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Subject: Re: [Patch v2] mm/hotplug: Only respect mem= parameter during boot
 stage
From:   David Hildenbrand <david@redhat.com>
To:     Balbir Singh <bsingharora@gmail.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>
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Organization: Red Hat GmbH
Message-ID: <c100d74b-1424-9852-b6ee-553a2ae48771@redhat.com>
Date:   Tue, 10 Dec 2019 10:37:05 +0100
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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.
> 
>>
>> Balbir
>>
> 
> 


-- 
Thanks,

David / dhildenb