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

[David Hildenbrand <david@redhat.com>]

On 27.07.21 11:48, David Hildenbrand wrote:
> 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>
> 

Pavel just mentioned uswsusp, and I wonder if it would be a possible 
alternative to this patch.

-- 
Thanks,

David / dhildenb