Re: [PATCH 1/2] Documentation: clarify limitations of hibernation

[Luigi Semenzato]

On Mon, Jan 27, 2020 at 8:28 AM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
>
> On Mon, Jan 27, 2020 at 5:13 PM Luigi Semenzato <semenzato@xxxxxxxxxx> wrote:
> >
> > On Mon, Jan 27, 2020 at 6:16 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > >
> > > On Fri 24-01-20 08:37:12, Luigi Semenzato wrote:
> > > [...]
> > > > The purpose of my documentation patch was to make it clearer that
> > > > hibernation may fail in situations in which suspend-to-RAM works; for
> > > > instance, when there is no swap, and anonymous pages are over 50% of
> > > > total RAM.  I will send a new version of the patch which hopefully
> > > > makes this clearer.
> > >
> > > I was under impression that s2disk is pretty much impossible without any
> > > swap.
> >
> > I am not sure what you mean by "swap" here.  S2disk needs a swap
> > partition for storing the image, but that partition is not used for
> > regular swap.
>
> That's not correct.
>
> The swap partition (or file) used by s2disk needs to be made active
> before it can use it and the mm subsystem is also able to use it for
> regular swap then.

OK---I had this wrong, thanks.

> >  If there is no swap, but more than 50% of RAM is free
> > or reclaimable, s2disk works fine.  If anonymous is more than 50%,
> > hibernation can still work, but swap needs to be set up (in addition
> > to the space for the hibernation image).  The setup is not obvious and
> > I don't think that the documentation is clear on this.
>
> Well, the entire contents of RAM must be preserved, this way or
> another, during hibernation.  That should be totally obvious to anyone
> using it really.

Yes, that's obvious.

> Some of the RAM contents is copies of data already there in the
> filesystems on persistent storage and that does not need to be saved
> again.  Everything else must be saved and s2disk (and Linux
> hibernation in general) uses active swap space to save these things.
> This implies that in order to hibernate the system, you generally need
> the amount of swap space equal to the size of RAM minus the size of
> files mapped into memory.
>
> So, to be on the safe side, the total amount of swap space to be used
> for hibernation needs to match the size of RAM (even though
> realistically it may be smaller than that in the majority of cases).

This all makes sense, but we do this:

-- add resume=/dev/sdc to the command line
-- attach a disk (/dev/sdc) with size equal to RAM
-- mkswap /dev/sdc
-- swapon /dev/sdc
-- echo disk > /sys/power/state

and the last operation fails with ENOMEM.  Are we doing something
wrong?  Are we hitting some other mm bug?

Thanks!