Re: [PATCH] mm: Correct guards for non_swap_entry()

[Steven Price]

On 06/03/2020 03:44, Andrew Morton wrote:
> On Thu,  5 Mar 2020 13:05:50 +0000 Steven Price <steven.price@xxxxxxx> wrote:
> 
>> If CONFIG_DEVICE_PRIVATE is defined, but neither CONFIG_MEMORY_FAILURE nor
>> CONFIG_MIGRATION, then non_swap_entry() will return 0, meaning that the
>> condition (non_swap_entry(entry) && is_device_private_entry(entry)) in
>> zap_pte_range() will never be true even if the entry is a device private
>> one.
>>
>> Equally any other code depending on non_swap_entry() will not function
>> as expected.
> 
> What are the user-visible runtime effects of this change?
> 
> Is a cc:stable needed?
> 

I originally spotted this just by looking at the code, I haven't
actually observed any problems.

Looking a bit more closely it appears that actually this situation
(currently at least) cannot occur:

DEVICE_PRIVATE depends on ZONE_DEVICE
ZONE_DEVICE depends on MEMORY_HOTREMOVE
MEMORY_HOTREMOVE depends on MIGRATION

So there's probably no need to back port.

Steve