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

[James Morse <james.morse@arm.com>]

Hi Eric,

On 15/04/2020 21:33, Eric W. Biederman wrote:
> James Morse <james.morse@arm.com> writes:
> 
>> An image loaded for kexec is not stored in place, instead its segments
>> are scattered through memory, and are re-assembled when needed. In the
>> meantime, the target memory may have been removed.
>>
>> Because mm is not aware that this memory is still in use, it allows it
>> to be removed.
>>
>> Add a memory notifier to prevent the removal of memory regions that
>> overlap with a loaded kexec image segment. e.g., when triggered from the
>> Qemu console:
>> | kexec_core: memory region in use
>> | memory memory32: Offline failed.
>>
>> Signed-off-by: James Morse <james.morse@arm.com>
> 
> Given that we are talking about the destination pages for kexec
> not where the loaded kernel is currently stored the description is
> confusing.

I think David has some better wording to cover this. I thought I had it with 'scattered
and re-assembled'.


> Beyond that I think it would be better to simply unload the loaded
> kernel at memory hotunplug time.

Unconditionally, or if it aliases the removed region?

I don't particular like it. User-space has asked for two impossible things, we are
changing the answer to the first when we see the second. Its a bit spooky. (maybe no one
will notice)


> Usually somewhere in the loaded image
> is a copy of the memory map at the time the kexec kernel was loaded.
> That will invalidate the memory map as well.

Ah, unconditionally. Sure, x86 needs this.
(arm64 re-discovers the memory map from firmware tables after kexec)

If that's an acceptable change in behaviour, sure, lets do that.


> All of this should be for a very brief window of a few seconds, as
> the loaded kexec image is quite short.

It seems I'm the outlier anticipating anything could happen between those syscalls.


Thanks,

James