Re: Changing shared_buffers without restart

> On Thu, Apr 17, 2025 at 07:05:36PM GMT, Ni Ku wrote:
> 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).

There is already a coordination infrastructure, implemented in the patch
0006, which will take care of this and prevent access to the shared
memory until everything is resized.