[PATCH v22 2/5] ring-buffer: Introducing ring-buffer mapping functions

[PATCH v22 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..fc66d01ff472 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 +6096,370 @@ 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). Finally,
+	 * prevent migration, GUP and dump (VM_IO).
+	 */
+	vm_flags_mod(vma, VM_DONTCOPY | 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 v22 2/5] ring-buffer: Introducing ring-buffer mapping functions

[David Hildenbrand]

On 30.04.24 13:13, 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

[...]

> +/*
> + *   +--------------+  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). Finally,
> +	 * prevent migration, GUP and dump (VM_IO).
> +	 */
> +	vm_flags_mod(vma, VM_DONTCOPY | 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);

Nit: I did not immediately understand if we could end here with p < 
nr_pages (IOW, pages[] not completely filled).

One source of confusion is the "s < nr_subbufs" check in the while loop: 
why is "p < nr_pages" insufficient?


For the MM bits:

Acked-by: David Hildenbrand <david@redhat.com>



-- 
Cheers,

David / dhildenb

Re: [PATCH v22 2/5] ring-buffer: Introducing ring-buffer mapping functions

[Vincent Donnefort]

On Thu, May 02, 2024 at 03:30:32PM +0200, David Hildenbrand wrote:
> On 30.04.24 13:13, 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
> 
> [...]
> 
> > +/*
> > + *   +--------------+  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). Finally,
> > +	 * prevent migration, GUP and dump (VM_IO).
> > +	 */
> > +	vm_flags_mod(vma, VM_DONTCOPY | 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);
> 
> Nit: I did not immediately understand if we could end here with p < nr_pages
> (IOW, pages[] not completely filled).
> 
> One source of confusion is the "s < nr_subbufs" check in the while loop: why
> is "p < nr_pages" insufficient?

Hum, indeed, the "s < nr_subbufs" check is superfluous, nr_pages, is already
capped by the number of subbufs, there's no way we can overflow subbuf_ids[].

> 
> 
> For the MM bits:
> 
> Acked-by: David Hildenbrand <david@redhat.com>

Thanks a lot for having a look at the series, very much appreciated!

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

Re: [PATCH v22 2/5] ring-buffer: Introducing ring-buffer mapping functions

[Steven Rostedt]

On Thu, 2 May 2024 14:38:32 +0100
Vincent Donnefort <vdonnefort@google.com> wrote:

> > > +	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);  
> > 
> > Nit: I did not immediately understand if we could end here with p < nr_pages
> > (IOW, pages[] not completely filled).
> > 
> > One source of confusion is the "s < nr_subbufs" check in the while loop: why
> > is "p < nr_pages" insufficient?  
> 
> Hum, indeed, the "s < nr_subbufs" check is superfluous, nr_pages, is already
> capped by the number of subbufs, there's no way we can overflow subbuf_ids[].

We can keep it as is, or perhaps change it to:

	while (p < nr_pages) {
		struct page *page;
		int off = 0;

		if (WARN_ON_ONCE(s >= nr_subbufs))
			break;

		page = virt_to_page(cpu_buffer->subbuf_ids[s]);
		for (; off < (1 << (subbuf_order)); off++, page++) {
			if (p >= nr_pages)
				break;

			pages[p++] = page;
		}
		s++;
	}

I don't like having an unchecked dependency between s and p.

-- Steve

Re: [PATCH v22 2/5] ring-buffer: Introducing ring-buffer mapping functions

[Steven Rostedt]

On Tue, 30 Apr 2024 12:13:51 +0100
Vincent Donnefort <vdonnefort@google.com> wrote:

> +#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). Finally,
> +	 * prevent migration, GUP and dump (VM_IO).
> +	 */
> +	vm_flags_mod(vma, VM_DONTCOPY | VM_DONTEXPAND | VM_IO, VM_MAYWRITE);

Do we really need the VM_IO?

When testing this in gdb, I would get:

(gdb) p tmap->map->subbuf_size
Cannot access memory at address 0x7ffff7fc2008

It appears that you can't ptrace IO memory. When I removed that flag,
gdb has no problem reading that memory.

I think we should drop that flag.

Can you send a v23 with that removed, Shuah's update, and also the
change below:

> +
> +	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++;
> +	}

The above can be made to:

	while (p < nr_pages) {
		struct page *page;
		int off = 0;

		if (WARN_ON_ONCE(s >= nr_subbufs))
			break;

		page = virt_to_page(cpu_buffer->subbuf_ids[s]);
		for (; off < (1 << (subbuf_order)); off++, page++) {
			if (p >= nr_pages)
				break;

			pages[p++] = page;
		}
		s++;
	}

Thanks.

