linux-kernel - Re: [PATCH v3] mm/hotplug: Only respect mem= parameter during boot stage

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Date:   Tue, 4 Feb 2020 08:53:30 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     Baoquan He <bhe@...hat.com>, linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org, mhocko@...nel.org,
        david@...hat.com, bsingharora@...il.com
Subject: Re: [PATCH v3] mm/hotplug: Only respect mem= parameter during boot
 stage

On 04.02.20 06:06, Baoquan He wrote:
> In commit 357b4da50a62 ("x86: respect memory size limiting via mem=
> parameter") a global varialbe max_mem_size is added to store
> the value parsed from 'mem= ', then checked when memory region is
> added. This truly stops those DIMMs from being added into system memory
> during boot-time.
> 
> However, it also limits the later memory hotplug functionality. Any
> DIMM can't be hotplugged any more if its region is beyond the
> max_mem_size. We will get errors like:
> 
> [  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
> 
> This will cause issue in a known use case where 'mem=' is added to
> the hypervisor. The memory that lies after 'mem=' boundary will be
> assigned to KVM guests. After commit 357b4da50a62 merged, memory
> can't be extended dynamically if system memory on hypervisor is not
> sufficient.
> 
> So fix it by also checking if it's during boot-time restricting to add
> memory. Otherwise, skip the restriction.
> 
> And also add this use case to document of 'mem=' kernel parameter.
> 
> Fixes: 357b4da50a62 ("x86: respect memory size limiting via mem= parameter")
> Signed-off-by: Baoquan He <bhe@...hat.com>

Reviewed-by: Juergen Gross <jgross@...e.com>


Juergen