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SubjectRe: [PATCH 5/5] tun: vringfd xmit support.
On Fri, 18 Apr 2008 14:43:24 +1000 Rusty Russell <rusty@rustcorp.com.au> wrote:

> This patch modifies tun to allow a vringfd to specify the send
> buffer. The user does a write to push out packets from the buffer.
>
> Again we use the 'struct virtio_net_hdr' to allow userspace to send
> GSO packets. In this case, it can hint how much to copy, and the
> other pages will be made into skb fragments.
>
>
> ...
>
> +/* We don't consolidate consecutive iovecs, so huge iovecs can break here.
> + * Users will learn not to do that. */
> +static int get_user_skb_frags(const struct iovec *iv, size_t len,
> + struct skb_frag_struct *f)
> +{
> + unsigned int i, j, num_pg = 0;
> + int err;
> + struct page *pages[MAX_SKB_FRAGS];
> +
> + down_read(&current->mm->mmap_sem);
> + while (len) {
> + int n, npages;
> + unsigned long base, len;
> + base = (unsigned long)iv->iov_base;
> + len = (unsigned long)iv->iov_len;
> +
> + if (len == 0) {
> + iv++;
> + continue;
> + }
> +
> + /* How many pages will this take? */
> + npages = 1 + (base + len - 1)/PAGE_SIZE - base/PAGE_SIZE;

Brain hurts. I hope you got that right.

> + if (unlikely(num_pg + npages > MAX_SKB_FRAGS)) {
> + err = -ENOSPC;
> + goto fail;
> + }
> + n = get_user_pages(current, current->mm, base, npages,
> + 0, 0, pages, NULL);

What is the maximum numbet of pages which an unpriviliged user can
concurrently pin with this code?

> + if (unlikely(n < 0)) {
> + err = n;
> + goto fail;
> + }
> +
> + /* Transfer pages to the frag array */
> + for (j = 0; j < n; j++) {
> + f[num_pg].page = pages[j];
> + if (j == 0) {
> + f[num_pg].page_offset = offset_in_page(base);
> + f[num_pg].size = min(len, PAGE_SIZE -
> + f[num_pg].page_offset);
> + } else {
> + f[num_pg].page_offset = 0;
> + f[num_pg].size = min(len, PAGE_SIZE);
> + }
> + len -= f[num_pg].size;
> + base += f[num_pg].size;
> + num_pg++;
> + }

This loop is a fancy way of doing

num_pg = n;

> + if (unlikely(n != npages)) {
> + err = -EFAULT;
> + goto fail;
> + }

why not do this immediately after running get_user_pages()?

> + }
> + up_read(&current->mm->mmap_sem);
> + return num_pg;
> +
> +fail:
> + for (i = 0; i < num_pg; i++)
> + put_page(f[i].page);

release_pages() could be a tad more efficient, but it's only error-path.

> + up_read(&current->mm->mmap_sem);
> + return err;
> +}
> +
>



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