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Subject: Re: [Patch v2] mm/hotplug: Only respect mem= parameter during boot
 stage
To:     David Hildenbrand <david@redhat.com>, Baoquan He <bhe@redhat.com>,
        linux-kernel@vger.kernel.org
Cc:     linux-mm@kvack.org, mhocko@kernel.org, jgross@suse.com,
        akpm@linux-foundation.org
References: <20191210084413.21957-1-bhe@redhat.com>
 <75188d0f-c609-5417-aa2e-354e76b7ba6e@gmail.com>
 <429622cf-f0f4-5d80-d39d-b0d8a6c6605f@redhat.com>
 <c100d74b-1424-9852-b6ee-553a2ae48771@redhat.com>
From:   Balbir Singh <bsingharora@gmail.com>
Message-ID: <618f5695-73e4-3f28-c874-4f12966dee51@gmail.com>
Date:   Tue, 10 Dec 2019 20:41:33 +1100
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On 10/12/19 8:37 pm, David Hildenbrand wrote:
> On 10.12.19 10:36, David Hildenbrand wrote:
>> On 10.12.19 10:24, Balbir Singh wrote:
>>>
>>>
>>> On 10/12/19 7:44 pm, Baoquan He wrote:
>>>> In commit 357b4da50a62 ("x86: respect memory size limiting via mem=
>>>> parameter") a global varialbe global max_mem_size is added to store
>>>                   typo ^^^
>>>> the value parsed from 'mem= ', then checked when memory region is
>>>> added. This truly stops those DIMM from being added into system memory
>>>> during boot-time.
>>>>
>>>> However, it also limits the later memory hotplug functionality. Any
>>>> memory board can't be hot added any more if its region is beyond the
>>>> max_mem_size. System will print error like below:
>>>>
>>>> [  216.387164] acpi PNP0C80:02: add_memory failed
>>>> [  216.389301] acpi PNP0C80:02: acpi_memory_enable_device() error
>>>> [  216.392187] acpi PNP0C80:02: Enumeration failure
>>>>
>>>> From document of 'mem= ' parameter, it should be a restriction during
>>>> boot, but not impact the system memory adding/removing after booting.
>>>>
>>>>   mem=nn[KMG]     [KNL,BOOT] Force usage of a specific amount of memory
>>>> 	          ...
>>>>
>>>> So fix it by also checking if it's during boot-time when restrict memory
>>>> adding. Otherwise, skip the restriction.
>>>>
>>>
>>> The fix looks reasonable, but I don't get the use case. Booting with mem= is
>>> generally a debug option, is this for debugging memory hotplug + limited memory?
>>
>> Some people/companies use "mem=" along with KVM e.g., to avoid
>> allocating memmaps for guest backing memory and to not expose it to the
>> buddy across kexec's. The excluded physical memory is then memmap into
> 
> s/memmap/mmaped/
> 
>> the hypervisor process and KVM can deal with that. I can imagine that
>> hotplug might be desirable as well for such use cases.

Makes sense, sounds hacky, but it seems like mem= is no longer a debug
option

Acked-by: Balbir Singh <bsingharora@gmail.com>

>>
>>>
>>> Balbir
>>>
>>
>>
> 
>