Horton

Re: Is this a problem in GenericXLogFinish()?

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>
Cc: Michael Paquier <michael@paquier.xyz>, Alexander Lakhin <exclusion@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Jeff Davis <pgsql@j-davis.com>, Heikki Linnakangas <hlinnaka@iki.fi>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2024-02-05T06:40:52Z
Lists: pgsql-hackers 
On Mon, Feb 5, 2024 at 10:00 AM Hayato Kuroda (Fujitsu)
<kuroda.hayato@fujitsu.com> wrote:
>
> >
> > Amit, this has been applied as of 861f86beea1c, and I got pinged about
> > the fact this triggers inconsistencies because we always set the LSN
> > of the write buffer (wbuf in _hash_freeovflpage) but
> > XLogRegisterBuffer() would *not* be called when the two following
> > conditions happen:
> > - When xlrec.ntups <= 0.
> > - When !xlrec.is_prim_bucket_same_wrt && !xlrec.is_prev_bucket_same_wrt
> >
> > And it seems to me that there is still a bug here: there should be no
> > point in setting the LSN on the write buffer if we don't register it
> > in WAL at all, no?
>
> Thanks for pointing out, I agreed your saying. PSA the patch for diagnosing the
> issue.
>

@@ -692,6 +697,9 @@ _hash_freeovflpage(Relation rel, Buffer bucketbuf,
Buffer ovflbuf,
  if (!xlrec.is_prev_bucket_same_wrt)
  wbuf_flags |= REGBUF_NO_CHANGE;
  XLogRegisterBuffer(1, wbuf, wbuf_flags);
+
+ /* Track the registration status for later use */
+ wbuf_registered = true;
  }

  XLogRegisterBuffer(2, ovflbuf, REGBUF_STANDARD);
@@ -719,7 +727,12 @@ _hash_freeovflpage(Relation rel, Buffer
bucketbuf, Buffer ovflbuf,

  recptr = XLogInsert(RM_HASH_ID, XLOG_HASH_SQUEEZE_PAGE);

- PageSetLSN(BufferGetPage(wbuf), recptr);
+ /* Set LSN to wbuf page buffer only when it is being registered */
+ if (wbuf_registered)
+ PageSetLSN(BufferGetPage(wbuf), recptr);

Why set LSN when the page is not modified (say when we use the flag
REGBUF_NO_CHANGE)?  I think you need to use a flag mod_wbuf and set it
in appropriate places during register buffer calls.

-- 
With Regards,
Amit Kapila.

Commits

  1. Fix inconsistency with replay of hash squeeze record for clean buffers

  2. Set LSN for wbuf in _hash_freeovflpage() iff wbuf is modified.

  3. Fix an uninitialized access in hash_xlog_squeeze_page().

  4. Use REGBUF_NO_CHANGE at one more place in the hash index.

  5. Assert that buffers are marked dirty before XLogRegisterBuffer().

  6. Fix bug in GenericXLogFinish().

  7. Improve hash index bucket split behavior.