btrfs-progs: filesystem: new subcommand mkswapfile

Add a command to create a new swapfile. The same can be achieved by
seandalone tools but they're just wrappers around the syscalls. The swap
format is simple enough to be created directly without mkswap command so
the swapfile can be created in one go.

The file must not exist before, this is to avoid problems with file
attributes or any other effects of existing extents. This also means the
command can't be used on block devices.

Default size is 2G, minimum size is 40KiB.

Signed-off-by: David Sterba <dsterba@suse.com>

Author: kdave

Documentation/btrfs-filesystem.rst

@@ -188,6 +188,23 @@ label [<device>|<mountpoint>] [<newlabel>]
                 The maximum allowable length shall be less than 256 chars and must not contain
                 a newline. The trailing newline is stripped automatically.
 
+mkswapfile [-s size] file
+        Create a new file that's suitable and formatted as a swapfile. Default
+        size is 2GiB, minimum size is 40KiB.
+
+        A swapfile must be created in a specific way: NOCOW and preallocated.
+        Subvolume containing a swapfile cannot be snapshotted and blocks of an
+        activated swapfile cannot be balanced.
+
+        Swapfile creation can be achieved by standalone commands too. Activation
+        needs to be done by command ``swapon(8)``.
+
+        ``Options``
+
+        -s|--size SIZE
+                Create swapfile of a given size SIZE (accepting k/m/g/e/p
+                suffix).
+
 resize [options] [<devid>:][+/-]<size>[kKmMgGtTpPeE]|[<devid>:]max <path>
         Resize a mounted filesystem identified by *path*. A particular device
         can be resized by specifying a *devid*.

cmds/filesystem.c

@@ -18,6 +18,7 @@
 #include <sys/ioctl.h>
 #include <sys/stat.h>
 #include <linux/version.h>
+#include <linux/fs.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -50,6 +51,7 @@
 #include "common/device-utils.h"
 #include "common/open-utils.h"
 #include "common/parse-utils.h"
+#include "common/string-utils.h"
 #include "common/filesystem-utils.h"
 #include "cmds/commands.h"
 #include "cmds/filesystem-usage.h"
@@ -1373,6 +1375,135 @@ static int cmd_filesystem_balance(const struct cmd_struct *unused,
 static DEFINE_COMMAND(filesystem_balance, "balance", cmd_filesystem_balance,
 		      cmd_filesystem_balance_usage, NULL, CMD_HIDDEN);
 
+static const char * const cmd_filesystem_mkswapfile_usage[] = {
+	"btrfs filesystem mkswapfile <file>",
+	"Force a sync on a filesystem",
+	HELPINFO_INSERT_GLOBALS,
+	HELPINFO_INSERT_VERBOSE,
+	HELPINFO_INSERT_QUIET,
+	NULL
+};
+
+/*
+ * Swap signature in the first 4KiB, v2:
+ *
+ * 00000400 .. = 01 00 00 00 ff ff 03 00  00 00 00 00 cb 70 8e 60
+ *                                                    ^^^^^^^^^^^
+ *                                                    uuid 4B
+ * 00000420 .. = 1d fb 4e ca be d4 3f 1f  6a 6b 0c 03 00 00 00 00
+ *               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+ *               uuid 8B
+ * 00000ff0 .. = 00 00 00 00 00 00 53 57  41 50 53 50 41 43 45 32
+ *                                  S  W   A  P  S  P  A  C  E  2
+ */
+static int write_swap_signature(int fd)
+{
+	int ret;
+	static unsigned char swap[4096] = {
+		[0x400] = 0x01,
+		[0x404] = 0xff,
+		[0x405] = 0xff,
+		[0x406] = 0x03,
+		/* 0x40c .. 0x42b UUID */
+		[0xff6] = 'S',
+		[0xff7] = 'W',
+		[0xff8] = 'A',
+		[0xff9] = 'P',
+		[0xffa] = 'S',
+		[0xffb] = 'P',
+		[0xffc] = 'A',
+		[0xffd] = 'C',
+		[0xffe] = 'E',
+		[0xfff] = '2',
+	};
+
+	uuid_generate(&swap[0x40c]);
+	ret = pwrite(fd, swap, 4096, 0);
+
+	return ret;
+}
+
+static int cmd_filesystem_mkswapfile(const struct cmd_struct *cmd, int argc, char **argv)
+{
+	int ret;
+	int fd;
+	const char *fname;
+	unsigned long flags;
+	u64 size = SZ_2G;
+
+	optind = 0;
+	while (1) {
+		int c;
+		static const struct option long_options[] = {
+			{ "size", required_argument, NULL, 's' },
+			{ NULL, 0, NULL, 0 }
+		};
+
+		c = getopt_long(argc, argv, "s:", long_options, NULL);
+		if (c < 0)
+			break;
+
+		switch (c) {
+		case 's':
+			size = parse_size_from_string(optarg);
+			/* Minimum limit reported by mkswap */
+			if (size < 40 * SZ_1K) {
+				error("swapfile needs to be at least 40 KiB");
+				return 1;
+			}
+			break;
+		default:
+			usage_unknown_option(cmd, argv);
+		}
+	}
+
+	if (check_argc_exact(argc - optind, 1))
+		return 1;
+
+	fname = argv[optind];
+	pr_verbose(LOG_INFO, "create file %s with mode 0600\n", fname);
+	fd = open(fname, O_RDWR | O_CREAT | O_EXCL, 0600);
+	if (fd < 0) {
+		error("cannot create new swapfile: %m");
+		return 1;
+	}
+	ret = ftruncate(fd, 0);
+	if (ret < 0) {
+		error("cannot truncate file: %m");
+		ret = 1;
+		goto out;
+	}
+	pr_verbose(LOG_INFO, "set NOCOW attribute\n");
+	flags = FS_NOCOW_FL;
+	ret = ioctl(fd, FS_IOC_SETFLAGS, &flags);
+	if (ret < 0) {
+		error("cannot set NOCOW flag: %m");
+		ret = 1;
+		goto out;
+	}
+	pr_verbose(LOG_INFO, "fallocate to size %llu\n", size);
+	ret = fallocate(fd, 0, 0, size);
+	if (ret < 0) {
+		error("cannot fallocate file: %m");
+		ret = 1;
+		goto out;
+	}
+	pr_verbose(LOG_INFO, "write swap signature\n");
+	ret = write_swap_signature(fd);
+	if (ret < 0) {
+		error("cannot write swap signature: %m");
+		ret = 1;
+		goto out;
+	}
+	pr_verbose(LOG_DEFAULT, "create swapfile %s size %s (%llu)\n",
+			fname, pretty_size_mode(size, UNITS_HUMAN), size);
+out:
+	close(fd);
+
+	return 0;
+}
+static DEFINE_SIMPLE_COMMAND(filesystem_mkswapfile, "mkswapfile");
+
 static const char filesystem_cmd_group_info[] =
 "overall filesystem tasks and information";
 
@@ -1387,6 +1518,7 @@ static const struct cmd_group filesystem_cmd_group = {
 		&cmd_struct_filesystem_resize,
 		&cmd_struct_filesystem_label,
 		&cmd_struct_filesystem_usage,
+		&cmd_struct_filesystem_mkswapfile,
 		NULL
 	}
 };