Add Linux posix_fadvise support

The purpose of this PR is to accepts fadvise ioctl from userland
to do read-ahead by demand.

It could dramatically improve sequential read performance especially
when primarycache is set to metadata or zfs_prefetch_disable is 1.

If the file is mmaped, generic_fadvise is also called for page cache
read-ahead besides dmu_prefetch.

Only POSIX_FADV_WILLNEED and POSIX_FADV_SEQUENTIAL are supported in
this PR currently.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Finix Yan <yancw@info2soft.com>
Closes #13694

tests/zfs-tests/cmd/.gitignore

@@ -4,6 +4,7 @@
 /devname2devid
 /dir_rd_update
 /draid
+/file_fadvise
 /file_append
 /file_check
 /file_trunc

tests/zfs-tests/cmd/Makefile.am

@@ -128,4 +128,7 @@ scripts_zfs_tests_bin_PROGRAMS  += %D%/read_dos_attributes %D%/write_dos_attribu
 
 scripts_zfs_tests_bin_PROGRAMS += %D%/randfree_file
 %C%_randfree_file_SOURCES       = %D%/file/randfree_file.c
+
+scripts_zfs_tests_bin_PROGRAMS += %D%/file_fadvise
+%C%_file_fadvise_SOURCES  = %D%/file/file_fadvise.c
 endif

tests/zfs-tests/cmd/file/file_fadvise.c

@@ -0,0 +1,97 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or https://opensource.org/licenses/CDDL-1.0.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+
+/*
+ * Copyright 2007 Sun Microsystems, Inc.  All rights reserved.
+ * Use is subject to license terms.
+ */
+
+/*
+ * Copyright (c) 2022 by Information2 Software, Inc. All rights reserved.
+ */
+
+#include "file_common.h"
+#include <sys/types.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <string.h>
+
+/*
+ * Call fadvise to prefetch data
+ */
+static const char *execname = "file_fadvise";
+
+static void
+usage(void)
+{
+	(void) fprintf(stderr,
+	    "usage: %s -f filename -a advise \n", execname);
+}
+
+int
+main(int argc, char *argv[])
+{
+	char *filename = NULL;
+	int advise = 0;
+	int fd, ch;
+	int	err = 0;
+
+	while ((ch = getopt(argc, argv, "a:f:")) != EOF) {
+		switch (ch) {
+		case 'a':
+			advise = atoll(optarg);
+			break;
+		case 'f':
+			filename = optarg;
+			break;
+		case '?':
+			(void) printf("unknown arg %c\n", optopt);
+			usage();
+			break;
+		}
+	}
+
+	if (!filename) {
+		(void) printf("Filename not specified (-f <file>)\n");
+		err++;
+	}
+
+	if (advise < POSIX_FADV_NORMAL || advise > POSIX_FADV_NOREUSE) {
+		(void) printf("advise is invalid\n");
+		err++;
+	}
+
+	if (err) {
+		usage(); /* no return */
+		return (1);
+	}
+
+	if ((fd = open(filename, O_RDWR, 0666)) < 0) {
+		perror("open");
+		return (1);
+	}
+
+	posix_fadvise(fd, 0, 0, advise);
+
+	close(fd);
+
+	return (0);
+}