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

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Date:   Thu, 6 Feb 2020 10:58:43 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Jürgen Groß <jgross@...e.com>
Cc:     linux-kernel@...r.kernel.org, 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 02/04/20 at 08:53am, Jürgen Groß wrote:
> 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>

Thanks, Juergen. Seems I should add more details to explain this. Will
post v4 with your 'Reviewed-by'.