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

[David Hildenbrand]

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@xxxxxxxxxx>
> ---
> v2->v3:
>   In discussion of v1 and v2, People have concern about the use case
>   related to the code change. So add the use case into patch log and
>   document of 'mem=' in kernel-parameters.txt.
> 
>  Documentation/admin-guide/kernel-parameters.txt | 13 +++++++++++--
>  mm/memory_hotplug.c                             |  8 +++++++-
>  2 files changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index ddc5ccdd4cd1..b809767e5f74 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2533,13 +2533,22 @@
>  			For details see: Documentation/admin-guide/hw-vuln/mds.rst
>  
>  	mem=nn[KMG]	[KNL,BOOT] Force usage of a specific amount of memory
> -			Amount of memory to be used when the kernel is not able
> -			to see the whole system memory or for test.
> +			Amount of memory to be used in cases as follows:
> +
> +			1 for test;
> +			2 when the kernel is not able to see the whole system memory;
> +			3 memory that lies after 'mem=' boundary is excluded from
> +			 the hypervisor, then assigned to KVM guests.

I remember that there were more use cases, but forgot where that was
documented :)

I do wonder if we want to change that now without anybody complaining.
Yes, I brought up a possible use case but don't know if it is relevant
in practice (IOW, nobody complained yet :) ).

Would like to get Michals opinion on this.

-- 
Thanks,

David / dhildenb