Re: [PATCH] mm/hotplug: Reorder memblock_[free|remove]() calls in try_remove_memory()

[Anshuman Khandual]

On 09/25/2019 08:43 AM, Andrew Morton wrote:
> On Mon, 23 Sep 2019 11:16:38 +0530 Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote:
> 
>>
>>
>> On 09/16/2019 11:17 AM, Anshuman Khandual wrote:
>>> In add_memory_resource() the memory range to be hot added first gets into
>>> the memblock via memblock_add() before arch_add_memory() is called on it.
>>> Reverse sequence should be followed during memory hot removal which already
>>> is being followed in add_memory_resource() error path. This now ensures
>>> required re-order between memblock_[free|remove]() and arch_remove_memory()
>>> during memory hot-remove.
>>>
>>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>>> Cc: Oscar Salvador <osalvador@xxxxxxx>
>>> Cc: Michal Hocko <mhocko@xxxxxxxx>
>>> Cc: David Hildenbrand <david@xxxxxxxxxx>
>>> Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
>>> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>>> ---
>>> Original patch https://lkml.org/lkml/2019/9/3/327
>>>
>>> Memory hot remove now works on arm64 without this because a recent commit
>>> 60bb462fc7ad ("drivers/base/node.c: simplify unregister_memory_block_under_nodes()").
>>>
>>> David mentioned that re-ordering should still make sense for consistency
>>> purpose (removing stuff in the reverse order they were added). This patch
>>> is now detached from arm64 hot-remove series.
>>>
>>> https://lkml.org/lkml/2019/9/3/326
>>
>> ...
>>
>> Hello Andrew,
>>
>> Any feedbacks on this, does it look okay ?
>>
> 
> Well.  I'd parked this for 5.4-rc1 processing because it looked like a
> cleanup.

This does not fix a serious problem. It just removes an inconsistency while
freeing resources during memory hot remove which for now does not pose a
real problem.

> 
> But waaaay down below the ^---$ line I see "Memory hot remove now works
> on arm64".  Am I correct in believing that 60bb462fc7ad broke arm64 mem
> hot remove?  And that this patch fixes a serious regression?  If so,

No. [Proposed] arm64 memory hot remove series does not anymore depend on
this particular patch because 60bb462fc7ad has already solved the problem.

> that should have been right there in the patch title and changelog!

V2 (https://patchwork.kernel.org/patch/11159939/) for this patch makes it
very clear in it's commit message.

- Anshuman