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From:   Luigi Semenzato <semenzato@google.com>
Date:   Tue, 22 Oct 2019 13:09:10 -0700
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Subject: is hibernation usable?
To:     linux-kernel <linux-kernel@vger.kernel.org>
Cc:     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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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:

 "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."

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.

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).

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.

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

Thank you in advance!