memfd_secret() in 5.14 [LWN.net]

memfd_secret() in 5.14

Posted Aug 7, 2021 15:31 UTC (Sat) by LtWorf (subscriber, #124958)
In reply to: memfd_secret() in 5.14 by khim
Parent article: memfd_secret() in 5.14

A suspended macbook will wake itself up every once in a while.

A software bug that makes it not suspend again, paired with a faulty temperature sensor might cause fire.

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memfd_secret() in 5.14

Posted Aug 7, 2021 15:35 UTC (Sat) by khim (subscriber, #9252) [Link] (7 responses)

What would prevent it from doing that if it's hibernated? How would you even know it went to hibernation when you requested it?

The fact that some OSes are buggy and need hibernation to make you feel safe doesn't mean all of them are like that.

ChromeOS doesn't support hibernation at all yet I never seen or heard about it causing fire. Have you heard about such cases?

memfd_secret() in 5.14

Posted Aug 7, 2021 16:10 UTC (Sat) by Wol (subscriber, #4433) [Link] (6 responses)

> What would prevent it from doing that if it's hibernated?

Because a hibernated laptop is SWITCHED OFF!

A sleeping laptop is still running, still burning battery, and will die after (some definition of) a few hours. Potentially losing your work. As mentioned, potentially waking up and causing a fire. Etc etc.

A hibernated laptop is switched off and doesn't wake until you press the power switch, or wake-on-lan, or some EXTERNAL event causes it to fire up.

That's another reason for wanting to hibernate. If I don't have access to a charger, and don't need my laptop, I DON'T want it burning battery!!!

Cheers,
Wol

memfd_secret() in 5.14

Posted Aug 7, 2021 16:22 UTC (Sat) by khim (subscriber, #9252) [Link] (1 responses)

> Because a hibernated laptop is SWITCHED OFF!

And how do you know it's actually switched off? Many (most?) modern laptops have all these LEDs (power, suspend, mic off and so on) software driven.

Thus, in the end, you have to trust your software anyway.

> As mentioned, potentially waking up and causing a fire.

Have you ever heard about EFI wakeup alarm clock? Your hibernated (and even, presumably, switched off) device can do that perfectly fine, too.

> A hibernated laptop is switched off and doesn't wake until you press the power switch, or wake-on-lan, or some EXTERNAL event causes it to fire up.

Who told you that? I don't even know how long ago we had systems which couldn't wake up on their own volition.

> If I don't have access to a charger, and don't need my laptop, I DON'T want it burning battery!!!

Power it off if it's really need to go to sleep for a few days. That's really rare need in today's world, though.

You would save much more power since after wake up there wouldn't be a browser with moder web pages which insist on draining your battery as fast as they can (LWN is rare exception).

memfd_secret() in 5.14

Posted Aug 7, 2021 17:41 UTC (Sat) by Wol (subscriber, #4433) [Link]

> Power it off if it's really need to go to sleep for a few days. That's really rare need in today's world, though.

Is it? Once again you're assuming everyone is the same as you.

It is NORMAL for me (living in a first world country) to have no access to mains power for several days at a time. Yes it's my choice. But if I can't recharge my laptop for a week, I want to save as much battery as I can, and that includes not having it wasting power sleeping.

(And while one person can't make much difference, why should I want to waste power? If EVERYBODY saved just a few watts here and there - and had the "let's save it" mindset), we might make a dent in global warming :-)

Cheers,
Wol

memfd_secret() in 5.14

Posted Aug 8, 2021 0:45 UTC (Sun) by mjg59 (subscriber, #23239) [Link] (2 responses)

> Because a hibernated laptop is SWITCHED OFF!

Modern hardware has basically the same set of wakeup events when powered down as it does when suspended. The embedded controller is powered and can turn the machine back on whenever it feels like it. Unless you actually pull the battery, "Switched off" doesn't mean what you think it means - the only meaningful distinction between suspend and off is whether or not the RAM is in self-refresh.

memfd_secret() in 5.14

Posted Aug 8, 2021 9:57 UTC (Sun) by Wol (subscriber, #4433) [Link] (1 responses)

Yup. And as I understand it, that's the difference between sleep and hibernate. Hibernate also means ram-refresh is OFF.

Which, when you're off-grid and battery life is critical, is important.

And I don't know which, in general, uses more battery. Rebooting? or restoring from hibernate? My gut feel is hibernate is likely to use less battery, because it doesn''t go through the (possibly) long start up sequence before getting to a usable system.

Cheers,
Wol

memfd_secret() in 5.14

Posted Aug 8, 2021 17:25 UTC (Sun) by mjg59 (subscriber, #23239) [Link]

That's relevant in terms of how long a device can keep state, but not relevant to whether it'll catch fire in your bag.

memfd_secret() in 5.14

Posted Aug 9, 2021 9:30 UTC (Mon) by chris_se (subscriber, #99706) [Link]

> As mentioned, potentially waking up and causing a fire.

Any device with a Lithium-Ion battery can catch fire, whether it's drawing power or not. And even if your device is shut down (not even hibernating), you could also trigger your power button accidentally. (Hasn't happened to me with a laptop yet, but has happened countless times with my cell phone, causing it to reboot.)

Was there a risk 30 years ago when there was no thermal throttling and no thermal cutoffs in devices, especially CPUs? Sure. Is there a non-zero risk that any device that contains energy storage catches fire? Absolutely. But hardware has come a long way since then to mitigate those risks and I think this concern is overblown. Heck, even 15 years ago with my then laptop I had a couple of times when it didn't even enter sleep (stochastic bugs in hardware and/or the kernel) and I didn't check before putting it into the bag because I was in a hurry. And while the inside of the bag got warm to the touch (at most 40C), nothing got really hot, the laptop throttled down and just drained its battery much quicker than in suspend mode, causing it to shut off and causing me some inconvenience.

Millions of people put their laptop in sleep mode in their bags daily, and I've yet to see an epidemic of cases where laptops catch fire due to this. The data just doesn't bear out that this is an issue that one should be concerned about. Your device catching fire because the manufacturer did a bad job with that specific model is by far a much likelier outcome than it catching fire due to being stuffed into a bag in suspend mode.

I'm not against hibernate, and I think that hibernating laptops is a very legitimate use case, especially if you want to leave it off (without draining battery) for a significant amount of time. But please don't argue for your position with fear-mongering against other legitimate ways of using your own device.