On Mon, Jul 25, 2022 at 2:22 AM Heikki Linnakangas <hlinnaka@iki.fi> wrote: > if (BufferIsLocal(recent_buffer)) > { > - bufHdr = GetBufferDescriptor(-recent_buffer - 1); > + bufHdr = GetLocalBufferDescriptor(-recent_buffer - 1); Aha, we're using the wrong buffer descriptors here. Currently this function is only called in XLogReadBufferExtended(), so the branch for local buffer cannot be reached. Maybe that's why it is not identified until now. > The code after that looks suspicious, too. It increases the usage count > even if the buffer was already pinned. That's different from what it > does for a shared buffer, and different from LocalBufferAlloc(). That's > pretty harmless, just causes the usage count to be bumped more > frequently, but I don't think it was intentional. The ordering of > bumping the usage count, the local ref count, and registration in the > resource owner are different too. As far as I can see, that makes no > difference, but I think we should keep this code as close as possible to > similar code used elsewhere, unless there's a particular reason to differ. Agree. Maybe we can wrap the codes in an inline function or macro and call that in both LocalBufferAlloc and here. Thanks Richard

Nice catch, LGTM. > On Jul 25, 2022, at 02:22, Heikki Linnakangas <hlinnaka@iki.fi> wrote: > > ReadRecentBuffer() doesn't work for local buffers, i.e. for temp tables. The bug is pretty clear if you look at the code: > > if (BufferIsLocal(recent_buffer)) > { > - bufHdr = GetBufferDescriptor(-recent_buffer - 1); > + bufHdr = GetLocalBufferDescriptor(-recent_buffer - 1); > > The code after that looks suspicious, too. It increases the usage count even if the buffer was already pinned. That's different from what it does for a shared buffer, and different from LocalBufferAlloc(). That's pretty harmless, just causes the usage count to be bumped more frequently, but I don't think it was intentional. The ordering of bumping the usage count, the local ref count, and registration in the resource owner are different too. As far as I can see, that makes no difference, but I think we should keep this code as close as possible to similar code used elsewhere, unless there's a particular reason to differ. > > I propose the attached to fix those things. > > I tested this by adding this little snippet to a random place where we have just read a page with ReadBuffer: > > diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c > index aab8d6fa4e5..c4abdbc96dd 100644 > --- a/src/backend/access/heap/heapam.c > +++ b/src/backend/access/heap/heapam.c > @@ -403,6 +403,14 @@ heapgetpage(TableScanDesc sscan, BlockNumber page) > RBM_NORMAL, scan->rs_strategy); > scan->rs_cblock = page; > > + { > + bool still_ok; > + > + still_ok = ReadRecentBuffer(scan->rs_base.rs_rd->rd_locator, MAIN_FORKNUM, page, scan->rs_cbuf); > + Assert(still_ok); > + ReleaseBuffer(scan->rs_cbuf); > + } > + > if (!(scan->rs_base.rs_flags & SO_ALLOW_PAGEMODE)) > return; > > Without the fix, the assertion is fails quickly on "make check". > > - Heikki<0001-Fix-ReadRecentBuffer-for-local-buffers.patch>

On Mon, Jul 25, 2022 at 6:22 AM Heikki Linnakangas <hlinnaka@iki.fi> wrote: > ReadRecentBuffer() doesn't work for local buffers, i.e. for temp tables. > The bug is pretty clear if you look at the code: - bufHdr = GetBufferDescriptor(-recent_buffer - 1); + int b = -recent_buffer - 1; + + bufHdr = GetLocalBufferDescriptor(b); Ugh, right. Obviously this code path is not reached currently. I added the local path for completeness but I didn't think of the idea of testing it the way you suggested, hence thinko escaped into the wild. That way of testing seems good and the patch indeed fixes the problem. - /* Bump local buffer's ref and usage counts. */ + /* + * Bump buffer's ref and usage counts. This is equivalent of + * PinBuffer for a shared buffer. + */ + if (LocalRefCount[b] == 0) + { + if (BUF_STATE_GET_USAGECOUNT(buf_state) < BM_MAX_USAGE_COUNT) + { + buf_state += BUF_USAGECOUNT_ONE; + pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state); + } + } + LocalRefCount[b]++; ResourceOwnerRememberBuffer(CurrentResourceOwner, recent_buffer); - LocalRefCount[-recent_buffer - 1]++; - if (BUF_STATE_GET_USAGECOUNT(buf_state) < BM_MAX_USAGE_COUNT) - pg_atomic_write_u32(&bufHdr->state, - buf_state + BUF_USAGECOUNT_ONE); +1, it makes sense to do it only if it wasn't pinned already, and it really should look identical to the code in LocalBufferAlloc, and perhaps the comment should even say so. LGTM.