Support huge_pages and huge_page_size on FreeBSD.

FreeBSD often uses huge (super) pages due to automatic promotion, but it
may be useful to be able to request that explicitly, and to be able to
experiment with different page sizes by explicit request.

Discussion: https://postgr.es/m/3043b674-46d6-a8e9-3811-5a3007c8dceb%40ww-it.cn

Author: macdice

doc/src/sgml/config.sgml

@@ -1850,9 +1850,9 @@ include_dir 'conf.d'
        </para>
 
        <para>
-        At present, this setting is supported only on Linux and Windows. The
+        At present, this setting is supported only on Linux, FreeBSD and Windows. The
         setting is ignored on other systems when set to
-        <literal>try</literal>.  On Linux, it is only supported when
+        <literal>try</literal>.  On Linux and FreeBSD, it is only supported when
         <varname>shared_memory_type</varname> is set to <literal>mmap</literal>
         (the default).
        </para>
@@ -1914,7 +1914,7 @@ include_dir 'conf.d'
         about usage and support, see <xref linkend="linux-huge-pages"/>.
        </para>
        <para>
-        Non-default settings are currently supported only on Linux.
+        Non-default settings are currently supported only on Linux and FreeBSD.
        </para>
       </listitem>
      </varlistentry>

src/backend/port/sysv_shmem.c

@@ -578,8 +578,9 @@ GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 bool
 check_huge_page_size(int *newval, void **extra, GucSource source)
 {
-#if !(defined(MAP_HUGE_MASK) && defined(MAP_HUGE_SHIFT))
-	/* Recent enough Linux only, for now.  See GetHugePageSize(). */
+#if !(defined(MAP_HUGE_MASK) && defined(MAP_HUGE_SHIFT)) && \
+	!defined(SHM_LARGEPAGE_ALLOC_DEFAULT)
+	/* Recent enough Linux and FreeBSD only, for now. */
 	if (*newval != 0)
 	{
 		GUC_check_errdetail("\"huge_page_size\" must be 0 on this platform.");
@@ -604,12 +605,9 @@ CreateAnonymousSegment(Size *size)
 	int			mmap_errno = 0;
 	int			mmap_flags = MAP_SHARED | MAP_ANONYMOUS | MAP_HASSEMAPHORE;
 
-#ifndef MAP_HUGETLB
-	/* PGSharedMemoryCreate should have dealt with this case */
-	Assert(huge_pages != HUGE_PAGES_ON);
-#else
 	if (huge_pages == HUGE_PAGES_ON || huge_pages == HUGE_PAGES_TRY)
 	{
+#ifdef MAP_HUGETLB
 		/*
 		 * Round up the request size to a suitable large value.
 		 */
@@ -627,8 +625,52 @@ CreateAnonymousSegment(Size *size)
 		if (huge_pages == HUGE_PAGES_TRY && ptr == MAP_FAILED)
 			elog(DEBUG1, "mmap(%zu) with MAP_HUGETLB failed, huge pages disabled: %m",
 				 allocsize);
-	}
 #endif
+#ifdef SHM_LARGEPAGE_ALLOC_DEFAULT
+		int			nsizes;
+		size_t	   *page_sizes;
+		size_t		page_size;
+		int			page_size_index = -1;
+
+		/*
+		 * Find the matching page size index, or if huge_page_size wasn't set,
+		 * then skip the smallest size and take the next one after that.
+		 */
+		nsizes = getpagesizes(NULL, 0);
+		page_sizes = palloc(nsizes * sizeof(*page_sizes));
+		getpagesizes(page_sizes, nsizes);
+		for (int i = 0; i < nsizes; ++i)
+		{
+			if (huge_page_size * 1024 == page_sizes[i] ||
+				(huge_page_size == 0 && i > 0))
+			{
+				page_size = page_sizes[i];
+				page_size_index = i;
+				if (allocsize % page_size != 0)
+					allocsize += page_size - (allocsize % page_size);
+				break;
+			}
+		}
+		pfree(page_sizes);
+		if (index >= 0)
+		{
+			int			fd;
+
+			fd = shm_create_largepage(SHM_ANON, O_RDWR, page_size_index,
+									  SHM_LARGEPAGE_ALLOC_DEFAULT, 0);
+			if (fd >= 0)
+			{
+				if (ftruncate(fd, allocsize) == 0)
+				{
+					ptr = mmap(NULL, allocsize, PROT_READ | PROT_WRITE,
+							   MAP_SHARED, fd, 0);
+					mmap_errno = errno;
+				}
+				close(fd);
+			}
+		}
+#endif
+	}
 
 	/*
 	 * Report whether huge pages are in use.  This needs to be tracked before
@@ -720,7 +762,7 @@ PGSharedMemoryCreate(Size size,
 						DataDir)));
 
 	/* Complain if hugepages demanded but we can't possibly support them */
-#if !defined(MAP_HUGETLB)
+#if !defined(MAP_HUGETLB) && !defined(SHM_LARGEPAGE_ALLOC_DEFAULT)
 	if (huge_pages == HUGE_PAGES_ON)
 		ereport(ERROR,
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),