Re: Fix memory counter update in reorderbuffer

On Wed, Aug 7, 2024 at 3:17 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Wed, Aug 7, 2024 at 7:42 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Tue, Aug 6, 2024 at 1:12 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > > On Sat, Aug 3, 2024 at 1:21 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > > >
> > > > I found a bug in the memory counter update in reorderbuffer. It was
> > > > introduced by commit 5bec1d6bc5e, so pg17 and master are affected.
> > > >
> > > > In ReorderBufferCleanupTXN() we zero the transaction size and then
> > > > free the transaction entry as follows:
> > > >
> > > > /* Update the memory counter */
> > > > ReorderBufferChangeMemoryUpdate(rb, NULL, txn, false, txn->size);
> > > >
> > > > /* deallocate */
> > > > ReorderBufferReturnTXN(rb, txn);
> > > >
> > >
> > > Why do we need to zero the transaction size explicitly? Shouldn't it
> > > automatically become zero after freeing all the changes?
> >
> > It will become zero after freeing all the changes. However, since
> > updating the max-heap when freeing each change could bring some
> > overhead, we freed the changes without updating the memory counter,
> > and then zerod it.
> >
>
> I think this should be covered in comments as it is not apparent.

Agreed.

>
> >
> > > BTW, commit 5bec1d6bc5e also introduced
> > > ReorderBufferChangeMemoryUpdate() in ReorderBufferTruncateTXN() which
> > > is also worth considering while fixing the reported problem. It may
> > > not have the same problem as you have reported but we can consider
> > > whether setting txn size as zero explicitly is required or not.
> >
> > The reason why it introduced ReorderBufferChangeMemoryUpdate() is the
> > same as I mentioned above. And yes, as you mentioned, it doesn't have
> > the same problem that I reported here.
> >
>
> I checked again and found that ReorderBufferResetTXN() first calls
> ReorderBufferTruncateTXN() and then ReorderBufferToastReset(). After
> that, it also tries to free spec_insert change which should have the
> same problem. So, what saves this path from the same problem?

Good catch. I've not created a test case for that but it seems to be
possible to happen.

I think that subtracting txn->size to reduce the memory counter to
zero seems to be a wrong idea in the first place. If we want to save
updating memory counter and max-heap, we should use the exact memory
size that we freed. In other words, just change the memory usage
update to a batch operation.

>
> *
> + /*
> + * Update the memory counter of the transaction, removing it from
> + * the max-heap.
> + */
> + ReorderBufferChangeMemoryUpdate(rb, NULL, txn, false, txn->size);
> + Assert(txn->size == 0);
> +
> pfree(txn);
>
> Just before freeing the TXN, updating the size looks odd though I
> understand the idea is to remove TXN from max_heap. Anyway, let's
> first discuss whether the same problem exists in
> ReorderBufferResetTXN() code path, and if so, how we want to fix it.

Agreed.

Regards,

--
Masahiko Sawada
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