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

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Message-ID: <Z9BdwFZ-ZRpMSabG@gourry-fedora-PF4VCD3F>
Date: Tue, 11 Mar 2025 11:58:56 -0400
From: Gregory Price <gourry@...rry.net>
To: Zi Yan <ziy@...dia.com>
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 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.

~Gregory