Re: Allow aggressive VACUUM to freeze without a cleanup lock

On Tue, Aug 25, 2026 at 6:32 AM Jingtang Zhang <mrdrivingduck(at)gmail(dot)com> wrote:
>
> The 2011 discussion [2] raised the related question of whether VACUUM
> could make progress after failing to acquire a cleanup lock. It found that
> changing line pointers while holding only an exclusive buffer content lock
> would be unsafe, because another backend can inspect a line pointer while
> holding only a buffer pin. However, freezing tuple headers is different: an
> exclusive buffer content lock is sufficient for that.

I think this proposal (in the email) should explain why freezing xids
and mxids is safe with only an exclusive lock. AFAICT that 2011 thread
doesn't get specific. And today, lazy_scan_noprune() specifically
returns false and requires a cleanup lock during an aggressive vacuum
when freezing would be required. If this is not necessary, you should
explain why. Was it never required and we were being over-cautious?
Did something change in the code to make it safe and we forgot to
update lazy_scan_noprune()?

- Melanie