Re: Changing shared_buffers without restart

On Mon, Apr 14, 2025 at 12:50 PM Dmitry Dolgov <9erthalion6@gmail.com> wrote:
>
> > 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?

ProcessProcSignalBarrier() is not within an interrupt handler but it
responds to a flag set by an interrupt handler. After calling
pg_atomic_write_u64(&MyProcSignalSlot->pss_barrierReceivedGeneration,
shared_gen); it will enter the loop

while (flags != 0)
where it may process many barriers before processing
PROCSIGNAL_BARRIER_SHMEM_RESIZE. Nothing stops the other barrier
processing code from touching buffers. Right now it's just smgrrelease
that gets called in the other barrier. But that's not guaranteed in
future.

>
> > 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.

Right. That's what I thought. But I didn't see the corresponding code.
So we have to keep track of two mappings for every segment - 1 for
allocation and one for reserving space and resize those two while
shrinking and expanding buffers. Am I correct?


--
Best Wishes,
Ashutosh Bapat