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

[Gregory Price <gourry@gourry.net>]

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@gourry.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
>