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

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Message-ID: <bd6f2d09-f4e2-0a63-3511-e0f9bf283fe3@linux.ibm.com>
Date:   Thu, 10 Sep 2020 13:35:32 +0200
From:   Laurent Dufour <ldufour@...ux.ibm.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     akpm@...ux-foundation.org, David Hildenbrand <david@...hat.com>,
        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 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.