linux-kernel - Re: [PATCH] mm: don't rely on system state to detect hot-plug operations

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Date:   Thu, 10 Sep 2020 14:38:00 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Laurent Dufour <ldufour@...ux.ibm.com>,
        Michal Hocko <mhocko@...e.com>
Cc:     akpm@...ux-foundation.org, Oscar Salvador <osalvador@...e.de>,
        rafael@...nel.org, nathanl@...ux.ibm.com, cheloha@...ux.ibm.com,
        stable@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: don't rely on system state to detect hot-plug
 operations

On 10.09.20 14:36, Laurent Dufour wrote:
> Le 10/09/2020 à 14:00, David Hildenbrand a écrit :
>> On 10.09.20 13:35, Laurent Dufour wrote:
>>> Le 10/09/2020 à 13:12, Michal Hocko a écrit :
>>>> On Thu 10-09-20 09:51:39, Laurent Dufour wrote:
>>>>> Le 10/09/2020 à 09:23, Michal Hocko a écrit :
>>>>>> On Wed 09-09-20 18:07:15, Laurent Dufour wrote:
>>>>>>> Le 09/09/2020 à 12:59, Michal Hocko a écrit :
>>>>>>>> On Wed 09-09-20 11:21:58, Laurent Dufour wrote:
>>>>>> [...]
>>>>>>>>> For the point a, using the enum allows to know in
>>>>>>>>> register_mem_sect_under_node() if the link operation is due to a hotplug
>>>>>>>>> operation or done at boot time.
>>>>>>>>
>>>>>>>> Yes, but let me repeat. We have a mess here and different paths check
>>>>>>>> for the very same condition by different ways. We need to unify those.
>>>>>>>
>>>>>>> What are you suggesting to unify these checks (using a MP_* enum as
>>>>>>> suggested by David, something else)?
>>>>>>
>>>>>> We do have system_state check spread at different places. I would use
>>>>>> this one and wrap it behind a helper. Or have I missed any reason why
>>>>>> that wouldn't work for this case?
>>>>>
>>>>> That would not work in that case because memory can be hot-added at the
>>>>> SYSTEM_SCHEDULING system state and the regular memory is also registered at
>>>>> that system state too. So system state is not enough to discriminate between
>>>>> the both.
>>>>
>>>> If that is really the case all other places need a fix as well.
>>>> Btw. could you be more specific about memory hotplug during early boot?
>>>> How that happens? I am only aware of https://lkml.kernel.org/r/20200818110046.6664-1-osalvador@suse.de
>>>> and that doesn't happen as early as SYSTEM_SCHEDULING.
>>>
>>> That points has been raised by David, quoting him here:
>>>
>>>> IIRC, ACPI can hotadd memory while SCHEDULING, this patch would break that.
>>>>
>>>> Ccing Oscar, I think he mentioned recently that this is the case with ACPI.
>>>
>>> Oscar told that he need to investigate further on that.
>>>
>>> On my side I can't get these ACPI "early" hot-plug operations to happen so I
>>> can't check that.
>>>
>>> If this is clear that ACPI memory hotplug doesn't happen at SYSTEM_SCHEDULING,
>>> the patch I proposed at first is enough to fix the issue.
>>>
>>
>> Booting a qemu guest with 4 coldplugged DIMMs gives me:
>>
>> :/root# dmesg | grep link_mem
>> [    0.302247] link_mem_sections() during 1
>> [    0.445086] link_mem_sections() during 1
>> [    0.445766] link_mem_sections() during 1
>> [    0.446749] link_mem_sections() during 1
>> [    0.447746] link_mem_sections() during 1
>>
>> So AFAICs everything happens during SYSTEM_SCHEDULING - boot memory and
>> ACPI (cold)plug.
>>
>> To make forward progress with this, relying on the system_state is
>> obviously not sufficient.
>>
>> 1. We have to fix this instance and the instance directly in
>> get_nid_for_pfn() by passing in the context (I once had a patch to clean
>> that up, to not have two state checks, but it got lost somewhere).
>>
>> 2. The "system_state < SYSTEM_RUNNING" check in
>> register_memory_resource() is correct. Actual memory hotplug after boot
>> is not impacted. (I remember we discussed this exact behavior back then)
>>
>> 3. build_all_zonelists() should work as expected, called from
>> start_kernel() before sched_init().
> 
> I'm bit confused now.
> Since hotplug operation is happening at SYSTEM_SCHEDULING like the regular 
> memory registration, would it be enough to add a parameter to 
> register_mem_sect_under_node() (reworking the memmap_context enum)?
> That way the check is not based on the system state but on the calling path.
> 

That would have been my suggestion to definitely fix it - maybe
Michal/Oscar have a better suggestion know that we know what's going on.

-- 
Thanks,

David / dhildenb