On 29.06.24 10:32, Wei Yang wrote:
On Sat, Jun 29, 2024 at 08:19:49AM +0200, David Hildenbrand wrote:
On 29.06.24 03:33, Wei Yang wrote:
Initialize all pages reserved is an ancient behavior.
Since commit 92923ca3aace ("mm: meminit: only set page reserved in the
memblock region"), SetPageReserved is removed from
__init_single_page(). Only those reserved pages are marked PG_reserved.
But we still set PG_reserved on offline and check it on online.
Following two commits removed both of them:
* Commit 0ee5f4f31d36 ("mm/page_alloc.c: don't set pages PageReserved()
when offlining") removed the set on offline.
* Commit 5ecae6359e3a ("mm/memory_hotplug: drop PageReserved() check in
online_pages_range()") removed the check on online.
This means we set PG_reserved for hot-plugged memory at initialization
is not helpful and a little different from bootmem initialization path.
Now we can remove it.
It's not that easy for ZONE_DEVICE.
Also, see mm/mm-stable
commit 3dadec1babf9eee0c67c967df931d6f0cb124a04
Author: David Hildenbrand <david@xxxxxxxxxx>
Date: Fri Jun 7 11:09:36 2024 +0200
mm: pass meminit_context to __free_pages_core()
Patch series "mm/memory_hotplug: use PageOffline() instead of
PageReserved() for !ZONE_DEVICE".
commit b873faaa609ab44c223b2327f55d2b6a2ba4ca9c
Author: David Hildenbrand <david@xxxxxxxxxx>
Date: Fri Jun 7 11:09:37 2024 +0200
mm/memory_hotplug: initialize memmap of !ZONE_DEVICE with PageOffline()
instead of PageReserved()
Let me try to understand this.
You also tries to get rid of PG_reserved but you want PG_offline instead,
because this benefit virtio-mem, right?
We now make proper use of PG_offline. All hotplugged pages start out
PG_offline once we turn the section online. Only the ones that actually
get exposed to the buddy -- actually get onlined -- get PG_offline
cleared. A side effect of that is less hacks for virtio-mem, and more
natural handling for the other ballooning drivers that hotplug memory.
In the future, I'm planning on moving more fake-offlining code from
virtio-mem the core, making use of more PG_offline in memory.
For now, it's stops the PG_reserved use while maintaining the same
semantics as before: the page content and "struct page" is not to be
touched by anybody except the "owner".
But I don't get why PG_offline is wrong for ZONE_DEVICE. I may miss some
knowledge for it.
I suggest you take a look at the PG_offline documentation. ZONE_DEVICE
are certainly not logically offline pages. They will never be considered
online as part of online sections. But they will never be handed to the
buddy.
Maybe we want a dedicate page type for them in the future, not sure. We
can right now identify them reliably using the zone idx.
Using a page type right now is very likely not possible, because we
might be using the page->_mapcount in rmap code when mapping some of
them to user space.
If we want to get rid of the PG_reserved for them right now, we'll have
to make sure all existing PageReserved checks won't be degraded. For
example, drivers/vfio/vfio_iommu_type1.c might need some work (no sure).
The KVM one in kvm_pfn_to_refcounted_page() should already be fine,
because they really want to refcount them.
A lot of other ones like can_gather_numa_stats(), already refuse
is_zone_device_page() manually, and maybe we want to factor both checks
out into a separate function like "is_special_reserved_page()" or sth
like that.
--
Cheers,
David / dhildenb
