linux-kernel - Re: [PATCH 1/3] mm/memory_hotplug: Allow an override for the memmap_on

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Date:   Thu, 22 Jun 2023 14:37:53 +0100
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     David Hildenbrand <david@...hat.com>
CC:     Vishal Verma <vishal.l.verma@...el.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        "Dan Williams" <dan.j.williams@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        <linux-acpi@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>, <nvdimm@...ts.linux.dev>,
        <linux-cxl@...r.kernel.org>, Huang Ying <ying.huang@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 1/3] mm/memory_hotplug: Allow an override for the
 memmap_on_memory param

On Fri, 16 Jun 2023 09:46:59 +0200
David Hildenbrand <david@...hat.com> wrote:

> On 16.06.23 00:00, Vishal Verma wrote:
> > For memory hotplug to consider MHP_MEMMAP_ON_MEMORY behavior, the
> > 'memmap_on_memory' module parameter was a hard requirement.
> > 
> > In preparation for the dax/kmem driver to use memmap_on_memory
> > semantics, arrange for the module parameter check to be bypassed via the
> > appropriate mhp_flag.
> > 
> > Recall that the kmem driver could contribute huge amounts of hotplugged
> > memory originating from special purposes devices such as CXL memory
> > expanders. In some cases memmap_on_memory may be the /only/ way this new
> > memory can be hotplugged. Hence it makes sense for kmem to have a way to
> > force memmap_on_memory without depending on a module param, if all the
> > other conditions for it are met.  
> 
> Just let the admin configure it. After all, an admin is involved in 
> configuring the dax/kmem device to begin with. If add_memory() fails you 
> could give a useful hint to the admin.
> 

Agreed. If it were just the default then fine, but making it the only option
limits admin choices.

Jonathan