Re: [PATCH v2] mm: Enable suspend-only swap spaces

[Andrew Morton]

On Fri,  9 Jul 2021 10:50:48 -0700 Evan Green <evgreen@xxxxxxxxxxxx> 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?