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

[Andrew Morton]

On Fri, 9 Jul 2021 16:23:18 -0700 Evan Green <evgreen@xxxxxxxxxxxx> wrote:

> On Fri, Jul 9, 2021 at 3:20 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > 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?
> 
> There's no other patch, in the kernel at least. SWAP_FLAG_* is exposed
> to usermode, which would set it when calling swapon(2). Once this
> patch is accepted, I'll have to add the option into util-linux [1], so
> that I can use it in my init scripts.
> 
> Said a different way, this patch isn't about altering how hibernate
> behaves, but about giving usermode the freedom to set up hibernate and
> swap independently.

OK, can we please get this into the changelog?  And it would be helpful
to describe how this will be invoked via swapon(8).

And I expect an update to the swapon syscall's manpage will be in order.