Re: swapfile warnings

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Mar 25, 2021 at 04:41:29PM +0100, Christian Brauner wrote:
> Hey,
> 
> I've been running xfstests on a v5.12-rc3 kernel using a swapfile:
> 
> ubuntu@edfu|~
> > cat /etc/fstab
> # /etc/fstab: static file system information.
> #
> # Use 'blkid' to print the universally unique identifier for a
> # device; this may be used with UUID= as a more robust way to name devices
> # that works even if disks are added and removed. See fstab(5).
> #
> # <file system> <mount point>   <type>  <options>       <dump>  <pass>
> # / was on /dev/sdb1 during curtin installation
> /dev/disk/by-uuid/f2c4df25-b55c-4d1b-95fa-059f72ef86eb / ext4 defaults 0 0
> /swap.img       none    swap    sw      0       0
> 
> where my rootfs is ext4:
> 
> ubuntu@edfu|~
> > findmnt | grep -i ext4
> /                                     /dev/sdb1              ext4       rw,relatime
> 
> In addition to the /dev/sdb disk I have three other disks attached:
> 
> ubuntu@edfu|~
> > lsblk
> NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
> loop0    7:0    0  55.5M  1 loop /snap/core18/1988
> loop1    7:1    0  69.9M  1 loop /snap/lxd/19188
> loop2    7:2    0  70.4M  1 loop /snap/lxd/19647
> loop3    7:3    0  32.3M  1 loop /snap/snapd/11107
> loop4    7:4    0  32.3M  1 loop /snap/snapd/11402
> sda      8:0    0 931.5G  0 disk
> sdb      8:16   0 223.6G  0 disk
> └─sdb1   8:17   0 223.6G  0 part /
> sdc      8:32   0 279.5G  0 disk
> sdd      8:48   0 931.5G  0 disk
> 
> I'm using two of them /dev/sdc and /dev/sdb for xfstests:
> 
> ubuntu@edfu|~/src/git/xfstests
> > cat local.config
> # Ideally define at least these 4 to match your environment
> # The first 2 are required.
> # See README for other variables which can be set.
> #
> # Note: SCRATCH_DEV >will< get overwritten!
> 
> export FSTYP=xfs
> export TEST_DEV=/dev/sdc
> export TEST_DIR=/mnt/test
> export SCRATCH_DEV=/dev/sdd
> export SCRATCH_MNT=/mnt/scratch
> 
> when the system gets under load during test runs I assume it sometimes
> swaps and so sometimes I see the following messages pop up:
> 
> > sudo journalctl | grep -i swapon
> Mar 21 00:00:58 edfu kernel: swapon: file has shared extents
> Mar 21 02:25:52 edfu kernel: swapon: file has unallocated extents
> Mar 21 21:38:12 edfu kernel: swapon: file has shared extents
> Mar 22 00:16:03 edfu kernel: swapon: file has unallocated extents
> Mar 23 11:49:46 edfu kernel: swapon: file has shared extents
> Mar 23 12:34:38 edfu kernel: swapon: file has unallocated extents
> 
> I don't know enough about swapfiles to make any deep observations but I
> wonder what this can be caused by and whether this is something to worry
> about.

That's ... probably just fstests performing regression tests of the
swapfile activation code to make sure that we only ever feed the mm
file extents that are mapped and stable.

If you run only these tests:
generic/356 generic/357 generic/472 generic/493 generic/494 generic/495
generic/496 generic/497 generic/554 generic/569 generic/570

Do you get the kernel messages right away?  Those are the swapfile
stressors that I know about...

--D

> 
> Christian