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 14:00:58 +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 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().

-- 
Thanks,

David / dhildenb