Date:   Mon, 21 Oct 2019 15:25:08 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     mark.rutland@....com, david@...hat.com, linux-mm@...ck.org,
        arunks@...eaurora.org, cpandya@...eaurora.org, ira.weiny@...el.com,
        will@...nel.org, steven.price@....com, valentin.schneider@....com,
        suzuki.poulose@....com, Robin.Murphy@....com, broonie@...nel.org,
        cai@....pw, ard.biesheuvel@....com, dan.j.williams@...el.com,
        linux-arm-kernel@...ts.infradead.org, osalvador@...e.de,
        steve.capper@....com, logang@...tatee.com,
        linux-kernel@...r.kernel.org, James Morse <james.morse@....com>,
        akpm@...ux-foundation.org, mgorman@...hsingularity.net
Subject: Re: [PATCH V9 2/2] arm64/mm: Enable memory hot remove



On 10/21/2019 03:23 PM, Anshuman Khandual wrote:
> 
> On 10/18/2019 03:18 PM, Catalin Marinas wrote:
>> On Fri, Oct 11, 2019 at 08:26:32AM +0530, Anshuman Khandual wrote:
>>> On 10/10/2019 05:04 PM, Catalin Marinas wrote:
>>>> Mark Rutland mentioned at some point that, as a preparatory patch to
>>>> this series, we'd need to make sure we don't hot-remove memory already
>>>> given to the kernel at boot. Any plans here?
>>> Hmm, this series just enables platform memory hot remove as required from
>>> generic memory hotplug framework. The path here is triggered either from
>>> remove_memory() or __remove_memory() which takes physical memory range
>>> arguments like (nid, start, size) and do the needful. arch_remove_memory()
>>> should never be required to test given memory range for anything including
>>> being part of the boot memory.
>> Assuming arch_remove_memory() doesn't (cannot) check, is there a risk on
> Platform can definitely enumerate boot memory ranges. But checking on it in
> arch_remove_memory() which deals with actual procedural details might not be
> ideal IMHO. Refusing a requested removal attempt should have been done up in
> the call chain. This will require making generic hot plug reject any removal
> request which falls within enumerated boot memory. IFAICS currently there is
> no generic way to remember which memory came as part of the boot process.
> Probably be a new MEMBLOCK flag will do.
> 
>> arm64 that, for example, one removes memory available at boot and then
>> kexecs a new kernel? Does the kexec tool present the new kernel with the
>> original memory map?
> I dont know, probably James can help here. But as I had mentioned earlier,
> the callers of remove_memory() should be able to control that. ACPI should
> definitely be aware about which ranges were part of boot memory and refrain
> from removing any subset, if the platform is known to have problems with
> any subsequent kexec operation because the way boot memory map get used.
> 
> Though I am not much aware about kexec internals, it should inherit the
> memory state at given point in time accommodating all previous memory hot
> and remove operations. As an example cloud environment scenario, memory
> resources might have increased or decreased during a guest lifetime, so
> when the guest needs to have new OS image why should not it have all the
> memory ? I dont know if it's feasible for the guest to expect previous hot
> add or remove operations to be played again after the kexec.
> 
> There is another fundamental question here. Is there a notion of a minimum
> subset of boot memory which cannot be hot removed no matter what ? If yes,
> how that is being conveyed to the kernel currently ?
> 
> The point is that all these need to be established between ACPI, EFI and
> kernel. AFAICS this problem is for MM subsystem (including the platform

s/is for/is not for/          ^^^^^^^^^^

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