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Date:   Thu, 22 Jun 2023 14:37:53 +0100
From:   Jonathan Cameron <Jonathan.Cameron@Huawei.com>
To:     David Hildenbrand <david@redhat.com>
CC:     Vishal Verma <vishal.l.verma@intel.com>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Len Brown <lenb@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.de>,
        "Dan Williams" <dan.j.williams@intel.com>,
        Dave Jiang <dave.jiang@intel.com>,
        <linux-acpi@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <linux-mm@kvack.org>, <nvdimm@lists.linux.dev>,
        <linux-cxl@vger.kernel.org>, Huang Ying <ying.huang@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>
Subject: Re: [PATCH 1/3] mm/memory_hotplug: Allow an override for the
 memmap_on_memory param
Message-ID: <20230622143753.00000282@Huawei.com>
In-Reply-To: <0ea4728a-8601-bf75-1921-bcde0818aac3@redhat.com>
References: <20230613-vv-kmem_memmap-v1-0-f6de9c6af2c6@intel.com>
        <20230613-vv-kmem_memmap-v1-1-f6de9c6af2c6@intel.com>
        <0ea4728a-8601-bf75-1921-bcde0818aac3@redhat.com>
Organization: Huawei Technologies Research and Development (UK) Ltd.
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On Fri, 16 Jun 2023 09:46:59 +0200
David Hildenbrand <david@redhat.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