-- Steve

> +
> +	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

Re: [PATCH v22 2/5] ring-buffer: Introducing ring-buffer mapping functions

[Vincent Donnefort]

On Tue, May 07, 2024 at 10:34:02PM -0400, Steven Rostedt wrote:
> On Tue, 30 Apr 2024 12:13:51 +0100
> Vincent Donnefort <vdonnefort@google.com> wrote:
> 
> > +#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). Finally,
> > +	 * prevent migration, GUP and dump (VM_IO).
> > +	 */
> > +	vm_flags_mod(vma, VM_DONTCOPY | VM_DONTEXPAND | VM_IO, VM_MAYWRITE);
> 
> Do we really need the VM_IO?
> 
> When testing this in gdb, I would get:
> 
> (gdb) p tmap->map->subbuf_size
> Cannot access memory at address 0x7ffff7fc2008
> 
> It appears that you can't ptrace IO memory. When I removed that flag,
> gdb has no problem reading that memory.

Yeah, VM_IO indeed implies DONTDUMP. VM_IO was part of Linus recommendations.
But perhaps, VM_DONTEXPAND and MIXEDMAP (implicitely set by vm_insert_pages) are
enough protection?

I don't see how anything could use GUP there and as David pointed-out on the
previous version, it doesn't event prevent the GUP-fast path.

> 
> I think we should drop that flag.
> 
> Can you send a v23 with that removed, Shuah's update, and also the
> change below:

Ack.

[...]

Re: [PATCH v22 2/5] ring-buffer: Introducing ring-buffer mapping functions

[David Hildenbrand]

On 09.05.24 13:05, Vincent Donnefort wrote:
> On Tue, May 07, 2024 at 10:34:02PM -0400, Steven Rostedt wrote:
>> On Tue, 30 Apr 2024 12:13:51 +0100
>> Vincent Donnefort <vdonnefort@google.com> wrote:
>>
>>> +#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). Finally,
>>> +	 * prevent migration, GUP and dump (VM_IO).
>>> +	 */
>>> +	vm_flags_mod(vma, VM_DONTCOPY | VM_DONTEXPAND | VM_IO, VM_MAYWRITE);
>>
>> Do we really need the VM_IO?
>>
>> When testing this in gdb, I would get:
>>
>> (gdb) p tmap->map->subbuf_size
>> Cannot access memory at address 0x7ffff7fc2008
>>
>> It appears that you can't ptrace IO memory. When I removed that flag,
>> gdb has no problem reading that memory.
> 
> Yeah, VM_IO indeed implies DONTDUMP. VM_IO was part of Linus recommendations.

Yes, the VM should recognize that memory to some degree as being special 
already due to VM_MIXEDMAP and VM_DONTEXPAND.

#define VM_SPECIAL (VM_IO | VM_DONTEXPAND | VM_PFNMAP | VM_MIXEDMAP)

So any of these flag achieve that (e.g., mlock_fixup() checks 
VM_SPECIAL). KSM similarly skips VM_DONTEXPAND and VM_MIXEDMAP (likely 
we should be using VM_SPECIAL in vma_ksm_compatible()). Not sure about 
page migration, likely its fine.

Thinking about MADV_DONTNEED, I can spot in 
madvise_dontneed_free_valid_vma() only that we disallow primarily VM_PFNMAP.

... I assume if user space MADV_DONTNEED's some pages we'll simply get a 
page fault later on access that will SIGBUS, handling that gracefully 
(we should double-check!).


> But perhaps, VM_DONTEXPAND and MIXEDMAP (implicitely set by vm_insert_pages) are
> enough protection?

Do we want to dump these pages? VM_DONTDUMP might be reasonabe then.

> 
> I don't see how anything could use GUP there and as David pointed-out on the
> previous version, it doesn't event prevent the GUP-fast path.

Yes, GUP-fast would still have worked under some conditions.

-- 
Cheers,

David / dhildenb

Re: [PATCH v22 2/5] ring-buffer: Introducing ring-buffer mapping functions

[Vincent Donnefort]

On Fri, May 10, 2024 at 11:15:59AM +0200, David Hildenbrand wrote:
> On 09.05.24 13:05, Vincent Donnefort wrote:
> > On Tue, May 07, 2024 at 10:34:02PM -0400, Steven Rostedt wrote:
> > > On Tue, 30 Apr 2024 12:13:51 +0100
> > > Vincent Donnefort <vdonnefort@google.com> wrote:
> > > 
> > > > +#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). Finally,
> > > > +	 * prevent migration, GUP and dump (VM_IO).
> > > > +	 */
> > > > +	vm_flags_mod(vma, VM_DONTCOPY | VM_DONTEXPAND | VM_IO, VM_MAYWRITE);
> > > 
> > > Do we really need the VM_IO?
> > > 
> > > When testing this in gdb, I would get:
> > > 
> > > (gdb) p tmap->map->subbuf_size
> > > Cannot access memory at address 0x7ffff7fc2008
> > > 
> > > It appears that you can't ptrace IO memory. When I removed that flag,
> > > gdb has no problem reading that memory.
> > 
> > Yeah, VM_IO indeed implies DONTDUMP. VM_IO was part of Linus recommendations.
> 
> Yes, the VM should recognize that memory to some degree as being special
> already due to VM_MIXEDMAP and VM_DONTEXPAND.
> 
> #define VM_SPECIAL (VM_IO | VM_DONTEXPAND | VM_PFNMAP | VM_MIXEDMAP)
> 
> So any of these flag achieve that (e.g., mlock_fixup() checks VM_SPECIAL).
> KSM similarly skips VM_DONTEXPAND and VM_MIXEDMAP (likely we should be using
> VM_SPECIAL in vma_ksm_compatible()). Not sure about page migration, likely
> its fine.
> 
> Thinking about MADV_DONTNEED, I can spot in
> madvise_dontneed_free_valid_vma() only that we disallow primarily VM_PFNMAP.
> 
> ... I assume if user space MADV_DONTNEED's some pages we'll simply get a
> page fault later on access that will SIGBUS, handling that gracefully (we
> should double-check!).

