linux-kernel - Re: [PATCH V2] arm64/mm/hotplug: Warn when memory limit has been reduced

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Date:   Mon, 3 Jan 2022 13:57:28 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        James Morse <james.morse@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] arm64/mm/hotplug: Warn when memory limit has been
 reduced


>> Via which mechanism would the unplug of that memory happen? On arm64,
>> this should only be possible via ACPI, when unplugging a DIMM that was
>> available since boot.
>>
>> But won't acpi_memory_enable_device() try adding that memory while
>> ignoring the memory limit? And adding should work, no?
> 
> Adding that memory via hotplug into the kernel first ? In that case
> removal would still go via the kernel and user would know about it.

Can we please add details on how to actually trigger it (below) to the
patch description? Otherwise it's really hard to get about which senario
we do care, and about which we don't care.

> 
>>
>> Can you share some details on how to trigger this on arm64?
> 
> The primary scenario this proposal is targeted towards is when boot
> memory is set aside from the host, hot-plugged back into the kernel
> and repurposed (via hotplug-hotremove path) for guest kernel usage.
> This new warning would reassert that "mem=" cmdline option is debug
> only on arm64 platform, and should not be used for production.
Let me get this straight:

1. Restrict physical memory to use via "mem="

-> Some boot memory is !present and, therefore !early

2. Hotplug that memory to the kernel

-> How?

a) dax/kmem? Does not apply I think.
b) DIMM? Does not apply I think.
c) CONFIG_ARCH_MEMORY_PROBE ?

3. Trigger physical hotunplug and actually remove the memory

-> How?

4. kexec; will try using hotunplugged memory


-- 
Thanks,

David / dhildenb