diff --git a/src/backend/storage/ipc/shmem.c b/src/backend/storage/ipc/shmem.c
index 9fe3ea251bc..ca3b31d9b9c 100644
--- a/src/backend/storage/ipc/shmem.c
+++ b/src/backend/storage/ipc/shmem.c
@@ -336,6 +336,8 @@ ShmemRequestStructWithOpts(const ShmemStructOpts *options)
 /*
  * Internal workhorse of ShmemRequestStruct() and ShmemRequestHash().
  *
+ * Validates the shared memory request options and registers the request.
+ * 
  * Note: Unlike in the public ShmemRequestStruct() and ShmemRequestHash()
  * functions, 'options' is *not* copied.  It must be allocated in
  * TopMemoryContext by the caller, and will be freed after the init/attach
@@ -356,6 +358,10 @@ ShmemRequestInternal(ShmemStructOpts *options, ShmemRequestKind kind)
 	if (options->name == NULL)
 		elog(ERROR, "shared memory request is missing 'name' option");
 
+	/*
+	 * A request with SHMEM_ATTACH_UNKNOWN_SIZE is valid after startup. Accept it
+	 * here.
+	 */
 	if (options->size <= 0 && options->size != SHMEM_ATTACH_UNKNOWN_SIZE)
 		elog(ERROR, "invalid size %zd for shared memory request for \"%s\"",
 			 options->size, options->name);
@@ -387,8 +393,8 @@ ShmemRequestInternal(ShmemStructOpts *options, ShmemRequestKind kind)
  *	ShmemGetRequestedSize() --- estimate the total size of all registered shared
  *                              memory structures.
  *
- * This is called when sizing a new segment at postmaster or standalone
- * startup, including a postmaster crash restart.
+ * This is called before creating the main shared memory segment at postmaster or
+ * standalone startup, and a postmaster crash restart.
  */
 size_t
 ShmemGetRequestedSize(void)
@@ -405,6 +411,7 @@ ShmemGetRequestedSize(void)
 	{
 		size_t		alignment = request->options->alignment;
 
+		/* We can't estimate the total size when the size is unknown. */
 		if (request->options->size == SHMEM_ATTACH_UNKNOWN_SIZE)
 			elog(ERROR, "SHMEM_ATTACH_UNKNOWN_SIZE cannot be used during startup");
 
diff --git a/src/test/modules/test_shmem/t/001_late_shmem_alloc.pl b/src/test/modules/test_shmem/t/001_late_shmem_alloc.pl
index 18e4b2f33bd..06796a7934c 100644
--- a/src/test/modules/test_shmem/t/001_late_shmem_alloc.pl
+++ b/src/test/modules/test_shmem/t/001_late_shmem_alloc.pl
@@ -21,29 +21,24 @@ $node->stop;
 ###
 $node->start;
 
-# This first call to the function after startup loads the library
-# and initializes the shmem area.
+# Check that the attach counter is incremented on a new connection
 my $attach_count1 =
   $node->safe_psql("postgres", "SELECT get_test_shmem_attach_count();");
-
-# Check that the attach counter is incremented on a new connection
 my $attach_count2 =
   $node->safe_psql("postgres", "SELECT get_test_shmem_attach_count();");
 cmp_ok($attach_count2, '>', $attach_count1,
 	"attach callback is called in each backend");
 
-# Allocate another shmem area, after the library is loaded.
-my $stderr;
+# Allocate and attach to a new shared memory area after startup
 my $res = $node->safe_psql("postgres",
 	"SELECT test_shmem_register('test_shmem after startup', 20, 1);");
 is($res, 0, 'allocate after startup');
-
-# Test attaching to it again
 $res = $node->safe_psql("postgres",
 	"SELECT test_shmem_register('test_shmem after startup', 20, 2);");
 is($res, 1, 'attach after startup');
 
 # If the size doesn't match when attaching, you get an error
+my $stderr;
 (undef, undef, $stderr) =
   $node->psql("postgres", "SELECT test_shmem_register('test_shmem after startup', 25, 3);");
 like(
@@ -70,7 +65,7 @@ like($stderr, qr/shared memory size must be at least \d+ bytes/,
 $node->stop;
 
 ###
-# Test that loading via shared_preload_libraries works
+# Test that loading via shared_preload_libraries also works
 ###
 $node->append_conf('postgresql.conf',
 	"shared_preload_libraries = 'test_shmem'");
@@ -99,7 +94,7 @@ else
 }
 
 $node->stop;
-$node->adjust_conf('postgresql.conf', 'shared_preload_libraries', undef);
+$node->adjust_conf('postgresql.conf', "shared_preload_libraries", undef);
 
 ###
 # Test a failure in initializing the shared memory area
@@ -175,6 +170,7 @@ SKIP:
 	skip 'single-user test is not supported by this platform', 5
 	  if $windows_os;
 
+	# Common part of single-user mode command
 	my @command = (
 		'postgres', '--single', '-F',
 		'-D' => $node->data_dir);
@@ -221,8 +217,4 @@ SELECT test_shmem_register('test_shmem after error', -1, 8);
 		"unknown-size requests are rejected during single-user startup");
 }
 
-# clean up
-$node->stop;
-$node->adjust_conf('postgresql.conf', "shared_preload_libraries", undef);
-
 done_testing();