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

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Date:   Mon, 16 Sep 2019 14:20:47 +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 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.