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 14:36:13 +0200
From:   Laurent Dufour <ldufour@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.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

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.