docs: unify mkswap(8) with swapon(8) about holes warning

Reviewed-by: Dave Rutherford <dave@evilpettingzoo.com>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>

diff --git a/disk-utils/mkswap.8 b/disk-utils/mkswap.8
index 0a0b715dd58b6cde4ea4f84a020aeca9d0fc5857..3655357bd03616c6fc66b0cfd66e0ecb8ebc5dd7 100644 (file)
--- a/disk-utils/mkswap.8
+++ b/disk-utils/mkswap.8
@@ -128,9 +128,19 @@ e.g.\& using a command like
 .RE
 .fi
 
-Note that a swap file must not contain any holes (so, using
+Note that a swap file must not contain any holes.  Using
 .BR cp (1)
-to create the file is not acceptable).
+to create the file is not acceptable.  Neither is use of
+.BR fallocate (1)
+on file systems that support preallocated files, such as
+.BR XFS " or " ext4 ,
+or on copy-on-write filesystems like
+.BR btrfs .
+It is recommended to use
+.BR dd (1)
+and /dev/zero in these cases.  Please read notes from
+.BR swapon (8)
+before adding a swap file to copy-on-write filesystems.
 
 .SH ENVIRONMENT
 .IP LIBBLKID_DEBUG=all