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

David Hildenbrand
2024-05-02T13:30:38Z

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