On Fri,  9 Jul 2021 10:50:48 -0700 Evan Green [off-list ref] wrote:

Currently it's not possible to enable hibernation without also enabling
generic swap for a given swap area. These two use cases are not the
same. For example there may be users who want to enable hibernation,
but whose drives don't have the write endurance for generic swap
activities.

Add a new SWAP_FLAG_NOSWAP that adds a swap region but refuses to allow
generic swapping to it. This region can still be wired up for use in
suspend-to-disk activities, but will never have regular pages swapped to
it.

Swap regions with SWAP_FLAG_NOSWAP set will not appear in /proc/meminfo
under SwapTotal and SwapFree, since they are not usable as general swap.

This patch doesn't appear to set SWAP_FLAG_NOSWAP anywhere.  Perhaps
there's another patch somewhere which changes the hibernation code?  If
so, can we please have both patches in a series?

Once we have a description of how this thing gets set, please let's
discuss what happens if someone tries to enable generic swap onto that
device after hibernation has set SWAP_FLAG_NOSWAP (I'm basically
guessing now).  Will it work?  Is there a backward-compatibility issue
here?