Re: [PATCH 1/3] kexec: Prevent removal of memory in use by a loaded kexec image

[David Hildenbrand <david@redhat.com>]

>>> Sounds doable to me, and not complicated.
>>>
>>>> images. It would apply to
>>>>
>>>> - arm64 and filter out all hotadded memory (IIRC, only boot memory can
>>>>   be used).
>>>
>>> Do you mean hot added memory after boot can't be recognized and added
>>> into system RAM on arm64?
>>
>> See patch #3 of this patch set, which wants to avoid placing kexec
>> binaries on hotplugged memory. But I have no idea what the current plan
>> regarding arm64 is (this thread exploded :) ).
>>
>> I would assume that we don't want to place kexec images on any
>> hotplugged (or rather: hot(un)pluggable) memory - on any architecture.
> 
> Yes, noticed that and James replied to DaveY.
> 
> Later, when I was considering to make a draft patch to do the picking of
> memory from normal zone, and add a notifier, as we discussed at above, I
> suddenly realized that kexec_file_load doesn't have this issue. It
> traverse system RAM bottom up to get an available region to put
> kernel/initrd/boot_param, etc. I can't think of a system where its
> low memory could be unavailable.

kexec_walk_memblock() has the option for "kbuf->top_down". Only
kexec_walk_resources() seems to ignore it.

So I think in case of memblocks (e.g., arm64), this still applies?

>>
>>>
>>>
>>>> - powerpc to filter out all LMBs that can be removed (assuming not all
>>>>   memory corresponds to LMBs that can be removed, otherwise we're in
>>>>   trouble ... :) )
>>>> - virtio-mem to filter out all memory it added.
>>>> - hyper-v to filter out partially backed memory blocks (esp. the last
>>>>   memory block it added and only partially backed it by memory).
>>>>
>>>> This would make it work for kexec_file_load(), however, I do wonder how
>>>> we would want to approach that from userspace kexec-tools when handling
>>>> it from kexec_load().
>>>
>>> Let's make kexec_file_load work firstly. Since this work is only first
>>> step to make kexec-ed kernel not break memory hotplug. After kexec
>>> rebooting, the KASLR may locate kernel into hotpluggable area too.
>>
>> Can you elaborate how that would work?
> 
> Well, boot memory can be hotplugged or not after boot, they are marked
> in uefi tables, the current kexec doesn't save and pass them into 2nd
> kenrel, when kexec kernel bootup, it need read them and avoid them to
> randomize kernel into.

What about e.g., memory hotplugged by ACPI? I would assume, that the
kexec kernel will not make use of that (IOW detected that) until the
ACPI driver comes up and re-detects + adds that memory.

Or how would that machinery work in case we have a DIMM hotplugged via ACPI?

-- 
Thanks,

David / dhildenb