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

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Message-ID: <20200910111246.GE28354@dhcp22.suse.cz>
Date:   Thu, 10 Sep 2020 13:12:46 +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 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.

-- 
Michal Hocko
SUSE Labs