Re: [PATCH v2 2/5] memblock: improve MEMBLOCK_HOTPLUG documentation

[Mike Rapoport <rppt@kernel.org>]

On Mon, Oct 04, 2021 at 11:36:02AM +0200, David Hildenbrand wrote:
> The description of MEMBLOCK_HOTPLUG is currently short and consequently
> misleading: we're actually dealing with a memory region that might get
> hotunplugged later (i.e., the platform+firmware supports it), yet it is
> indicated in the firmware-provided memory map as system ram that will just
> get used by the system for any purpose when not taking special care. The
> firmware marked this memory region as a hot(un)plugged (e.g., hotplugged
> before reboot), implying that it might get hotunplugged again later.
> 
> Whether we consider this information depends on the "movable_node" kernel
> commandline parameter: only with "movable_node" set, we'll try keeping
> this memory hotunpluggable, for example, by not serving early allocations
> from this memory region and by letting the buddy manage it using the
> ZONE_MOVABLE.
> 
> Let's make this clearer by extending the documentation.
> 
> Note: kexec *has to* indicate this memory to the second kernel. With
> "movable_node" set, we don't want to place kexec-images on this memory.
> Without "movable_node" set, we don't care and can place kexec-images on
> this memory. In both cases, after successful memory hotunplug, kexec has to
> be re-armed to update the memory map for the second kernel and to place the
> kexec-images somewhere else.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>

> ---
>  include/linux/memblock.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> index 34de69b3b8ba..4ee8dd2d63a7 100644
> --- a/include/linux/memblock.h
> +++ b/include/linux/memblock.h
> @@ -28,7 +28,11 @@ extern unsigned long long max_possible_pfn;
>  /**
>   * enum memblock_flags - definition of memory region attributes
>   * @MEMBLOCK_NONE: no special request
> - * @MEMBLOCK_HOTPLUG: hotpluggable region
> + * @MEMBLOCK_HOTPLUG: memory region indicated in the firmware-provided memory
> + * map during early boot as hot(un)pluggable system RAM (e.g., memory range
> + * that might get hotunplugged later). With "movable_node" set on the kernel
> + * commandline, try keeping this memory region hotunpluggable. Does not apply
> + * to memblocks added ("hotplugged") after early boot.
>   * @MEMBLOCK_MIRROR: mirrored region
>   * @MEMBLOCK_NOMAP: don't add to kernel direct mapping and treat as
>   * reserved in the memory map; refer to memblock_mark_nomap() description
> -- 
> 2.31.1
> 

-- 
Sincerely yours,
Mike.