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

[David Hildenbrand]

On 04.06.24 11:35, Mike Rapoport wrote:
> On Mon, Jun 03, 2024 at 10:53:03PM +0200, David Hildenbrand wrote:
>> On 03.06.24 12:43, Mike Rapoport wrote:
>>> On Mon, Jun 03, 2024 at 11:14:00AM +0200, David Hildenbrand wrote:
>>>> 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 at 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
>>>
>>> You are welcome to send a patch :-P
>>
>> I'll try once I understood what changed ever since you documented that in
>> 2018 -- or if we missed that detail back then already.
>   
>>>> 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.
>>>
>>> The commit that added memblock_free() at the first place (f9126ab9241f
>>> ("memory-hotplug: fix wrong edge when hot add a new node")) does not really
>>> describe why that was required :(
>>>
>>> But at a quick glance it looks completely spurious.
>>
>> There are more details [1] but I also did not figure out why the
>> memblock_free() was really required to resolve that issue.
>>
>> [1] https://marc.info/?l=linux-kernel&m=142961156129456&w=2
>   
> The tinkering with memblock there and in f9126ab9241f seem bogus in the
> context of memory hotplug on x86.
> 
> I believe that dropping that memblock_phys_free() is right thing to do
> regardless of this series. There's no corresponding memblock_alloc() and it
> was added as part of a fix for hotunplug on x86 that anyway had memblock
> discarded at that point.

So when we re-add that memory, we might have still ranges as "reserved". 
It does sound weird, but you're the boss :)

-- 
Cheers,

David / dhildenb