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Subject: Re: [PATCH V11 1/5] mm/hotplug: Introduce arch callback validating
 the hot remove range
To:     Anshuman Khandual <anshuman.khandual@arm.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
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From:   David Hildenbrand <david@redhat.com>
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Message-ID: <80ab5f55-77ef-4719-52fc-2b23c0ecb866@redhat.com>
Date:   Fri, 10 Jan 2020 09:42:21 +0100
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On 10.01.20 04:09, Anshuman Khandual wrote:
> Currently there are two interfaces to initiate memory range hot removal=
 i.e
> remove_memory() and __remove_memory() which then calls try_remove_memor=
y().
> Platform gets called with arch_remove_memory() to tear down required ke=
rnel
> page tables and other arch specific procedures. But there are platforms
> like arm64 which might want to prevent removal of certain specific memo=
ry
> ranges irrespective of their present usage or movability properties.

Why? Is this only relevant for boot memory? I hope so, otherwise the
arch code needs fixing IMHO.

If it's only boot memory, we should disallow offlining instead via a
memory notifier - much cleaner.

>=20
> Current arch call back arch_remove_memory() is too late in the process =
to
> abort memory hot removal as memory block devices and firmware memory ma=
p
> entries would have already been removed. Platforms should be able to ab=
ort
> the process before taking the mem_hotplug_lock with mem_hotplug_begin()=
.
> This essentially requires a new arch callback for memory range validati=
on.

I somewhat dislike this very much. Memory removal should never fail if
used sanely. See e.g., __remove_memory(), it will BUG() whenever
something like that would strike.

>=20
> This differentiates memory range validation between memory hot add and =
hot
> remove paths before carving out a new helper check_hotremove_memory_ran=
ge()
> which incorporates a new arch callback. This call back provides platfor=
ms
> an opportunity to refuse memory removal at the very onset. In future th=
e
> same principle can be extended for memory hot add path if required.
>=20
> Platforms can choose to override this callback in order to reject speci=
fic
> memory ranges from removal or can just fallback to a default implementa=
tion
> which allows removal of all memory ranges.

I suspect we want really want to disallow offlining instead. E.g., I
remember s390x does that with certain areas needed for dumping/kexec.

Somebody who added memory via add_memory() should always be able to
remove the memory via remove_memory() again. Only boot memory can be
treated in a special way, but boot memory is initially always online.

--=20
Thanks,

David / dhildenb