Re: [PATCH v4] mm: Enable suspend-only swap spaces

[David Hildenbrand <david@redhat.com>]

On 27.07.21 02:12, Evan Green wrote:
> Add a new SWAP_FLAG_HIBERNATE_ONLY 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 hibernate /dev/sda2.
> 
> Currently it's not possible to enable hibernation without also enabling
> generic swap for a given area. One semi-workaround for this is to delay
> the call to swapon() until just before attempting to hibernate, and then
> call swapoff() just after hibernate completes. This is somewhat kludgy,
> and also doesn't really work to keep swap out of the hibernate region.
> When hibernate begins, it starts by allocating a large chunk of memory
> for itself. This often ends up forcing a lot of data out into swap. By
> this time the hibernate region is eligible for generic swap, so swap
> ends up leaking into the hibernate region even with the workaround.
> 
> There are a few reasons why usermode might want to be able to
> exclusively steer swap and hibernate. One reason relates to SSD wearing.
> Hibernate's endurance and speed requirements are different from swap.
> It may for instance be advantageous to keep hibernate in primary
> storage, but put swap in an SLC namespace. These namespaces are faster
> and have better endurance, but cost 3-4x in terms of capacity.
> Exclusively steering hibernate and swap enables system designers to
> accurately partition their storage without either wearing out their
> primary storage, or overprovisioning their fast swap area.
> 
> Another reason to allow exclusive steering has to do with security.
> The requirements for designing systems with resilience against
> offline attacks are different between swap and hibernate. Swap
> effectively requires a dictionary of hashes, as pages can be added and
> removed arbitrarily, whereas hibernate only needs a single hash for the
> entire image. If you've set up block-level integrity for swap and
> image-level integrity for hibernate, then allowing swap blocks to
> possibly leak out to the hibernate region is problematic, since it
> creates swap pages not protected by any integrity.
> 
> Swap regions with SWAP_FLAG_HIBERNATE_ONLY set will not appear in
> /proc/meminfo under SwapTotal and SwapFree, since they are not usable as
> general swap. These regions do still appear in /proc/swaps.

Right, and they also don't account towards the memory overcommit 
calculations.

Thanks for extending the patch description!

[...]

> +	if (swap_flags & SWAP_FLAG_HIBERNATE_ONLY) {
> +		if (IS_ENABLED(CONFIG_HIBERNATION)) {
> +			if (swap_flags & ~SWAP_HIBERNATE_ONLY_VALID_FLAGS)
> +				return -EINVAL;
> +
> +		} else {
> +			return -EINVAL;
> +		}
> +	}

We could do short

if ((swap_flags & SWAP_FLAG_HIBERNATE_ONLY) &&
      (!IS_ENABLED(CONFIG_HIBERNATION) ||
       (swap_flags & ~SWAP_HIBERNATE_ONLY_VALID_FLAGS)))
	return -EINVAL;

or

if (swap_flags & SWAP_FLAG_HIBERNATE_ONLY))
	if (!IS_ENABLED(CONFIG_HIBERNATION) ||
             (swap_flags & ~SWAP_HIBERNATE_ONLY_VALID_FLAGS))
		return -EINVAL;

> +
>   	if (!capable(CAP_SYS_ADMIN))
>   		return -EPERM;
>   
> @@ -3335,16 +3366,20 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>   	if (swap_flags & SWAP_FLAG_PREFER)
>   		prio =
>   		  (swap_flags & SWAP_FLAG_PRIO_MASK) >> SWAP_FLAG_PRIO_SHIFT;
> +
> +	if (swap_flags & SWAP_FLAG_HIBERNATE_ONLY)
> +		p->flags |= SWP_HIBERNATE_ONLY;
>   	enable_swap_info(p, prio, swap_map, cluster_info, frontswap_map);
>   
> -	pr_info("Adding %uk swap on %s.  Priority:%d extents:%d across:%lluk %s%s%s%s%s\n",
> +	pr_info("Adding %uk swap on %s.  Priority:%d extents:%d across:%lluk %s%s%s%s%s%s\n",
>   		p->pages<<(PAGE_SHIFT-10), name->name, p->prio,
>   		nr_extents, (unsigned long long)span<<(PAGE_SHIFT-10),
>   		(p->flags & SWP_SOLIDSTATE) ? "SS" : "",
>   		(p->flags & SWP_DISCARDABLE) ? "D" : "",
>   		(p->flags & SWP_AREA_DISCARD) ? "s" : "",
>   		(p->flags & SWP_PAGE_DISCARD) ? "c" : "",
> -		(frontswap_map) ? "FS" : "");
> +		(frontswap_map) ? "FS" : "",
> +		(p->flags & SWP_HIBERNATE_ONLY) ? "H" : "");
>   
>   	mutex_unlock(&swapon_mutex);
>   	atomic_inc(&proc_poll_event);
> 

Looks like the cleanest alternative to me, as long as we don't want to 
invent new interfaces.

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb