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Subject: Re: [Patch v2] mm/hotplug: Only respect mem= parameter during boot
 stage
To:     Baoquan He <bhe@redhat.com>, linux-kernel@vger.kernel.org
Cc:     linux-mm@kvack.org, mhocko@kernel.org, david@redhat.com,
        jgross@suse.com, akpm@linux-foundation.org
References: <20191210084413.21957-1-bhe@redhat.com>
From:   Balbir Singh <bsingharora@gmail.com>
Message-ID: <75188d0f-c609-5417-aa2e-354e76b7ba6e@gmail.com>
Date:   Tue, 10 Dec 2019 20:24:55 +1100
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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?

Balbir