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Subject: Re: [Patch v2] mm/hotplug: Only respect mem= parameter during boot
 stage
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>
 <75188d0f-c609-5417-aa2e-354e76b7ba6e@gmail.com>
From:   David Hildenbrand <david@redhat.com>
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Message-ID: <429622cf-f0f4-5d80-d39d-b0d8a6c6605f@redhat.com>
Date:   Tue, 10 Dec 2019 10:36:19 +0100
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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
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