Re: Confine vacuum skip logic to lazy_scan_skip

On Fri, Mar 8, 2024 at 10:41 AM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
>
> On Fri, Mar 8, 2024 at 8:49 AM Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
> > ISTM we should revert the above hunk, and backpatch it to v16. I'm a
> > little wary because I don't understand why that change was made in the
> > first place, though. I think it was just an ill-advised attempt at
> > tidying up the code as part of the larger commit, but I'm not sure.
> > Peter, do you remember?
>
> I think that it makes sense to set the VM when indicated by
> lazy_scan_prune, independent of what either the visibility map or the
> page's PD_ALL_VISIBLE marking say. The whole point of
> DISABLE_PAGE_SKIPPING is to deal with VM corruption, after all.

Not that it will be fun to maintain another special case in the VM
update code in lazy_scan_prune(), but we could have a special case
that checks if DISABLE_PAGE_SKIPPING was passed to vacuum and if
all_visible_according_to_vm is true and all_visible is true, we update
the VM but don't dirty the page. The docs on DISABLE_PAGE_SKIPPING say
it is meant to deal with VM corruption -- it doesn't say anything
about dealing with incorrectly set PD_ALL_VISIBLE markings.

- Melanie