Re: [PATCH v3] mm: Enable suspend-only swap spaces

On Thu, Jul 22, 2021 at 11:58 PM David Hildenbrand [off-list ref] wrote:

On 22.07.21 20:00, Evan Green wrote:

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On Thu, Jul 22, 2021 at 12:12 AM David Hildenbrand [off-list ref] wrote:

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On 21.07.21 23:40, Evan Green wrote:

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Currently it's not possible to enable hibernation without also enabling
generic swap for a given swap area. These two use cases are not the
same. For example there may be users who want to enable hibernation,
but whose drives don't have the write endurance for generic swap
activities. Swap and hibernate also have different security/integrity
requirements, prompting folks to possibly set up something like block-level
integrity for swap and image-level integrity for hibernate. Keeping swap
and hibernate separate in these cases becomes not just a matter of
preference, but correctness.

Add a new SWAP_FLAG_NOSWAP that adds a swap region but refuses to allow
generic swapping to it. This region can still be wired up for use in
suspend-to-disk activities, but will never have regular pages swapped to
it. This flag will be passed in by utilities like swapon(8), usage would
probably look something like: swapon -o noswap /dev/sda2.

Just a minor comment, I'd call it rather SWAP_FLAG_HIBERNATE_ONLY and
SWAP_FLAG_HIBERNATE_ONLY -- that calls the child by its name.

I went back and forth on this too. It seemed pretty close to toss-up
to me. I went with NOSWAP ultimately because it seemed more closely
tied to what the flag was actually doing, rather than building in my
one expected use case into the name. In some world years from now
where either hibernate has diverged, been deleted, or maybe some new
usage has been invented for swap space, the NOSWAP name felt like it
had a better chance of holding up. The argument is weak though, as
these features are pretty well cast in stone, and the likelihood of
any of those outcomes seems low. I can change it if you feel strongly,
but would probably keep it as-is otherwise.

Just imagine technology Z popping up and using also the swap
infrastructure. What would be the semantics of NOSWAP? With
HIBERNATE_ONLY it's clear -- enable that device only for hibernation,
nothing else.

But you raise a good point: if hibernation isn't even possible in a
configuration (e.g., not configured into the kernel), we should simply
reject that flag. So if hibernation would vanish at some point
completely from the system, it would all be handled accordingly.

That would result in quite a consistent definition of
SWAP_FLAG_HIBERNATE_ONLY IMHO.

Makes sense?

Ok, I'll change the name, and reject the flag if hibernation is not enabled.

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I think some other flags might not apply with that new flag set, right?
For example, does SWAP_FLAG_DISCARD_ONCE or SWP_AREA_DISCARD still have
any meaning with the new flag being set?

We should most probably disallow enabling any flag that doesn't make any
sense in combination.

Good point, I can send a followup patch for that. From my reading

I'd actually enjoy if we'd have that logic in the introducing patch.

Ok.

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SWAP_FLAG_DISCARD and SWAP_FLAG_DISCARD_ONCE are still valid, since
the discard can be run at swapon() time. SWAP_FLAG_PREFER (specifying
the priority) doesn't make sense, and SWAP_FLAG_DISCARD_PAGES never
kicks in because it's called at the cluster level. Hm, that sort of
seems like a bug that freed hibernate swap doesn't get discarded. I
can disallow it now as unsupported, but might send a patch to fix it
later.

Might be worth fixing, indeed.

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Apart from that, I'd love to see a comment in here why the workaround
suggested by Michal isn't feasible -- essentially a summary of what we
discussed.

Ah sorry, I had tried to clarify that in the commit text, but didn't
explicitly address the workaround. To summarize, the workaround keeps
generic swap out of your hibernate region... until hibernate time. But
once hibernate starts, a lot of swapping tends to happen when the
hiber-image is allocated. At this point the hibernate region is
eligible for general swap even with the workaround. The reasons I gave
for wanting to exclusively steer swap and hibernate are SSD write
wearing, different integrity solutions for swap vs hibernate, and our
own security changes that no-op out the swapon/swapoff syscalls after
init.

That would be nice to have in the patch description :)

Sure, I'll add that as well and send out a v4 shortly.
-Evan