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

On Fri, Jul 8, 2016 at 10:24 PM, Amit Kapila <amit.kapila16@gmail.com> wrote:
> On Thu, Jul 7, 2016 at 12:34 PM, Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>> 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.
>
> + /* Clear only the all-frozen bit on visibility map if needed */
>
> + if (PageIsAllVisible(BufferGetPage(buffer)) &&
>
> + VM_ALL_FROZEN(relation, block, &vmbuffer))
> + {
> + visibilitymap_clear_extended(relation, block, vmbuffer,
> + VISIBILITYMAP_ALL_FROZEN);
> + }
> +
>
> + if (RelationNeedsWAL(relation))
> + {
> ..
>
> + XLogBeginInsert();
> + XLogRegisterBuffer(0, buffer, REGBUF_STANDARD);
> +
> + xlrec.offnum = ItemPointerGetOffsetNumber(&oldtup.t_self);
> + xlrec.locking_xid = xmax_old_tuple;
> + xlrec.infobits_set = compute_infobits(oldtup.t_data->t_infomask,
> +   oldtup.t_data->t_infomask2);
> + XLogRegisterData((char *) &xlrec, SizeOfHeapLock);
> + recptr = XLogInsert(RM_HEAP_ID, XLOG_HEAP_LOCK);
> ..
>
> One thing that looks awkward in this code is that it doesn't record
> whether the frozen bit is actually cleared during the actual operation
> and then during replay, it always clear the frozen bit irrespective of
> whether it has been cleared by the actual operation or not.
>

I changed it so that we look all-frozen bit up first, and then clear
it if needed.

> + /* Clear only the all-frozen bit on visibility map if needed */
> + if (PageIsAllVisible(page) &&
> + VM_ALL_FROZEN(relation, BufferGetBlockNumber(*buffer), &vmbuffer))
> + {
> + BlockNumber block = BufferGetBlockNumber(*buffer);
> +
> + visibilitymap_pin(relation, block, &vmbuffer);
>
> I think it is not right to call visibilitymap_pin after holding a
> buffer lock (visibilitymap_pin can perform I/O).  Refer heap_update
> for how to pin the visibility map.
>

Thank you for your advice!
Fixed.

Attached separated two patched, please give me feedbacks.

Regards,

--
Masahiko Sawada