Add GUC parameter "huge_pages_status"

This is useful to show the allocation state of huge pages when setting
up a server with "huge_pages = try", where allocating huge pages would
be attempted but the server would continue its startup sequence even if
the allocation fails.  The effective status of huge pages is not easily
visible without OS-level tools (or for instance, a lookup at
/proc/N/smaps), and the environments where Postgres runs may not
authorize that.  Like the other GUCs related to huge pages, this works
for Linux and Windows.

This GUC can report as values:
- "on", if huge pages were allocated.
- "off", if huge pages were not allocated.
- "unknown", a special state that could only be seen when using for
example postgres -C because it is only possible to know if the shared
memory allocation worked after we can check for the GUC values, even if
checking a runtime-computed GUC.  This value should never be seen when
querying for the GUC on a running server.  An assertion is added to
check that.

The discussion has also turned around having a new function to grab this
status, but this would have required more tricks for -DEXEC_BACKEND,
something that GUCs already handle.

Noriyoshi Shinoda has initiated the thread that has led to the result of
this commit.

Author: Justin Pryzby
Reviewed-by: Nathan Bossart, Kyotaro Horiguchi, Michael Paquier
Discussion: https://postgr.es/m/TU4PR8401MB1152EBB0D271F827E2E37A01EECC9@TU4PR8401MB1152.NAMPRD84.PROD.OUTLOOK.COM

doc/src/sgml/config.sgml

@@ -1727,7 +1727,9 @@ include_dir 'conf.d'
         server will try to request huge pages, but fall back to the default if
         that fails. With <literal>on</literal>, failure to request huge pages
         will prevent the server from starting up. With <literal>off</literal>,
-        huge pages will not be requested.
+        huge pages will not be requested. The actual state of huge pages is
+        indicated by the server variable
+        <xref linkend="guc-huge-pages-status"/>.
        </para>
 
        <para>
@@ -10738,6 +10740,25 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-huge-pages-status" xreflabel="huge_pages_status">
+      <term><varname>huge_pages_status</varname> (<type>enum</type>)
+      <indexterm>
+       <primary><varname>huge_pages_status</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the state of huge pages in the current instance:
+        <literal>on</literal>, <literal>off</literal>, or
+        <literal>unknown</literal> (if displayed with
+        <literal>postgres -C</literal>).
+        This parameter is useful to determine whether allocation of huge pages
+        was successful under <literal>huge_pages=try</literal>.
+        See <xref linkend="guc-huge-pages"/> for more information.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-integer-datetimes" xreflabel="integer_datetimes">
       <term><varname>integer_datetimes</varname> (<type>boolean</type>)
       <indexterm>

src/backend/port/sysv_shmem.c

@@ -627,6 +627,14 @@ CreateAnonymousSegment(Size *size)
 	}
 #endif
 
+	/*
+	 * Report whether huge pages are in use.  This needs to be tracked before
+	 * the second mmap() call if attempting to use huge pages failed
+	 * previously.
+	 */
+	SetConfigOption("huge_pages_status", (ptr == MAP_FAILED) ? "off" : "on",
+					PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
+
 	if (ptr == MAP_FAILED && huge_pages != HUGE_PAGES_ON)
 	{
 		/*
@@ -737,8 +745,14 @@ PGSharedMemoryCreate(Size size,
 		sysvsize = sizeof(PGShmemHeader);
 	}
 	else
+	{
 		sysvsize = size;
 
+		/* huge pages are only available with mmap */
+		SetConfigOption("huge_pages_status", "off",
+						PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
+	}
+
 	/*
 	 * Loop till we find a free IPC key.  Trust CreateDataDirLockFile() to
 	 * ensure no more than one postmaster per data directory can enter this

src/backend/port/win32_shmem.c

@@ -401,6 +401,11 @@ retry:
 	on_shmem_exit(pgwin32_SharedMemoryDelete, PointerGetDatum(hmap2));
 
