Re: swapfile warnings

[Christian Brauner <christian.brauner@ubuntu.com>]

On Thu, Mar 25, 2021 at 08:49:26AM -0700, Darrick J. Wong wrote:
> 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...

Ah, that sounds good:

generic/357 -> "swapon: file has shared extents"
generic/495 -> "swapon: file has unallocated extents"

Ok, I'm a bit careful since I've been bitten by the swapfile thing and
wanted to make sure!

Thanks for the pointer!
Christian