On 19/08/2020 12:29, David Sterba wrote:

How often could the snapshot deletion and swapfile activation happen at
the same time? Snapshotting subvolume with the swapfile requires
deactivation, snapshot/send/whatever and then activation. This sounds
like a realistic usecase.

It is very likely when the swapfile is one that is only used
occasionally (for example, when running a particular program which needs
a massive amount of virtual memory, or having to stop using a different
swapfile because a disk looks like it is starting to fail).

If the swapfile is not normally used, it is not unlikely it got
snapshotted (as part of a larger operation, presumably) while
deactivated. When the user tries to use it, they realise it isn't
working because it is snapshotted, so they delete the snapshot and then
immediately try to activate it again -- causing confusion when it still
fails.