 	*shim = hdr;
+
+	/* Report whether huge pages are in use */
+	SetConfigOption("huge_pages_status", (flProtect & SEC_LARGE_PAGES) ?
+					"on" : "off", PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
+
 	return hdr;
 }
 

src/backend/storage/ipc/ipci.c

@@ -190,6 +190,13 @@ CreateSharedMemoryAndSemaphores(void)
 		 */
 		seghdr = PGSharedMemoryCreate(size, &shim);
 
+		/*
+		 * Make sure that huge pages are never reported as "unknown" while the
+		 * server is running.
+		 */
+		Assert(strcmp("unknown",
+					  GetConfigOption("huge_pages_status", false, false)) != 0);
+
 		InitShmemAccess(seghdr);
 
 		/*

src/backend/utils/misc/guc_tables.c

@@ -365,6 +365,13 @@ static const struct config_enum_entry huge_pages_options[] = {
 	{NULL, 0, false}
 };
 
+static const struct config_enum_entry huge_pages_status_options[] = {
+	{"off", HUGE_PAGES_OFF, false},
+	{"on", HUGE_PAGES_ON, false},
+	{"unknown", HUGE_PAGES_UNKNOWN, false},
+	{NULL, 0, false}
+};
+
 static const struct config_enum_entry recovery_prefetch_options[] = {
 	{"off", RECOVERY_PREFETCH_OFF, false},
 	{"on", RECOVERY_PREFETCH_ON, false},
@@ -550,6 +557,7 @@ int			ssl_renegotiation_limit;
  */
 int			huge_pages = HUGE_PAGES_TRY;
 int			huge_page_size;
+int			huge_pages_status = HUGE_PAGES_UNKNOWN;
 
 /*
  * These variables are all dummies that don't do anything, except in some
@@ -4876,6 +4884,17 @@ struct config_enum ConfigureNamesEnum[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"huge_pages_status", PGC_INTERNAL, PRESET_OPTIONS,
+			gettext_noop("Indicates the status of huge pages."),
+			NULL,
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+		},
+		&huge_pages_status,
+		HUGE_PAGES_UNKNOWN, huge_pages_status_options,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"recovery_prefetch", PGC_SIGHUP, WAL_RECOVERY,
 			gettext_noop("Prefetch referenced blocks during recovery."),

src/include/storage/pg_shmem.h

@@ -46,12 +46,13 @@ extern PGDLLIMPORT int shared_memory_type;
 extern PGDLLIMPORT int huge_pages;
 extern PGDLLIMPORT int huge_page_size;
 
-/* Possible values for huge_pages */
+/* Possible values for huge_pages and huge_pages_status */
 typedef enum
 {
 	HUGE_PAGES_OFF,
 	HUGE_PAGES_ON,
-	HUGE_PAGES_TRY
+	HUGE_PAGES_TRY,				/* only for huge_pages */
+	HUGE_PAGES_UNKNOWN			/* only for huge_pages_status */
 }			HugePagesType;
 
 /* Possible values for shared_memory_type */

src/test/authentication/t/003_peer.pl

@@ -100,6 +100,12 @@ my $system_user =
   $node->safe_psql('postgres',
 	q(select (string_to_array(SYSTEM_USER, ':'))[2]));
 
+# While on it, check the status of huge pages, that can be either on
+# or off, but never unknown.
+my $huge_pages_status =
+  $node->safe_psql('postgres', q(SHOW huge_pages_status;));
+isnt($huge_pages_status, 'unknown', "check huge_pages_status");
+
 # Tests without the user name map.
 # Failure as connection is attempted with a database role not mapping
 # to an authorized system user.

src/test/authentication/t/005_sspi.pl

@@ -21,6 +21,10 @@ $node->init;
 $node->append_conf('postgresql.conf', "log_connections = on\n");
 $node->start;
 
+my $huge_pages_status =
+  $node->safe_psql('postgres', q(SHOW huge_pages_status;));
+isnt($huge_pages_status, 'unknown', "check huge_pages_status");
+
 # SSPI is set up by default.  Make sure it interacts correctly with
 # require_auth.
 $node->connect_ok("require_auth=sspi",