From 217b01b7eddabb5d863d50b1d4ca8e1019d9413c Mon Sep 17 00:00:00 2001
From: Justin Pryzby <pryzbyj@telsasoft.com>
Date: Mon, 23 Jan 2023 18:33:51 -0600
Subject: [PATCH v2] add GUC: huge_pages_active

This is useful to show the current state of huge pages when
huge_pages=try.  The effective status is not otherwise visible without
OS level tools like gdb or /proc/N/smaps.
---
 doc/src/sgml/config.sgml            | 17 ++++++++++++++++-
 src/backend/port/sysv_shmem.c       |  5 ++++-
 src/backend/port/win32_shmem.c      |  5 ++++-
 src/backend/utils/misc/guc_tables.c | 13 +++++++++++++
 4 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index d190be1925d..fcef4e3ce8e 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -1708,7 +1708,8 @@ 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-active"/>.
        </para>
 
        <para>
@@ -10687,6 +10688,20 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-huge-pages-active" xreflabel="huge_pages_active">
+      <term><varname>huge_pages_active</varname> (<type>boolean</type>)
+      <indexterm>
+       <primary><varname>huge_pages_active</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports whether huge pages are in use by the current process.
+        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>
diff --git a/src/backend/port/sysv_shmem.c b/src/backend/port/sysv_shmem.c
index eaba244bc9c..9b9b25d53ba 100644
--- a/src/backend/port/sysv_shmem.c
+++ b/src/backend/port/sysv_shmem.c
@@ -621,7 +621,10 @@ CreateAnonymousSegment(Size *size)
 		ptr = mmap(NULL, allocsize, PROT_READ | PROT_WRITE,
 				   PG_MMAP_FLAGS | mmap_flags, -1, 0);
 		mmap_errno = errno;
-		if (huge_pages == HUGE_PAGES_TRY && ptr == MAP_FAILED)
+		if (ptr != MAP_FAILED)
+			SetConfigOption("huge_pages_active", "on",
+					PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
+		else if (huge_pages == HUGE_PAGES_TRY)
 			elog(DEBUG1, "mmap(%zu) with MAP_HUGETLB failed, huge pages disabled: %m",
 				 allocsize);
 	}
diff --git a/src/backend/port/win32_shmem.c b/src/backend/port/win32_shmem.c
index 62e08074770..73472a18f25 100644
--- a/src/backend/port/win32_shmem.c
+++ b/src/backend/port/win32_shmem.c
@@ -290,7 +290,10 @@ retry:
 								 size_low,	/* Size Lower 32 bits */
 								 szShareMem);
 
-		if (!hmap)
+		if (hmap)
+			SetConfigOption("huge_pages_active", "on",
+					PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
+		else
 		{
 			if (GetLastError() == ERROR_NO_SYSTEM_RESOURCES &&
 				huge_pages == HUGE_PAGES_TRY &&
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index b21698934c8..1e8e67a4c54 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -571,6 +571,7 @@ static int	shared_memory_size_mb;
 static int	shared_memory_size_in_huge_pages;
 static int	wal_block_size;
 static bool data_checksums;
+static bool huge_pages_active = false; /* dynamically set */
 static bool integer_datetimes;
 
 #ifdef USE_ASSERT_CHECKING
@@ -1013,6 +1014,18 @@ struct config_bool ConfigureNamesBool[] =
 		true,
 		NULL, NULL, NULL
 	},
+
+	{
+		{"huge_pages_active", PGC_INTERNAL, PRESET_OPTIONS,
+			gettext_noop("Indicates whether huge pages are in use."),
+			NULL,
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
+		},
+		&huge_pages_active,
+		false,
+		NULL, NULL, NULL
+	},
+
 	{
 		/* Not for general use --- used by SET SESSION AUTHORIZATION */
 		{"is_superuser", PGC_INTERNAL, UNGROUPED,
-- 
2.25.1