On Wed, 2017-02-22 at 11:38 -0500, Willem de Bruijn wrote:
> From: Willem de Bruijn <will...@google.com>
>
> Refine skb_copy_ubufs to support compount pages. With upcoming TCP
> and UDP zerocopy sendmsg, such fragments may appear.
>
> These skbuffs can have both kernel and zerocopy fragments, e.g., when
> corking. Avoid unnecessary copying of fragments that have no userspace
> reference.
>
> It is not safe to modify skb frags when the skbuff is shared. This
> should not happen. Fail loudly if we find an unexpected edge case.
>
> Signed-off-by: Willem de Bruijn <will...@google.com>
> ---
> net/core/skbuff.c | 24 +++++++++++++++++++++++-
> 1 file changed, 23 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index f3557958e9bf..67e4216fca01 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -944,6 +944,9 @@ EXPORT_SYMBOL_GPL(skb_morph);
> * If this function is called from an interrupt gfp_mask() must be
> * %GFP_ATOMIC.
> *
> + * skb_shinfo(skb) can only be safely modified when not accessed
> + * concurrently. Fail if the skb is shared or cloned.
> + *
> * Returns 0 on success or a negative error code on failure
> * to allocate kernel memory to copy to.
> */
> @@ -954,11 +957,29 @@ int skb_copy_ubufs(struct sk_buff *skb, gfp_t gfp_mask)
> struct page *page, *head = NULL;
> struct ubuf_info *uarg = skb_shinfo(skb)->destructor_arg;
>
> + if (skb_shared(skb) || skb_cloned(skb)) {
> + WARN_ON_ONCE(1);
> + return -EINVAL;
> + }
> +
> for (i = 0; i < num_frags; i++) {
> u8 *vaddr;
> + unsigned int order = 0;
> + gfp_t mask = gfp_mask;
> skb_frag_t *f = &skb_shinfo(skb)->frags[i];
>
> - page = alloc_page(gfp_mask);
> + page = skb_frag_page(f);
> + if (page_count(page) == 1) {
> + skb_frag_ref(skb, i);
This could be : get_page(page);
> + goto copy_done;
> + }
> +
> + if (f->size > PAGE_SIZE) {
> + order = get_order(f->size);
> + mask |= __GFP_COMP;
Note that this would probably fail under memory pressure.
We could instead try to explode the few segments into order-0 only
pages.
Hopefully this case should not be frequent.
