Thanks for the information and discussions, it's been very helpful.
I also have a related question about how ftruncate() is used in the patch. In my testing I also see that when using ftruncate to shrink a shared segment, the memory is freed immediately after the call, even if other processes still have that memory mapped, and they will hit SIGBUS if they try to access that memory again as the manpage says.
So am I correct to think that, to support the bufferpool shrinking case, it would not be safe to call ftruncate in AnonymousShmemResize as-is, since at that point other processes may still be using pages that belong to the truncated memory? It appears that for shrinking we should only call ftruncate when we're sure no process will access those pages again (eg, all processes have handled the resize interrupt signal barrier). I suppose this can be done by the resize coordinator after synchronizing with all the other processes. But in that case it seems we cannot use the postmaster as the coordinator then? b/c I see some code comments saying the postmaster does not have waiting infrastructure... (maybe even if the postmaster has waiting infra we don't want to use it anyway since it can be blocked for a long time and won't be able to serve other requests).
