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SubjectRe: [PATCH 1/2] Documentation: clarify limitations of hibernation
On Mon, Jan 6, 2020 at 4:53 AM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Thu 26-12-19 14:02:04, Luigi Semenzato wrote:
> [...]
> > +Limitations of Hibernation
> > +==========================
> > +
> > +When entering hibernation, the kernel tries to allocate a chunk of memory large
> > +enough to contain a copy of all pages in use, to use it for the system
> > +snapshot. If the allocation fails, the system cannot hibernate and the
> > +operation fails with ENOMEM. This will happen, for instance, when the total
> > +amount of anonymous pages (process data) exceeds 1/2 of total RAM.
> > +
> > +One possible workaround (besides terminating enough processes) is to force
> > +excess anonymous pages out to swap before hibernating. This can be achieved
> > +with memcgroups, by lowering memory usage limits with ``echo <new limit> >
> > +/dev/cgroup/memory/<group>/memory.mem.usage_in_bytes``. However, the latter
> > +operation is not guaranteed to succeed.
>
> I am not familiar with the hibernation process much. But what prevents
> those allocations to reclaim memory and push out the anonymous memory to
> the swap on demand during the hibernation's allocations?

Good question, thanks.

The hibernation image is stored into a swap device (or partition), so
I suppose one could set up two swap devices, giving a lower priority
to the hibernation device, so that it remains unused while the kernel
reclaims pages for the hibernation image.

If that works, then it may be appropriate to describe this technique
in Documentation/power/swsusp.rst. There's a brief mention of this
situation in the Q/A section, but maybe this deserves more visibility.

In my experience, the page allocator is prone to giving up in this
kind of situation. But my experience is up to 4.X kernels. Is this
guaranteed to work now?

Note that I removed the cgroup suggestion in later versions of this patch:
https://marc.info/?l=linux-pm&m=157800718520897

Also, there was some related discussion here:
https://marc.info/?l=linux-kernel&m=157177497015315

Thanks!



> --
> Michal Hocko
> SUSE Labs

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