Re: VM map freeze corruption

On Wed, Apr 18, 2018 at 7:37 AM, Wood, Dan <hexpert(at)amazon(dot)com> wrote:

>
>
> My analysis is that heap_prepare_freeze_tuple->FreezeMultiXactId()
> returns FRM_NOOP if the MultiXACT locked rows haven't committed. This
> results in changed=false and totally_frozen=true(as initialized). When
> this returns to lazy_scan_heap(), no rows are added to the frozen[] array.
> Yet, tuple_totally_frozen is true. This means the page is marked frozen in
> the VM, even though the MultiXACT row wasn't left untouch.
>
> A fix to heap_prepare_freeze_tuple() that seems to do the trick is:
> else
> {
> Assert(flags & FRM_NOOP);
> + totally_frozen = false;
> }
>

That's a great find! This can definitely lead to various problems and could
be one of the reasons behind the issue reported here [1]. For example, if
we change the script slightly at the end, we can get the same error
reported in the bug report.

sleep 4; # Wait for share locks to be released

# See if another vacuum freeze advances relminmxid beyond xmax present in
the
# heap
echo "vacuum (verbose, freeze) t;" | $p
echo "select pg_check_frozen('t');" | $p

# See if a vacuum freeze scanning all pages corrects the problem
echo "vacuum (verbose, freeze, disable_page_skipping) t;" | $p
echo "select pg_check_frozen('t');" | $p

Thanks,
Pavan

[1]
https://www.postgresql.org/message-id/CAGewt-ujGpMLQ09gXcUFMZaZsGJC98VXHEFbF-tpPB0fB13K%2BA%40mail.gmail.com

--
Pavan Deolasee http://www.2ndQuadrant.com/
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