Re: [RFC PATCH 8/8] HACK: mm: memory_hotplug: Drop memblock_phys_free() call in try_remove_memory()

[David Hildenbrand <david@redhat.com>]

On 03.06.24 09:57, Mike Rapoport wrote:
> On Fri, May 31, 2024 at 09:49:32AM +0200, David Hildenbrand wrote:
>> On 29.05.24 19:12, Jonathan Cameron wrote:
>>> I'm not sure what this is balancing, but it if is necessary then the reserved
>>> memblock approach can't be used to stash NUMA node assignments as after the
>>> first add / remove cycle the entry is dropped so not available if memory is
>>> re-added at the same HPA.
>>>
>>> This patch is here to hopefully spur comments on what this is there for!
>>>
>>> Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
>>> ---
>>>    mm/memory_hotplug.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>>> index 431b1f6753c0..3d8dd4749dfc 100644
>>> --- a/mm/memory_hotplug.c
>>> +++ b/mm/memory_hotplug.c
>>> @@ -2284,7 +2284,7 @@ static int __ref try_remove_memory(u64 start, u64 size)
>>>    	}
>>>    	if (IS_ENABLED(CONFIG_ARCH_KEEP_MEMBLOCK)) {
>>> -		memblock_phys_free(start, size);
>>> +		//		memblock_phys_free(start, size);
>>>    		memblock_remove(start, size);
>>>    	}
>>
>> memblock_phys_free() works on memblock.reserved, memblock_remove() works  on
>> memblock.memory.
>>
>> If you take a look at the doc at the top of memblock.c:
>>
>> memblock.memory: physical memory available to the system
>> memblock.reserved: regions that were allocated [during boot]
>>
>>
>> memblock.memory is supposed to be a superset of memblock.reserved. Your
> 
> No it's not.
> memblock.reserved is more of "if there is memory, don't touch it".

Then we should certainly clarify that in the comments! :P

But for the memory hotunplug case, that's most likely why that code was 
added. And it only deals with ordinary system RAM, not weird 
reservations you describe below.

> Some regions in memblock.reserved are boot time allocations and they are indeed a
> subset of memblock.memory, but some are reservations done by firmware (e.g.
> reserved memory in DT) that just might not have a corresponding regions in
> memblock.memory. It can happen for example, when the same firmware runs on
> devices with different memory configuration, but still wants to preserve
> some physical addresses.

Could this happen with a good old BIOS as well? Just curious.

> 
>> "hack" here indicates that you somehow would be relying on the opposite
>> being true, which indicates that you are doing the wrong thing.
>   
> I'm not sure about that, I still have to digest the patches :)

In any case, using "reserved" to store persistent data across 
plug/unplug sounds wrong; but maybe I'm wrong :)

-- 
Cheers,

David / dhildenb