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

From: David Hildenbrand <david@redhat.com>
To: Mike Rapoport <rppt@kernel.org>
Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>,
	Dan Williams <dan.j.williams@intel.com>,
	linux-cxl@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	Sudeep Holla <sudeep.holla@arm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Will Deacon <will@kernel.org>, Jia He <justin.he@arm.com>,
	Mike Rapoport <rppt@linux.ibm.com>,
	linuxarm@huawei.com, catalin.marinas@arm.com,
	Anshuman.Khandual@arm.com,
	Yuquan Wang <wangyuquan1236@phytium.com.cn>,
	Oscar Salvador <osalvador@suse.de>,
	Lorenzo Pieralisi <lpieralisi@kernel.org>,
	James Morse <james.morse@arm.com>
Subject: Re: [RFC PATCH 8/8] HACK: mm: memory_hotplug: Drop memblock_phys_free() call in try_remove_memory()
Date: Mon, 3 Jun 2024 22:53:03 +0200	[thread overview]
Message-ID: <da88fa70-203e-49e6-bf4d-22cd161ef8d1@redhat.com> (raw)
In-Reply-To: <Zl2eNQF7S9zlUFne@kernel.org>

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@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

-- 
Cheers,

David / dhildenb

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