• Willem de Bruijn's avatar
    udp: elide zerocopy operation in hot path · 52900d22
    Willem de Bruijn authored
    With MSG_ZEROCOPY, each skb holds a reference to a struct ubuf_info.
    Release of its last reference triggers a completion notification.
    
    The TCP stack in tcp_sendmsg_locked holds an extra ref independent of
    the skbs, because it can build, send and free skbs within its loop,
    possibly reaching refcount zero and freeing the ubuf_info too soon.
    
    The UDP stack currently also takes this extra ref, but does not need
    it as all skbs are sent after return from __ip(6)_append_data.
    
    Avoid the extra refcount_inc and refcount_dec_and_test, and generally
    the sock_zerocopy_put in the common path, by passing the initial
    reference to the first skb.
    
    This approach is taken instead of initializing the refcount to 0, as
    that would generate error "refcount_t: increment on 0" on the
    next skb_zcopy_set.
    
    Changes
      v3 -> v4
        - Move skb_zcopy_set below the only kfree_skb that might cause
          a premature uarg destroy before skb_zerocopy_put_abort
          - Move the entire skb_shinfo assignment block, to keep that
            cacheline access in one place
    Signed-off-by: default avatarWillem de Bruijn <willemb@google.com>
    Acked-by: default avatarPaolo Abeni <pabeni@redhat.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    52900d22
ip_output.c 40.1 KB