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

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Date: Tue, 11 Jun 2024 11:42:56 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-hyperv@...r.kernel.org,
 virtualization@...ts.linux.dev, xen-devel@...ts.xenproject.org,
 kasan-dev@...glegroups.com, Mike Rapoport <rppt@...nel.org>,
 Oscar Salvador <osalvador@...e.de>, "K. Y. Srinivasan" <kys@...rosoft.com>,
 Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>,
 Dexuan Cui <decui@...rosoft.com>, "Michael S. Tsirkin" <mst@...hat.com>,
 Jason Wang <jasowang@...hat.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
 Eugenio Pérez <eperezma@...hat.com>,
 Juergen Gross <jgross@...e.com>, Stefano Stabellini
 <sstabellini@...nel.org>,
 Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
 Alexander Potapenko <glider@...gle.com>, Marco Elver <elver@...gle.com>,
 Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v1 2/3] mm/memory_hotplug: initialize memmap of
 !ZONE_DEVICE with PageOffline() instead of PageReserved()

On 07.06.24 11:09, 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.
> 

@Andrew, can we add here:

"Note that self-hosted vmemmap pages will no longer be marked as 
reserved. This matches ordinary vmemmap pages allocated from the buddy 
during memory hotplug. Now, really only vmemmap pages allocated from 
memblock during early boot will be marked reserved. Existing 
PageReserved() checks seem to be handling all relevant cases correctly 
even after this change."

-- 
Cheers,

David / dhildenb