Re: [PATCH] mm: fix movable_node kernel command-line

[Michal Hocko <mhocko@kernel.org>]

On Mon 23-10-17 09:03:14, Sharath Kumar Bhat wrote:
> On Mon, Oct 23, 2017 at 02:52:13PM +0200, Michal Hocko wrote:
> > On Fri 20-10-17 16:32:09, Sharath Kumar Bhat wrote:
> > > Currently when booted with the 'movable_node' kernel command-line the user
> > > can not have both the functionality of 'movable_node' and at the same time
> > > specify more movable memory than the total size of hotpluggable memories.
> > > 
> > > This is a problem because it limits the total amount of movable memory in
> > > the system to the total size of hotpluggable memories and in a system the
> > > total size of hotpluggable memories can be very small or all hotpluggable
> > > memories could have been offlined. The 'movable_node' parameter was aimed
> > > to provide the entire memory of hotpluggable NUMA nodes to applications
> > > without any kernel allocations in them. The 'movable_node' option will be
> > > useful if those hotpluggable nodes have special memory like MCDRAM as in
> > > KNL which is a high bandwidth memory and the user would like to use all of
> > > it for applications. But in doing so the 'movable_node' command-line poses
> > > this limitation and does not allow the user to specify more movable memory
> > > in addition to the hotpluggable memories.
> > > 
> > > With this change the existing 'movablecore=' and 'kernelcore=' command-line
> > > parameters can be specified in addition to the 'movable_node' kernel
> > > parameter. This allows the user to boot the kernel with an increased amount
> > > of movable memory in the system and still have only movable memory in
> > > hotpluggable NUMA nodes.
> > 
> > I really detest making the already cluttered kernelcore* handling even
> > more so. Why cannot your MCDRAM simply announce itself as hotplugable?
> > Also it is not really clear to me how can you control that only your
> > specific memory type gets into movable zone.
> > -- 
> > Michal Hocko
> > SUSE Labs
> 
> In the example MCDRAM is already being announced as hotpluggable and
> 'movable_node' is also used to ensure that there is no kernel allocations
> in that. This is a required functionality but when done so user can not have
> movable zone in other non-hotpluggable memories in addition to hotpluggable
> memory.
> 
> This change wont affect any of the present use cases such as 'kernelcore='
> or 'movablecore=' or using only 'movable_node'. They continue to work as
> before.
> 
> In addition to those it lets admin to specify 'kernelcore=' or
> 'movablecore=' when using 'movable_node' command-line

So, why exactly do we need this functionality? kernelcore is an ugly
interface, I am not entirely thrilled into extending it even more.

-- 
Michal Hocko
SUSE Labs

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