How to fix a bad swapfile

I recently saw a bunch of those warnings when booting Arch Linux:

$ systemctl status swapfile.swap | grep -i invalid
Jun 12 12:34:25 m604 swapon[22623]: swapon: /swapfile: swapon failed: Invalid argument

This was weird since the swapfile worked fine before and the file existed and had secure permissions:

$ stat /swapfile
  File: /swapfile
  Size: 8589934592      Blocks: 16777224   IO Block: 4096   regular file
Device: fe01h/65025d    Inode: 16          Links: 1
Access: (0600/-rw-------)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2020-06-12 08:52:50.974137402 +0200
Modify: 2019-12-28 10:16:06.886603444 +0100
Change: 2019-12-28 10:16:06.886603444 +0100
 Birth: 2016-12-28 12:24:34.180000236 +0100

journalctl came to my rescue and showed an error message the indicated what was wrong:

$ journalctl --boot --priority 0..3 | grep holes | head -n1
Jun 13 13:10:27 m604 kernel: swapon: swapfile has holes

Never heard of files having holes before, but everyday you learn something knew. The man pages of swapon and mkswap cleared things up:

$ man swapon | grep holes -A4
   Files with holes
       The  swap file implementation in the kernel expects to be able to write to the file directly, without the assistance of the
       filesystem.  This is a problem on files with holes or on copy-on-write files on filesystems like Btrfs.
       Commands like cp(1) or truncate(1) create files with holes.  These files will be rejected by swapon.
       Preallocated files created by fallocate(1) may be interpreted as files with holes too depending of the filesystem.   Preal‐
       located swap files are supported on XFS since Linux 4.18.
       The most portable solution to create a swap file is to use dd(1) and /dev/zero.

From man mkswap:

 To set up a swap file, it is necessary to create that file before initializing it with mkswap, e.g. using a command like

        # dd if=/dev/zero of=swapfile bs=1MiB count=$((8*1024))

 to create 8GiB swapfile.

So, you can not use fallocate to preallocate the space for a swap file on ext4, at least it is not reliable. I used this command to recreate the file:

$ sudo dd if=/dev/zero of=swapfile bs=1MiB count=$((8*1024)) status=progress && sync && sudo chmod 0600 /swapfile && sudo mkswap /swapfile

klingtnet/swapon-failed.md

How to fix a bad swapfile