From: Karel Zak <kzak@redhat.com>
Date: Wed, 29 Jan 2025 11:05:10 +0000 (+0100)
Subject: mkswap: improve --file option for use on btrfs
X-Git-Tag: v2.42-start~65^2~3
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=ec3ecd0141c62a93a8ff221746251e96bdbe2975;p=thirdparty%2Futil-linux.git

mkswap: improve --file option for use on btrfs

We will now try to set the "nocow" attribute, but will ignore it if it
is not supported.

Suggested-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Signed-off-by: Karel Zak <kzak@redhat.com>
---

diff --git a/disk-utils/mkswap.8.adoc b/disk-utils/mkswap.8.adoc
index 2aba655de..70d0fb315 100644
--- a/disk-utils/mkswap.8.adoc
+++ b/disk-utils/mkswap.8.adoc
@@ -114,6 +114,8 @@ If you don't know the page size that your machine uses, you can look it up with
 
 Aside from *mkswap --file*, it is also possible to create the swapfile manually before initializing it with *mkswap*, e.g. using a command like
 
+Since version 2.41, *mkswap --file* sets the nocow attribute for newly created files to support swapfiles on Btrfs.
+
 ....
 # dd if=/dev/zero of=swapfile bs=1MiB count=$((8*1024))
 ....
diff --git a/disk-utils/mkswap.c b/disk-utils/mkswap.c
index b1f3a3285..08db89b2c 100644
--- a/disk-utils/mkswap.c
+++ b/disk-utils/mkswap.c
@@ -33,8 +33,10 @@
 # include <selinux/context.h>
 # include "selinux-utils.h"
 #endif
-#ifdef HAVE_LINUX_FIEMAP_H
+#ifdef HAVE_LINUX_FS_H
 # include <linux/fs.h>
+#endif
+#ifdef HAVE_LINUX_FIEMAP_H
 # include <linux/fiemap.h>
 #endif
 
@@ -405,7 +407,21 @@ static void open_device(struct mkswap_control *ctl)
 	}
 	if (ctl->fd < 0)
 		err(EXIT_FAILURE, _("cannot open %s"), ctl->devname);
+
 	if (ctl->file && ctl->filesz) {
+		int attr = 0;
+
+		/* Let's attempt to set the "nocow" attribute for Btrfs, etc.
+		 * Ignore if unsupported. */
+#if defined(FS_IOC_GETFLAGS) && defined(FS_NOCOW_FL)
+		if (ioctl(ctl->fd, FS_IOC_GETFLAGS, &attr) == 0) {
+			attr |= FS_NOCOW_FL;
+			if (ioctl(ctl->fd, FS_IOC_SETFLAGS, &attr) < 0 &&
+			    (errno != EOPNOTSUPP && errno != ENOSYS && errno != EINVAL))
+				warn(_("failed to set 'nocow' attribute"));
+		}
+#endif
+
 		if (ftruncate(ctl->fd, ctl->filesz) < 0)
 			err(EXIT_FAILURE, _("couldn't allocate swap file %s"), ctl->devname);
 #ifdef HAVE_POSIX_FALLOCATE
diff --git a/sys-utils/swapon.8.adoc b/sys-utils/swapon.8.adoc
index 4335d7f86..a25309d18 100644
--- a/sys-utils/swapon.8.adoc
+++ b/sys-utils/swapon.8.adoc
@@ -152,6 +152,8 @@ The most portable solution to create a swap file is to use *dd*(1) and _/dev/zer
 
 Swap files on Btrfs are supported since Linux 5.0 on files with *nocow* attribute. See the *btrfs*(5) manual page for more details.
 
+Since version 2.41, the command *mkswap --file* can create a new swap file with the *nocow* attribute.
+
 === NFS
 
 Swap over *NFS* may not work.