linux-kernel - Re: [PATCH] mm/hotplug: Reorder memblock_[free|remove]() calls in try_remove

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Message-ID: <cc4193ec-a760-539b-099e-4aa1816dcea6@arm.com>
Date:   Tue, 17 Sep 2019 08:40:03 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Mike Rapoport <rppt@...ux.ibm.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, Oscar Salvador <osalvador@...e.de>,
        Michal Hocko <mhocko@...e.com>,
        David Hildenbrand <david@...hat.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH] mm/hotplug: Reorder memblock_[free|remove]() calls in
 try_remove_memory()



On 09/16/2019 02:20 PM, Anshuman Khandual wrote:
> 
> 
> On 09/16/2019 12:06 PM, Mike Rapoport wrote:
>> On Mon, Sep 16, 2019 at 11:17:37AM +0530, 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@...ux-foundation.org>
>>> Cc: Oscar Salvador <osalvador@...e.de>
>>> Cc: Michal Hocko <mhocko@...e.com>
>>> Cc: David Hildenbrand <david@...hat.com>
>>> Cc: Pavel Tatashin <pasha.tatashin@...een.com>
>>> Cc: Dan Williams <dan.j.williams@...el.com>
>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>>> ---
>>> 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
>>>
>>>  mm/memory_hotplug.c | 4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>>> index c73f09913165..355c466e0621 100644
>>> --- a/mm/memory_hotplug.c
>>> +++ b/mm/memory_hotplug.c
>>> @@ -1770,13 +1770,13 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size)
>>>
>>>  	/* remove memmap entry */
>>>  	firmware_map_remove(start, start + size, "System RAM");
>>> -	memblock_free(start, size);
>>> -	memblock_remove(start, size);
>>>
>>>  	/* remove memory block devices before removing memory */
>>>  	remove_memory_block_devices(start, size);
>>>
>>>  	arch_remove_memory(nid, start, size, NULL);
>>> +	memblock_free(start, size);
>>
>> I don't see memblock_reserve() anywhere in memory_hotplug.c, so the
>> memblock_free() call here seems superfluous. I think it can be simply
>> dropped.
> 
> I had observed that previously but was not sure whether or not there are
> still scenarios where it might be true. Error path in add_memory_resource()
> even just calls memblock_remove() not memblock_free(). Unless there is any
> objection, can just drop it.

Hello Mike,

Looks like we might need memblock_free() here as well. As you mentioned
before there might not be any memblock_reserve() in mm/memory_hotplug.c
but that does not guarantee that there could not have been a previous
memblock_reserve() or memblock_alloc_XXX() allocation which came from
the current hot remove range.

memblock_free() followed by memblock_remove() on the entire hot-remove
range ensures that memblock.memory and memblock.reserve are in sync and
the entire range is guaranteed to be removed from both the memory types.
Or am I missing something here.

- Anshuman