I've just tested and indeed, I get a SIGBUS! All good there.

> 
> 
> > But perhaps, VM_DONTEXPAND and MIXEDMAP (implicitely set by vm_insert_pages) are
> > enough protection?
> 
> Do we want to dump these pages? VM_DONTDUMP might be reasonabe then. 

Somehow I thought this would prevent ptrace as well, but I've just tested it and
this is not the case as well. So let's keep DONTDUMP.

Thanks!

> 
> > 
> > I don't see how anything could use GUP there and as David pointed-out on the
> > previous version, it doesn't event prevent the GUP-fast path.
> 
> Yes, GUP-fast would still have worked under some conditions.
> 
> -- 
> Cheers,
> 
> David / dhildenb
> 

Re: [PATCH v22 2/5] ring-buffer: Introducing ring-buffer mapping functions

[David Hildenbrand]

On 08.05.24 04:34, Steven Rostedt wrote:
> On Tue, 30 Apr 2024 12:13:51 +0100
> Vincent Donnefort <vdonnefort@google.com> wrote:
> 
>> +#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). Finally,
>> +	 * prevent migration, GUP and dump (VM_IO).
>> +	 */
>> +	vm_flags_mod(vma, VM_DONTCOPY | VM_DONTEXPAND | VM_IO, VM_MAYWRITE);
> 
> Do we really need the VM_IO?
> 
> When testing this in gdb, I would get:
> 
> (gdb) p tmap->map->subbuf_size
> Cannot access memory at address 0x7ffff7fc2008
> 
> It appears that you can't ptrace IO memory. When I removed that flag,
> gdb has no problem reading that memory.
> 
> I think we should drop that flag.
> 
> Can you send a v23 with that removed, Shuah's update, and also the
> change below:
> 
>> +
>> +	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++;
>> +	}
> 
> The above can be made to:
> 
> 	while (p < nr_pages) {
> 		struct page *page;
> 		int off = 0;
> 
> 		if (WARN_ON_ONCE(s >= nr_subbufs))
> 			break;

I'm not particularly happy about us calling vm_insert_pages with NULL 
pointers stored in pages.

Should we instead do

if (WARN_ON_ONCE(s >= nr_subbufs)) {
	err = -EINVAL;
	goto out;
}

?

-- 
Cheers,

David / dhildenb

Re: [PATCH v22 2/5] ring-buffer: Introducing ring-buffer mapping functions

[Vincent Donnefort]

[...]

> > > +
> > > +	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++;
> > > +	}
> > 
> > The above can be made to:
> > 
> > 	while (p < nr_pages) {
> > 		struct page *page;
> > 		int off = 0;
> > 
> > 		if (WARN_ON_ONCE(s >= nr_subbufs))
> > 			break;
> 
> I'm not particularly happy about us calling vm_insert_pages with NULL
> pointers stored in pages.
> 
> Should we instead do
> 
> if (WARN_ON_ONCE(s >= nr_subbufs)) {
> 	err = -EINVAL;
> 	goto out;
> }
> 
> ?

I could also nr_pages = p in the event of s >= nr_subbufs... but that
really that shouldn't happen so let's return an error.

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

Re: [PATCH v22 2/5] ring-buffer: Introducing ring-buffer mapping functions

[Steven Rostedt]

On Fri, 10 May 2024 12:03:12 +0100
Vincent Donnefort <vdonnefort@google.com> wrote:

> > I'm not particularly happy about us calling vm_insert_pages with NULL
> > pointers stored in pages.
> > 
> > Should we instead do
> > 
> > if (WARN_ON_ONCE(s >= nr_subbufs)) {
> > 	err = -EINVAL;
> > 	goto out;
> > }
> > 
> > ?  
> 
> I could also nr_pages = p in the event of s >= nr_subbufs... but that
> really that shouldn't happen so let's return an error.

I'm good with this. It should never happen anyway.

-- Steve

[PATCH v22 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