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

[David Hildenbrand <david@redhat.com>]

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