Re: [PATCH stable 5.10] mm/memory_hotplug: extend offline_and_remove_memory() to handle more than one memory block

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Jun 19, 2023 at 02:51:21PM +0800, Wupeng Ma wrote:
> From: David Hildenbrand <david@redhat.com>
> 
> commit 8dc4bb58a146655eb057247d7c9d19e73928715b upstream.
> 
> virtio-mem soon wants to use offline_and_remove_memory() memory that
> exceeds a single Linux memory block (memory_block_size_bytes()). Let's
> remove that restriction.
> 
> Let's remember the old state and try to restore that if anything goes
> wrong. While re-onlining can, in general, fail, it's highly unlikely to
> happen (usually only when a notifier fails to allocate memory, and these
> are rather rare).
> 
> This will be used by virtio-mem to offline+remove memory ranges that are
> bigger than a single memory block - for example, with a device block
> size of 1 GiB (e.g., gigantic pages in the hypervisor) and a Linux memory
> block size of 128MB.
> 
> While we could compress the state into 2 bit, using 8 bit is much
> easier.
> 
> This handling is similar, but different to acpi_scan_try_to_offline():
> 
> a) We don't try to offline twice. I am not sure if this CONFIG_MEMCG
> optimization is still relevant - it should only apply to ZONE_NORMAL
> (where we have no guarantees). If relevant, we can always add it.
> 
> b) acpi_scan_try_to_offline() simply onlines all memory in case
> something goes wrong. It doesn't restore previous online type. Let's do
> that, so we won't overwrite what e.g., user space configured.
> 
> Reviewed-by: Wei Yang <richard.weiyang@linux.alibaba.com>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Jason Wang <jasowang@redhat.com>
> Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Link: https://lore.kernel.org/r/20201112133815.13332-28-david@redhat.com
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> Acked-by: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>
> ---
>  mm/memory_hotplug.c | 105 +++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 89 insertions(+), 16 deletions(-)
> 

Why is this needed in 5.10.y?  Looks like a new feature to me, what
problem does it solve there?

thanks,

greg k-h