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If we didn't have a routing cache, we would not be able to properly propagate certain kinds of dynamic path attributes, for example PMTU information and redirects. The reason is that if we didn't have a routing cache, then there would be no way to lookup all of the active cached routes hanging off of sockets, tunnels, IPSEC bundles, etc. Consider the case where we created a cached route, but no inetpeer entry existed and also we were not asked to pre-COW the route metrics and therefore did not force the creation a new inetpeer entry. If we later get a PMTU message, or a redirect, and store this information in a new inetpeer entry, there is no way to teach that cached route about the newly existing inetpeer entry. The facilities implemented here handle this problem. First we create a generation ID. When we create a cached route of any kind, we remember the generation ID at the time of attachment. Any time we force-create an inetpeer entry in response to new path information, we bump that generation ID. The dst_ops->check() callback is where the knowledge of this event is propagated. If the global generation ID does not equal the one stored in the cached route, and the cached route has not attached to an inetpeer yet, we look it up and attach if one is found. Now that we've updated the cached route's information, we update the route's generation ID too. This clears the way for implementing PMTU and redirects directly in the inetpeer cache. There is absolutely no need to consult cached route information in order to maintain this information. At this point nothing bumps the inetpeer genids, that comes in the later changes which handle PMTUs and redirects using inetpeers. Signed-off-by: David S. Miller <davem@davemloft.net> |
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