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SubjectRe: [PATCH 07/17] new helpers: skb_copy_datagram_from_iter() and zerocopy_sg_from_iter()
On 11/24/2014 08:02 AM, Ben Hutchings wrote:
> On Sat, 2014-11-22 at 04:33 +0000, Al Viro wrote:
> [...]
>> --- a/net/core/datagram.c
>> +++ b/net/core/datagram.c
>> @@ -572,6 +572,77 @@ fault:
>> }
>> EXPORT_SYMBOL(skb_copy_datagram_from_iovec);
>>
> Missing kernel-doc.
>
>> +int skb_copy_datagram_from_iter(struct sk_buff *skb, int offset,
>> + struct iov_iter *from,
>> + int len)
>> +{
>> + int start = skb_headlen(skb);
>> + int i, copy = start - offset;
>> + struct sk_buff *frag_iter;
>> +
>> + /* Copy header. */
>> + if (copy > 0) {
>> + if (copy > len)
>> + copy = len;
>> + if (copy_from_iter(skb->data + offset, copy, from) != copy)
>> + goto fault;
>> + if ((len -= copy) == 0)
>> + return 0;
>> + offset += copy;
>> + }
>> +
>> + /* Copy paged appendix. Hmm... why does this look so complicated? */
>> + for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
>> + int end;
>> + const skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
>> +
>> + WARN_ON(start > offset + len);
>> +
>> + end = start + skb_frag_size(frag);
>> + if ((copy = end - offset) > 0) {
>> + size_t copied;
> Blank line needed after a declaration.
>
>> + if (copy > len)
>> + copy = len;
>> + copied = copy_page_from_iter(skb_frag_page(frag),
>> + frag->page_offset + offset - start,
>> + copy, from);
>> + if (copied != copy)
>> + goto fault;
>> +
>> + if (!(len -= copy))
>> + return 0;
> The other two instances of this condition are written as:
>
> if ((len -= copy) == 0)
>
> Similarly in skb_copy_bits().
>
>> + offset += copy;
>> + }
>> + start = end;
>> + }
>> +
>> + skb_walk_frags(skb, frag_iter) {
>> + int end;
>> +
>> + WARN_ON(start > offset + len);
>> +
>> + end = start + frag_iter->len;
>> + if ((copy = end - offset) > 0) {
>> + if (copy > len)
>> + copy = len;
>> + if (skb_copy_datagram_from_iter(frag_iter,
>> + offset - start,
>> + from, copy))
>> + goto fault;
>> + if ((len -= copy) == 0)
>> + return 0;
>> + offset += copy;
>> + }
>> + start = end;
>> + }
>> + if (!len)
>> + return 0;
>> +
>> +fault:
>> + return -EFAULT;
>> +}
>> +EXPORT_SYMBOL(skb_copy_datagram_from_iter);
>> +
>> /**
>> * zerocopy_sg_from_iovec - Build a zerocopy datagram from an iovec
>> * @skb: buffer to copy
>> @@ -643,6 +714,50 @@ int zerocopy_sg_from_iovec(struct sk_buff *skb, const struct iovec *from,
>> }
>> EXPORT_SYMBOL(zerocopy_sg_from_iovec);
>>
> Missing kernel-doc.
>
>> +int zerocopy_sg_from_iter(struct sk_buff *skb, struct iov_iter *from)
>> +{
>> + int len = iov_iter_count(from);
>> + int copy = min_t(int, skb_headlen(skb), len);
>> + int i = 0;
>> +
>> + /* copy up to skb headlen */
>> + if (skb_copy_datagram_from_iter(skb, 0, from, copy))
>> + return -EFAULT;
>> +
>> + while (iov_iter_count(from)) {
>> + struct page *pages[MAX_SKB_FRAGS];
>> + size_t start;
>> + ssize_t copied;
>> + unsigned long truesize;
>> + int n = 0;
>> +
>> + copied = iov_iter_get_pages(from, pages, ~0U, MAX_SKB_FRAGS, &start);
>> + if (copied < 0)
>> + return -EFAULT;
>> +
>> + truesize = DIV_ROUND_UP(copied + start, PAGE_SIZE) * PAGE_SIZE;
> PAGE_ALIGN(copied + start) ?
>
>> + skb->data_len += copied;
>> + skb->len += copied;
>> + skb->truesize += truesize;
>> + atomic_add(truesize, &skb->sk->sk_wmem_alloc);
>> + while (copied) {
>> + int off = start;
> This variable seems redundant. Can't we use start directly and move the
> 'start = 0' to the bottom of the loop?
>
>> + int size = min_t(int, copied, PAGE_SIZE - off);
>> + start = 0;
>> + if (i < MAX_SKB_FRAGS)
>> + skb_fill_page_desc(skb, i, pages[n], off, size);
>> + else
>> + put_page(pages[n]);
> Why is this condition needed, given we told iov_iter_get_pages() to
> limit to MAX_SKB_FRAGS pages?

We don't want to send truncated packets and there's no other way to put
those pages since it was not in the frag array.


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