Re: [PATCH 0/3] kexec/memory_hotplug: Prevent removal and accidental use

[David Hildenbrand <david@redhat.com>]

On 26.03.20 19:07, James Morse wrote:
> Hello!
> 
> arm64 recently queued support for memory hotremove, which led to some
> new corner cases for kexec.
> 
> If the kexec segments are loaded for a removable region, that region may
> be removed before kexec actually occurs. This causes the first kernel to
> lockup when applying the relocations. (I've triggered this on x86 too).
> 
> The first patch adds a memory notifier for kexec so that it can refuse
> to allow in-use regions to be taken offline.

IIRC other architectures handle that by setting the affected pages
PageReserved. Any reason why to not stick to the same?

> 
> 
> This doesn't solve the problem for arm64, where the new kernel must
> initially rely on the data structures from the first boot to describe
> memory. These don't describe hotpluggable memory.
> If kexec places the kernel in one of these regions, it must also provide
> a DT that describes the region in which the kernel was mapped as memory.
> (and somehow ensure its always present in the future...)
> 
> To prevent this from happening accidentally with unaware user-space,
> patches two and three allow arm64 to give these regions a different
> name.
> 
> This is a change in behaviour for arm64 as memory hotadd and hotremove
> were added separately.
> 
> 
> I haven't tried kdump.
> Unaware kdump from user-space probably won't describe the hotplug
> regions if the name is different, which saves us from problems if
> the memory is no longer present at kdump time, but means the vmcore
> is incomplete.

Whenever memory is added/removed, kdump.service is to be restarted from
user space, which will fixup the data structures such that kdump will
not try to dump unplugged memory. Also, makedumpfile will check if the
sections are still around IIRC.

Not sure what you mean by "Unaware kdump from user-space".


-- 
Thanks,

David / dhildenb