Re: 回复：Bug in MultiXact replay compat logic for older minor version after crash-recovery

On 22/03/2026 15:09, 段坤仁(刻韧) wrote:
> On 20/03/2026 16:19, Heikki Linnakangas wrote:
>> it means that tracking the latest page we have zeroed is not merely
>> an optimization to avoid excessive SimpleLruDoesPhysicalPageExist()
>> calls, it's needed for correctness.
>
> Agreed.
>
> On 20/03/2026 18:14, Heikki Linnakangas wrote:
>> I also added another safety measure: before calling
>> SimpleLruDoesPhysicalPageExist(), flush all the SLRU buffers.
>
> This is more robust than scanning the SLRU buffers first and only
> calling SimpleLruDoesPhysicalPageExist() on a miss, which would
> rely on the SLRU eviction invariant.
>
> I walked through the scenarios I could think of. Let N be the last
> multixid on offset page P, so N+1 falls on page P+1.
>
> (a) Old-version WAL (CREATE_ID:N before ZERO_OFF_PAGE:P+1):
> last_initialized_offsets_page = P from earlier ZERO_OFF_PAGE.
> init_needed = (P == P) = true -> init P+1. Correct.
> Later ZERO_OFF_PAGE:P+1 is skipped via pre_initialized_offsets_page.
>
> (b) Crash-restart, page P+1 not on disk (the original bug):
> last_initialized_offsets_page = -1, fallback path fires.
> SimpleLruDoesPhysicalPageExist(P+1) = false -> init. Correct.
>
> (c) Crash-restart, page P+1 already on disk:
> Same fallback, SimpleLruDoesPhysicalPageExist(P+1) = true -> skip.
> last_initialized_offsets_page stays -1 until the next
> ZERO_OFF_PAGE switches back to the fast path.
>
> (d) Out-of-order CREATE_IDs (ZERO_PAGE:P+1 -> CREATE_ID:N+1 ->
> CREATE_ID:N+2 -> CREATE_ID:N):
> N+1 and N+2 don't cross a page boundary, compat logic not entered.
> CREATE_ID:N: init_needed = (P+1 == P) = false -> skip.
> Page P+1 is not re-zeroed, data from N+1/N+2 preserved.
>
> (e) Consecutive page crossings (N on page P, later M on page P+1):
> After init of P+1: last_initialized_offsets_page = P+1.
> CREATE_ID:M: init_needed = (P+1 == P+1) = true -> init P+2.
> Tracking advances monotonically across page boundaries.
>
> The logic looks correct to me in all the cases above.

Ok, committed. Thank you!

- Heikki