net: add NETDEV_PRECHANGEMTU to notify before mtu change happens
Currently, if a device changes its mtu, first the change happens (invloving all the side effects), and after that the NETDEV_CHANGEMTU is sent so that other devices can catch up with the new mtu. However, if they return NOTIFY_BAD, then the change is reverted and error returned. This is a really long and costy operation (sometimes). To fix this, add NETDEV_PRECHANGEMTU notification which is called prior to any change actually happening, and if any callee returns NOTIFY_BAD - the change is aborted. This way we're skipping all the playing with apply/revert the mtu. CC: "David S. Miller" <davem@davemloft.net> CC: Jiri Pirko <jiri@resnulli.us> CC: Eric Dumazet <edumazet@google.com> CC: Nicolas Dichtel <nicolas.dichtel@6wind.com> CC: Cong Wang <amwang@redhat.com> Signed-off-by: NVeaceslav Falico <vfalico@redhat.com> Acked-by: NJiri Pirko <jiri@resnulli.us> Signed-off-by: NDavid S. Miller <davem@davemloft.net>
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