Re: Reviewing freeze map code - Mailing list pgsql-hackers

Than you for reviewing!

On Thu, Jul 7, 2016 at 7:06 AM, Andres Freund <andres@anarazel.de> wrote:
> On 2016-07-05 23:37:59 +0900, Masahiko Sawada wrote:
>> diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
>> index 57da57a..fd66527 100644
>> --- a/src/backend/access/heap/heapam.c
>> +++ b/src/backend/access/heap/heapam.c
>> @@ -3923,6 +3923,17 @@ l2:
>>
>>       if (need_toast || newtupsize > pagefree)
>>       {
>> +             /*
>> +              * To prevent data corruption due to updating old tuple by
>> +              * other backends after released buffer
>
> That's not really the reason, is it? The prime problem is crash safety /
> replication. The row-lock we're faking (by setting xmax to our xid),
> prevents concurrent updates until this backend died.

Fixed.

>>                 , we need to emit that
>> +              * xmax of old tuple is set and clear visibility map bits if
>> +              * needed before releasing buffer. We can reuse xl_heap_lock
>> +              * for this purpose. It should be fine even if we crash midway
>> +              * from this section and the actual updating one later, since
>> +              * the xmax will appear to come from an aborted xid.
>> +              */
>> +             START_CRIT_SECTION();
>> +
>>               /* Clear obsolete visibility flags ... */
>>               oldtup.t_data->t_infomask &= ~(HEAP_XMAX_BITS | HEAP_MOVED);
>>               oldtup.t_data->t_infomask2 &= ~HEAP_KEYS_UPDATED;
>> @@ -3936,6 +3947,46 @@ l2:
>>               /* temporarily make it look not-updated */
>>               oldtup.t_data->t_ctid = oldtup.t_self;
>>               already_marked = true;
>> +
>> +             /* Clear PD_ALL_VISIBLE flags */
>> +             if (PageIsAllVisible(BufferGetPage(buffer)))
>> +             {
>> +                     all_visible_cleared = true;
>> +                     PageClearAllVisible(BufferGetPage(buffer));
>> +                     visibilitymap_clear(relation, BufferGetBlockNumber(buffer),
>> +                                                             vmbuffer);
>> +             }
>> +
>> +             MarkBufferDirty(buffer);
>> +
>> +             if (RelationNeedsWAL(relation))
>> +             {
>> +                     xl_heap_lock xlrec;
>> +                     XLogRecPtr recptr;
>> +
>> +                     /*
>> +                      * For logical decoding we need combocids to properly decode the
>> +                      * catalog.
>> +                      */
>> +                     if (RelationIsAccessibleInLogicalDecoding(relation))
>> +                             log_heap_new_cid(relation, &oldtup);
>
> Hm, I don't see that being necessary here. Row locks aren't logically
> decoded, so there's no need to emit this here.

Fixed.

>
>> +     /* Clear PD_ALL_VISIBLE flags */
>> +     if (PageIsAllVisible(page))
>> +     {
>> +             Buffer  vmbuffer = InvalidBuffer;
>> +             BlockNumber     block = BufferGetBlockNumber(*buffer);
>> +
>> +             all_visible_cleared = true;
>> +             PageClearAllVisible(page);
>> +             visibilitymap_pin(relation, block, &vmbuffer);
>> +             visibilitymap_clear(relation, block, vmbuffer);
>> +     }
>> +
>
> That clears all-visible unnecessarily, we only need to clear all-frozen.
>

Fixed.

>
>> @@ -8694,6 +8761,23 @@ heap_xlog_lock(XLogReaderState *record)
>>               }
>>               HeapTupleHeaderSetXmax(htup, xlrec->locking_xid);
>>               HeapTupleHeaderSetCmax(htup, FirstCommandId, false);
>> +
>> +             /* The visibility map need to be cleared */
>> +             if ((xlrec->infobits_set & XLHL_ALL_VISIBLE_CLEARED) != 0)
>> +             {
>> +                     RelFileNode     rnode;
>> +                     Buffer          vmbuffer = InvalidBuffer;
>> +                     BlockNumber     blkno;
>> +                     Relation        reln;
>> +
>> +                     XLogRecGetBlockTag(record, 0, &rnode, NULL, &blkno);
>> +                     reln = CreateFakeRelcacheEntry(rnode);
>> +
>> +                     visibilitymap_pin(reln, blkno, &vmbuffer);
>> +                     visibilitymap_clear(reln, blkno, vmbuffer);
>> +                     PageClearAllVisible(page);
>> +             }
>> +
>
>
>>               PageSetLSN(page, lsn);
>>               MarkBufferDirty(buffer);
>>       }
>> diff --git a/src/include/access/heapam_xlog.h b/src/include/access/heapam_xlog.h
>> index a822d0b..41b3c54 100644
>> --- a/src/include/access/heapam_xlog.h
>> +++ b/src/include/access/heapam_xlog.h
>> @@ -242,6 +242,7 @@ typedef struct xl_heap_cleanup_info
>>  #define XLHL_XMAX_EXCL_LOCK          0x04
>>  #define XLHL_XMAX_KEYSHR_LOCK        0x08
>>  #define XLHL_KEYS_UPDATED            0x10
>> +#define XLHL_ALL_VISIBLE_CLEARED 0x20
>
> Hm. We can't easily do that in the back-patched version; because a
> standby won't know to check for the flag . That's kinda ok, since we
> don't yet need to clear all-visible yet at that point of
> heap_update. But that better means we don't do so on the master either.
>

Attached latest version patch.
I changed visibilitymap_clear function so that it allows to specify
bits being cleared.
The function that needs to clear the only all-frozen bit on visibility
map calls visibilitymap_clear_extended function to clear particular
bit.
Other function can call visibilitymap_clear function to clear all bits
for one page.

Instead of adding XLHL_ALL_VISIBLE_CLEARED, we do vm loop up for back branches.
To reduce unnecessary looking up visibility map, I changed it so that
we check the PD_ALL_VISIBLE on heap page, and then look up all-frozen
bit on visibility map if necessary.

Regards,

--
Masahiko Sawada