Re: [Xen-devel] [PATCH v1 00/10] mm: Don't mark hotplugged pages PG_reserved (including ZONE_DEVICE)

[David Hildenbrand <david@xxxxxxxxxx>]

On 24.10.19 14:09, David Hildenbrand wrote:
> This is the result of a recent discussion with Michal ([1], [2]). Right
> now we set all pages PG_reserved when initializing hotplugged memmaps. This
> includes ZONE_DEVICE memory. In case of system memory, PG_reserved is
> cleared again when onlining the memory, in case of ZONE_DEVICE memory
> never.
>
> In ancient times, we needed PG_reserved, because there was no way to tell
> whether the memmap was already properly initialized. We now have
> SECTION_IS_ONLINE for that in the case of !ZONE_DEVICE memory. ZONE_DEVICE
> memory is already initialized deferred, and there shouldn't be a visible
> change in that regard.
>
> One of the biggest fears were side effects. I went ahead and audited all
> users of PageReserved(). The details can be found in "mm/memory_hotplug:
> Don't mark pages PG_reserved when initializing the memmap".
>
> This patch set adapts all relevant users of PageReserved() to keep the
> existing behavior in respect to ZONE_DEVICE pages. The biggest part part
> that needs changes is KVM, to keep the existing behavior (that's all I
> care about in this series).
>
> Note that this series is able to rely completely on pfn_to_online_page().
> No new is_zone_device_page() calles are introduced (as requested by Dan).
> We are currently discussing a way to mark also ZONE_DEVICE memmaps as
> active/initialized - pfn_active() - and lightweight locking to make sure
> memmaps remain active (e.g., using RCU). We might later be able to convert
> some suers of pfn_to_online_page() to pfn_active(). Details can be found
> in [3], however, this represents yet another cleanup/fix we'll perform
> on top of this cleanup.
>
> I only gave it a quick test with DIMMs on x86-64, but didn't test the
> ZONE_DEVICE part at all (any tips for a nice QEMU setup?). Also, I didn't
> test the KVM parts (especially with ZONE_DEVICE pages or no memmap at all).
> Compile-tested on x86-64 and PPC.
Jeff Moyer ran some NVDIMM test cases for me (thanks!!!), including 
xfstests, pmdk, and ndctl. No regressions found.
I will run some KVM tests, especially NDIMM passthrough, but will have 
to setup a test environment first.
I would appreciate some review in the meantime. :)

--

Thanks,

David / dhildenb


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