Messages in this thread | | | Date | Thu, 2 May 2024 15:30:32 +0200 | Subject | Re: [PATCH v22 2/5] ring-buffer: Introducing ring-buffer mapping functions | From | David Hildenbrand <> |
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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
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