Re: Changing shared_buffers without restart

> On Mon, Apr 14, 2025 at 10:40:28AM GMT, Ashutosh Bapat wrote:
>
> However, when we put back the patches to shrink buffers, we will evict
> the extra buffers, and shrink - if all the processes haven't
> participated in the barrier by then, some of them may try to access
> those buffers - re-installing them and then bad things can happen.

As I've mentioned above, I don't see how a process could try to access a
buffer, if it's on the path between receiving the ProcSignalBarrier and
attaching to the global shmem Barrier, even if we shrink buffers.
AFAICT interrupt handles should not touch buffers, and otherwise the
process doesn't have any point withing this window where it might do
this. Do you have some particular scenario in mind?

> I might have not noticed it, but are we putting two mappings one
> reserved and one allocated in the same address space, so that when the
> allocated mapping shrinks or expands, the reserved mapping continues
> to prohibit any other mapping from appearing there? I looked at some
> of the previous emails, but didn't find anything that describes how
> the reserved mapped space is managed.

I though so, but this turns out to be incorrect. Just have done a small
experiment -- looks like when reserving some space, mapping and
unmapping a small segment from it leaves a non-mapped gap. That would
mean for shrinking the new available space has to be reserved again.