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

Vincent Donnefort
2024-05-02T13:38:38Z

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
>