Re: [PATCH v20 2/5] ring-buffer: Introducing ring-buffer mapping functions

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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@xxxxxxxxx>
Signed-off-by: Vincent Donnefort <vdonnefort@xxxxxxxxxx>

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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