linux-kernel - Re: CXL Boot to Bash - Section 3: Memory (block) Hotplug

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Message-ID: <7A60A7B1-0BFF-45ED-8A11-06F0D5286723@nvidia.com>
Date: Tue, 11 Mar 2025 12:08:03 -0400
From: Zi Yan <ziy@...dia.com>
To: Gregory Price <gourry@...rry.net>
Cc: David Hildenbrand <david@...hat.com>, Yang Shi <shy828301@...il.com>,
 lsf-pc@...ts.linux-foundation.org, linux-mm@...ck.org,
 linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: CXL Boot to Bash - Section 3: Memory (block) Hotplug

On 11 Mar 2025, at 11:58, Gregory Price wrote:

> On Tue, Mar 11, 2025 at 10:53:41AM -0400, Zi Yan wrote:
>> On 20 Feb 2025, at 14:26, David Hildenbrand wrote:
>>
>>> Yes, the only way to get some 1 GiB pages is by using larger memory blocks (e.g., 2 GiB on x86-64), which comes with a different set of issues (esp. hotplug granularity).
>>
>> An alternative I can think of is to mark a hot-plugged memory block dedicated
>> to memmap and use it for new memory block’s memmap provision. In this way,
>> a 256MB memory block can be used for 256MB*(256MB/4MB)=16GB hot plugged memory.
>> Yes, it will waste memory before 256MB+16GB is online, but that might be
>> easier to handle than variable sized memory block, I suppose?
>>
>>>
>
> The devil is in the details here.  We'd need a way for the driver to
> tell hotplug "use this for memmap for some yet-to-be-mapped region" -
> rather than having that allocate naturally.  Either this, or a special
> ZONE specifically for memmap allocations.

Or a new option for memmap_on_memory like “use_whole_block”, then hotplug
code checks altmap is NULL or not when a memory block is plugged.
If altmap is NULL, the hot plugged memory block is used as memmap,
otherwise, the memory block is plugged as normal memory. The code also
needs to maintain which part of the altmap is used to tell whether
the memmap’d memory block can be offline or not.


Best Regards,
Yan, Zi