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Subject: Re: [PATCH] mm: don't rely on system state to detect hot-plug
 operations
To:     Michal Hocko <mhocko@suse.com>, Oscar Salvador <osalvador@suse.de>
Cc:     Laurent Dufour <ldufour@linux.ibm.com>, akpm@linux-foundation.org,
        rafael@kernel.org, nathanl@linux.ibm.com, cheloha@linux.ibm.com,
        stable@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>
References: <9faac1ce-c02d-7dbc-f79a-4aaaa5a73d28@linux.ibm.com>
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 <20200910120343.GA6635@linux> <20200910124755.GG28354@dhcp22.suse.cz>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <27e73d7d-c172-aa84-dad2-f97ed6123db4@redhat.com>
Date:   Thu, 10 Sep 2020 14:49:28 +0200
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On 10.09.20 14:47, Michal Hocko wrote:
> On Thu 10-09-20 14:03:48, Oscar Salvador wrote:
>> On Thu, Sep 10, 2020 at 01:35:32PM +0200, Laurent Dufour wrote:
>>  
>>> That points has been raised by David, quoting him here:
>>>
>>>> IIRC, ACPI can hotadd memory while SCHEDULING, this patch would break that.
>>>>
>>>> Ccing Oscar, I think he mentioned recently that this is the case with ACPI.
>>>
>>> Oscar told that he need to investigate further on that.
>>
>> I think my reply got lost.
>>
>> We can see acpi hotplugs during SYSTEM_SCHEDULING:
>>
>> $QEMU -enable-kvm -machine pc -smp 4,sockets=4,cores=1,threads=1 -cpu host -monitor pty \
>>         -m size=$MEM,slots=255,maxmem=4294967296k  \
>>         -numa node,nodeid=0,cpus=0-3,mem=512 -numa node,nodeid=1,mem=512 \
>>         -object memory-backend-ram,id=memdimm0,size=134217728 -device pc-dimm,node=0,memdev=memdimm0,id=dimm0,slot=0 \
>>         -object memory-backend-ram,id=memdimm1,size=134217728 -device pc-dimm,node=0,memdev=memdimm1,id=dimm1,slot=1 \
>>         -object memory-backend-ram,id=memdimm2,size=134217728 -device pc-dimm,node=0,memdev=memdimm2,id=dimm2,slot=2 \
>>         -object memory-backend-ram,id=memdimm3,size=134217728 -device pc-dimm,node=0,memdev=memdimm3,id=dimm3,slot=3 \
>>         -object memory-backend-ram,id=memdimm4,size=134217728 -device pc-dimm,node=1,memdev=memdimm4,id=dimm4,slot=4 \
>>         -object memory-backend-ram,id=memdimm5,size=134217728 -device pc-dimm,node=1,memdev=memdimm5,id=dimm5,slot=5 \
>>         -object memory-backend-ram,id=memdimm6,size=134217728 -device pc-dimm,node=1,memdev=memdimm6,id=dimm6,slot=6 \
>>
>> kernel: [    0.753643] __add_memory: nid: 0 start: 0100000000 - 0108000000 (size: 134217728)
>> kernel: [    0.756950] register_mem_sect_under_node: system_state= 1
>>
>> kernel: [    0.760811]  register_mem_sect_under_node+0x4f/0x230
>> kernel: [    0.760811]  walk_memory_blocks+0x80/0xc0
>> kernel: [    0.760811]  link_mem_sections+0x32/0x40
>> kernel: [    0.760811]  add_memory_resource+0x148/0x250
>> kernel: [    0.760811]  __add_memory+0x5b/0x90
>> kernel: [    0.760811]  acpi_memory_device_add+0x130/0x300
>> kernel: [    0.760811]  acpi_bus_attach+0x13c/0x1c0
>> kernel: [    0.760811]  acpi_bus_attach+0x60/0x1c0
>> kernel: [    0.760811]  acpi_bus_scan+0x33/0x70
>> kernel: [    0.760811]  acpi_scan_init+0xea/0x21b
>> kernel: [    0.760811]  acpi_init+0x2f1/0x33c
>> kernel: [    0.760811]  do_one_initcall+0x46/0x1f4
> 
> Is there any actual usecase for a configuration like this? What is the
> point to statically define additional memory like this when the same can
> be achieved on the same command line?

You can online it movable right away to unplug later.

Also, under QEMU, just do a reboot with hotplugged memory and you're in
the very same situation.

-- 
Thanks,

David / dhildenb