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

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Message-ID: <Z7TLwtQY3vGUw2bO@gourry-fedora-PF4VCD3F>
Date: Tue, 18 Feb 2025 13:04:50 -0500
From: Gregory Price <gourry@...rry.net>
To: Yang Shi <shy828301@...il.com>
Cc: 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, Feb 18, 2025 at 09:49:28AM -0800, Yang Shi wrote:
> On Mon, Feb 17, 2025 at 12:05 PM Gregory Price <gourry@...rry.net> wrote:
> > The node ID passed in as an argument is a "preferred node", which means
> > is insufficient space on that node exists to service the GFP_KERNEL
> > allocation, it will fall back to another node.
> >
> > If all hot-plugged memory is added to ZONE_MOVABLE, two things occur:
> >
> >   1) A portion of the memory block is carved out for to allocate memmap
> >      data (reducing usable size by 64b*nr_pages)
> >
> >   2) The memory is allocated on ZONE_NORMAL on another node..
> 
> Nice write-up, thanks for putting everything together. A follow up
> question on this. Do you mean the memmap memory will show up as a new
> node with ZONE_NORMAL only besides other hot-plugged memory blocks? So
> we will actually see two nodes are hot-plugged?
> 

No, it creates 1 ZONE_MOVABLE memory block of size

                   (BLOCK_SIZE - memmap_size)

and as far as i can tell the actual memory map allocations still
occur on ZONE_NORMAL (i.e. not CXL).

So you just lose the capacity, it's just stranded and unused.

> Thanks,
> Yang
>