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

[Baoquan He <bhe@redhat.com>]

On 02/06/20 at 09:55am, David Hildenbrand 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@redhat.com>
> > ---
> > 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 :)

In fact, as long as it's not used for test, hotplug will be helpful, e.g
the 2nd use case. We use 'mem=' to skip these bad part of boot memory DIMMs,
while hotplug can help us to extend RAM with good DIMMs. No need to
discard the partly broken memmory board.

> 
> 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 :) ).

Yes, I should hold it a while. Worry it's not clear enough. But in
kernel-parameters.txt, I can't write too many details about use case.

> 
> Would like to get Michals opinion on this.

Sure, will hold.  Thanks.