Re: [PATCH v1 2/3] mm/memory_hotplug: initialize memmap of !ZONE_DEVICE with PageOffline() instead of PageReserved()

[Oscar Salvador <osalvador@suse.de>]

On Fri, Jun 07, 2024 at 11:09:37AM +0200, David Hildenbrand wrote:
> We currently initialize the memmap such that PG_reserved is set and the
> refcount of the page is 1. In virtio-mem code, we have to manually clear
> that PG_reserved flag to make memory offlining with partially hotplugged
> memory blocks possible: has_unmovable_pages() would otherwise bail out on
> such pages.
> 
> We want to avoid PG_reserved where possible and move to typed pages
> instead. Further, we want to further enlighten memory offlining code about
> PG_offline: offline pages in an online memory section. One example is
> handling managed page count adjustments in a cleaner way during memory
> offlining.
> 
> So let's initialize the pages with PG_offline instead of PG_reserved.
> generic_online_page()->__free_pages_core() will now clear that flag before
> handing that memory to the buddy.
> 
> Note that the page refcount is still 1 and would forbid offlining of such
> memory except when special care is take during GOING_OFFLINE as
> currently only implemented by virtio-mem.
> 
> With this change, we can now get non-PageReserved() pages in the XEN
> balloon list. From what I can tell, that can already happen via
> decrease_reservation(), so that should be fine.
> 
> HV-balloon should not really observe a change: partial online memory
> blocks still cannot get surprise-offlined, because the refcount of these
> PageOffline() pages is 1.
> 
> Update virtio-mem, HV-balloon and XEN-balloon code to be aware that
> hotplugged pages are now PageOffline() instead of PageReserved() before
> they are handed over to the buddy.
> 
> We'll leave the ZONE_DEVICE case alone for now.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>

Acked-by: Oscar Salvador <osalvador@suse.de> # for the generic
memory-hotplug bits


-- 
Oscar Salvador
SUSE Labs