible to do real-time analysis without a copy.

A solution for that problem is to let the user-space map the ring-buffer
directly.

The mapping is exposed via the per-CPU file trace_pipe_raw. The first
element of the mapping is the meta-page. It is followed by each
subbuffer constituting the ring-buffer, ordered by their unique page ID:

  * Meta-page -- include/uapi/linux/trace_mmap.h for a description
  * Subbuf ID 0
  * Subbuf ID 1
     ...

It is therefore easy to translate a subbuf ID into an offset in the
mapping:

  reader_id = meta->reader->id;
  reader_offset = meta->meta_page_size + reader_id * meta->subbuf_size;

When new data is available, the mapper must call a newly introduced ioctl:
TRACE_MMAP_IOCTL_GET_READER. This will update the Meta-page reader ID to
point to the next reader containing unread data.

Mapping will prevent snapshot and buffer size modifications.

CC: <linux-mm@kvack.org>
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

diff --git a/include/uapi/linux/trace_mmap.h b/include/uapi/linux/trace_mmap.h
index b682e9925539..bd1066754220 100644
--- a/include/uapi/linux/trace_mmap.h
+++ b/include/uapi/linux/trace_mmap.h
@@ -43,4 +43,6 @@ struct trace_buffer_meta {
 	__u64	Reserved2;
 };
 
+#define TRACE_MMAP_IOCTL_GET_READER		_IO('T', 0x1)
+
 #endif /* _TRACE_MMAP_H_ */
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 233d1af39fff..a35e7f598233 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -1191,6 +1191,12 @@ static void tracing_snapshot_instance_cond(struct trace_array *tr,
 		return;
 	}
 
+	if (tr->mapped) {
+		trace_array_puts(tr, "*** BUFFER MEMORY MAPPED ***\n");
+		trace_array_puts(tr, "*** Can not use snapshot (sorry) ***\n");
+		return;
+	}
+
 	local_irq_save(flags);
 	update_max_tr(tr, current, smp_processor_id(), cond_data);
 	local_irq_restore(flags);
@@ -1323,7 +1329,7 @@ static int tracing_arm_snapshot_locked(struct trace_array *tr)
 	lockdep_assert_held(&trace_types_lock);
 
 	spin_lock(&tr->snapshot_trigger_lock);
-	if (tr->snapshot == UINT_MAX) {
+	if (tr->snapshot == UINT_MAX || tr->mapped) {
 		spin_unlock(&tr->snapshot_trigger_lock);
 		return -EBUSY;
 	}
@@ -6068,7 +6074,7 @@ static void tracing_set_nop(struct trace_array *tr)
 {
 	if (tr->current_trace == &nop_trace)
 		return;
-	
+
 	tr->current_trace->enabled--;
 
 	if (tr->current_trace->reset)
@@ -8194,15 +8200,32 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
 	return ret;
 }
 
-/* An ioctl call with cmd 0 to the ring buffer file will wake up all waiters */
 static long tracing_buffers_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 {
 	struct ftrace_buffer_info *info = file->private_data;
 	struct trace_iterator *iter = &info->iter;
+	int err;
+
+	if (cmd == TRACE_MMAP_IOCTL_GET_READER) {
+		if (!(file->f_flags & O_NONBLOCK)) {
+			err = ring_buffer_wait(iter->array_buffer->buffer,
+					       iter->cpu_file,
+					       iter->tr->buffer_percent,
+					       NULL, NULL);
+			if (err)
+				return err;
+		}
 
-	if (cmd)
-		return -ENOIOCTLCMD;
+		return ring_buffer_map_get_reader(iter->array_buffer->buffer,
+						  iter->cpu_file);
+	} else if (cmd) {
+		return -ENOTTY;
+	}
 
+	/*
+	 * An ioctl call with cmd 0 to the ring buffer file will wake up all
+	 * waiters
+	 */
 	mutex_lock(&trace_types_lock);
 
 	/* Make sure the waiters see the new wait_index */
@@ -8214,6 +8237,76 @@ static long tracing_buffers_ioctl(struct file *file, unsigned int cmd, unsigned
 	return 0;
 }
 
+#ifdef CONFIG_TRACER_MAX_TRACE
+static int get_snapshot_map(struct trace_array *tr)
+{
+	int err = 0;
+
+	/*
+	 * Called with mmap_lock held. lockdep would be unhappy if we would now
+	 * take trace_types_lock. Instead use the specific
+	 * snapshot_trigger_lock.
+	 */
+	spin_lock(&tr->snapshot_trigger_lock);
+
+	if (tr->snapshot || tr->mapped == UINT_MAX)
+		err = -EBUSY;
+	else
+		tr->mapped++;
+
+	spin_unlock(&tr->snapshot_trigger_lock);
+
+	/* Wait for update_max_tr() to observe iter->tr->mapped */
+	if (tr->mapped == 1)
+		synchronize_rcu();
+
+	return err;
+
+}
+static void put_snapshot_map(struct trace_array *tr)
+{
+	spin_lock(&tr->snapshot_trigger_lock);
+	if (!WARN_ON(!tr->mapped))
+		tr->mapped--;
+	spin_unlock(&tr->snapshot_trigger_lock);
+}
+#else
+static inline int get_snapshot_map(struct trace_array *tr) { return 0; }
+static inline void put_snapshot_map(struct trace_array *tr) { }
+#endif
+
+static void tracing_buffers_mmap_close(struct vm_area_struct *vma)
+{
+	struct ftrace_buffer_info *info = vma->vm_file->private_data;
+	struct trace_iterator *iter = &info->iter;
+
+	WARN_ON(ring_buffer_unmap(iter->array_buffer->buffer, iter->cpu_file));
+	put_snapshot_map(iter->tr);
+}
+
+static const struct vm_operations_struct tracing_buffers_vmops = {
+	.close		= tracing_buffers_mmap_close,
+};
+
+static int tracing_buffers_mmap(struct file *filp, struct vm_area_struct *vma)
+{
+	struct ftrace_buffer_info *info = filp->private_data;
+	struct trace_iterator *iter = &info->iter;
+	int ret = 0;
+
+	ret = get_snapshot_map(iter->tr);
+	if (ret)
+		return ret;
+
+	ret = ring_buffer_map(iter->array_buffer->buffer, iter->cpu_file, vma);
+	if (ret)
+		put_snapshot_map(iter->tr);
+
+	vma->vm_ops = &tracing_buffers_vmops;
+
+	return ret;
+}
+
 static const struct file_operations tracing_buffers_fops = {
 	.open		= tracing_buffers_open,
 	.read		= tracing_buffers_read,
@@ -8223,6 +8316,7 @@ static const struct file_operations tracing_buffers_fops = {
 	.splice_read	= tracing_buffers_splice_read,
 	.unlocked_ioctl = tracing_buffers_ioctl,
 	.llseek		= no_llseek,
+	.mmap		= tracing_buffers_mmap,
 };
 
 static ssize_t
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 64450615ca0c..749a182dab48 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -336,6 +336,7 @@ struct trace_array {
 	bool			allocated_snapshot;
 	spinlock_t		snapshot_trigger_lock;
 	unsigned int		snapshot;
+	unsigned int		mapped;
 	unsigned long		max_latency;
 #ifdef CONFIG_FSNOTIFY
 	struct dentry		*d_max_latency;
-- 
2.44.0.769.g3c40516874-goog