swapon: be more explicit about BTRFS

It seems nocow and nodatasum attributes are required.
See btrfs_swap_activate() in  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ed46ff3d423780fa5173b38a844bf0fdb210a2a7

Addresses: https://github.com/karelzak/util-linux/issues/633
Signed-off-by: Karel Zak <kzak@redhat.com>

diff --git a/sys-utils/swapon.8 b/sys-utils/swapon.8
index 60e4d4feb11c4f8823c84d1bc937b6ac47d0a9c0..d645850bb72ed705793ee988088fe927b534da67 100644 (file)
--- a/sys-utils/swapon.8
+++ b/sys-utils/swapon.8
@@ -207,9 +207,10 @@ The most portable solution to create a swap file is to use
 .BR dd (1)
 and /dev/zero in these cases.
 .SS Btrfs
-The swap files on btrfs are supported since Linux 5.0.  The previous versions use
-copy-on-write semantic for the swap files, it means that the file location is may not be static
-and corruption can result.
+The swap files on btrfs are supported since Linux 5.0 on files with nocow and
+nodatasum attributes and without compression.  The previous versions may use
+copy-on-write semantic for the swap files, it means that the file location is
+may not be static and corruption can result.
 .sp
 One possible workaround is to map the swap file to a loopback device.  This will allow the filesystem
 to determine the mapping properly but may come with a performance impact.