Handle ENOENT status when querying NUMA node

We've assumed that touching the memory is sufficient for a page to be
located on one of the NUMA nodes. But a page may be moved to a swap
after we touch it, due to memory pressure.

We touch the memory before querying the status, but there is no
guarantee it won't be moved to the swap in the meantime. The touching
happens only on the first call, so later calls are more likely to be
affected. And the batching increases the window too.

It's up to the kernel if/when pages get moved to swap. We have to accept
ENOENT (-2) as a valid result, and handle it without failing. This patch
simply treats it as an unknown node, and returns NULL in the two
affected views (pg_shmem_allocations_numa and pg_buffercache_numa).

Hugepages cannot be swapped out, so this affects only regular pages.

Reported by Christoph Berg, investigation and fix by me. Backpatch to
18, where the two views were introduced.

Reported-by: Christoph Berg <myon@debian.org>
Discussion: 18
Backpatch-through: https://postgr.es/m/aTq5Gt_n-oS_QSpL@msg.df7cb.de

contrib/pg_buffercache/pg_buffercache_pages.c

@@ -551,8 +551,16 @@ pg_buffercache_os_pages_internal(FunctionCallInfo fcinfo, bool include_numa)
 
 		if (fctx->include_numa)
 		{
-			values[2] = Int32GetDatum(fctx->record[i].numa_node);
-			nulls[2] = false;
+			/* status is valid node number */
+			if (fctx->record[i].numa_node >= 0)
+			{
+				values[2] = Int32GetDatum(fctx->record[i].numa_node);
+				nulls[2] = false;
+			} else {
+				/* some kind of error (e.g. pages moved to swap) */
+				values[2] = (Datum) 0;
+				nulls[2] = true;
+			}
 		}
 		else
 		{

src/backend/storage/ipc/shmem.c

@@ -599,7 +599,7 @@ pg_get_shmem_allocations_numa(PG_FUNCTION_ARGS)
 	InitMaterializedSRF(fcinfo, 0);
 
 	max_nodes = pg_numa_get_max_node();
-	nodes = palloc_array(Size, max_nodes + 1);
+	nodes = palloc_array(Size, max_nodes + 2);
 
 	/*
 	 * Shared memory allocations can vary in size and may not align with OS
@@ -635,7 +635,6 @@ pg_get_shmem_allocations_numa(PG_FUNCTION_ARGS)
 	hash_seq_init(&hstat, ShmemIndex);
 
 	/* output all allocated entries */
-	memset(nulls, 0, sizeof(nulls));
 	while ((ent = (ShmemIndexEnt *) hash_seq_search(&hstat)) != NULL)
 	{
 		int			i;
@@ -684,22 +683,33 @@ pg_get_shmem_allocations_numa(PG_FUNCTION_ARGS)
 			elog(ERROR, "failed NUMA pages inquiry status: %m");
 
 		/* Count number of NUMA nodes used for this shared memory entry */
-		memset(nodes, 0, sizeof(Size) * (max_nodes + 1));
+		memset(nodes, 0, sizeof(Size) * (max_nodes + 2));
 
 		for (i = 0; i < shm_ent_page_count; i++)
 		{
 			int			s = pages_status[i];
 
 			/* Ensure we are adding only valid index to the array */
-			if (s < 0 || s > max_nodes)
+			if (s >= 0 && s <= max_nodes)
+			{
+				/* valid NUMA node */
+				nodes[s]++;
+				continue;
+			}
+			else if (s == -2)
 			{
-				elog(ERROR, "invalid NUMA node id outside of allowed range "
-					 "[0, " UINT64_FORMAT "]: %d", max_nodes, s);
+				/* -2 means ENOENT (e.g. page was moved to swap) */
+				nodes[max_nodes + 1]++;
+				continue;
 			}
 
-			nodes[s]++;
+			elog(ERROR, "invalid NUMA node id outside of allowed range "
+				 "[0, " UINT64_FORMAT "]: %d", max_nodes, s);
 		}
 
+		/* no NULLs for regular nodes */
+		memset(nulls, 0, sizeof(nulls));
+
 		/*
 		 * Add one entry for each NUMA node, including those without allocated
 		 * memory for this segment.
@@ -713,6 +723,14 @@ pg_get_shmem_allocations_numa(PG_FUNCTION_ARGS)
 			tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc,
 								 values, nulls);
 		}
+
+		/* The last entry is used for pages without a NUMA node. */
+		nulls[1] = true;
+		values[0] = CStringGetTextDatum(ent->key);
+		values[2] = Int64GetDatum(nodes[max_nodes + 1] * os_page_size);
+
+		tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc,
+							 values, nulls);
 	}
 
 	LWLockRelease(ShmemIndexLock);