[question] difference between vm_extend and fsm_extend

Hey hacks,

I'm looking the code of free space map and visibility map, both the module
call `ExtendBufferedRelTo` to extend its file when necessary, what confused
me is that `vm_extend` has an extra step that send a shared message to force
other backends to close other smgr references.

My question is why does `fsm_extend` not need the invalidation step?

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Regards
Junwang Zhao