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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Tue, 22 Oct 2019 22:57:20 +0200
Message-ID: <CAJZ5v0gJWxLJTi7TjaRP-3aR3f4VnX1n9dRE_jxdS6e3SM46LQ@mail.gmail.com>
Subject: Re: is hibernation usable?
To:     Luigi Semenzato <semenzato@google.com>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Geoff Pike <gpike@google.com>, Bas Nowaira <bassem@google.com>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Sonny Rao <sonnyrao@google.com>,
        Brian Geffon <bgeffon@google.com>
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On Tue, Oct 22, 2019 at 10:09 PM Luigi Semenzato <semenzato@google.com> wrote:
>
> Following a thread in linux-pm
> (https://marc.info/?l=linux-mm&m=157012300901871) I have some issues
> that may be of general interest.
>
> 1. To the best of my knowledge, Linux hibernation is guaranteed to
> fail if more than 1/2 of total RAM is in use (for instance, by
> anonymous pages).  My knowledge is based on evidence, experiments,
> code inspection, the thread above, and a comment in
> Documentation/swsusp.txt, copied here:

So I use it on a regular basis (i.e. every day) on a system that often
has over 50% or RAM in use and it all works.

I also know about other people using it on a regular basis.

For all of these users, it is usable.

>  "Instead, we load the image into unused memory and then atomically
> copy it back to it original location. This implies, of course, a
> maximum image size of half the amount of memory."

That isn't right any more.  An image that is loaded during resume can,
in fact, be larger than 50% of RAM.  An image that is created during
hibernation, however, cannot.

> 2. There's no simple/general workaround.  Rafael suggested on the
> thread "Whatever doesn't fit into 50% of RAM needs to be swapped out
> before hibernation".  This is a good suggestion: I am actually close
> to achieving this using memcgroups, but it's a fair amount of work,
> and a fairly special case.  Not everybody uses memcgroups, and I don't
> know of other reliable ways of forcing swap from user level.

I don't need to do anything like that.

hibernate_preallocate_memory() manages to free a sufficient amount of
memory on my system every time.

> 3. A feature that works only when 1/2 of total RAM can be allocated
> is, in my opinion, not usable, except possibly under special
> circumstances, such as mine. Most of the available articles and
> documentation do not mention this important fact (but for the excerpt
> I mentioned, which is not in a prominent position).

It can be used with over 1/2 of RAM allocated and that is quite easy
to demonstrate.

Honestly, I'm not sure what your problem is really.

> Two questions then:
>
> A. Should the documentation be changed to reflect this fact more
> clearly?  I feel that the current situation is a disservice to the
> user community.

Propose changes.

> B. Would it be worthwhile to improve the hibernation code to remove
> this limitation?  Is this of interest to anybody (other than me)?

Again, propose specific changes.