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

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Message-ID: <20200910120123.GF28354@dhcp22.suse.cz>
Date:   Thu, 10 Sep 2020 14:01:23 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Laurent Dufour <ldufour@...ux.ibm.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

On Thu 10-09-20 13:35:32, 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.

: Please, note that upstream has fixed that differently (and unintentionally) by
: adding another boot state (SYSTEM_SCHEDULING), which is set before smp_init().
: That should happen before memory hotplug events even with memhp_default_state=online.
: Backporting that would be too intrusive.

Either I am confused or the above says that no hotplug should happen
during SYSTEM_SCHEDULING even in the above case. I really have hard time
to imagine how an early boot hotplug should even work. We start with a
memory layout provided by a BIOS/FW and intiailize it statically. How
would a hotplug even actually trigger that early?
-- 
Michal Hocko
SUSE Labs