On Wed, Aug 7, 2024 at 7:42 AM Masahiko Sawada <sawada.m...@gmail.com> wrote: > > On Tue, Aug 6, 2024 at 1:12 PM Amit Kapila <amit.kapil...@gmail.com> wrote: > > > > On Sat, Aug 3, 2024 at 1:21 AM Masahiko Sawada <sawada.m...@gmail.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. > > > 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? * + /* + * 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. -- With Regards, Amit Kapila.
