linux-kernel - Re: [PATCH 2/2] mm, memory_hotplug: remove zone restrictions

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Message-ID: <20170710123028.GH19185@dhcp22.suse.cz>
Date:   Mon, 10 Jul 2017 14:30:28 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Reza Arbab <arbab@...ux.vnet.ibm.com>,
        Yasuaki Ishimatsu <yasu.isimatu@...il.com>,
        qiuxishi@...wei.com, Kani Toshimitsu <toshi.kani@....com>,
        slaoub@...il.com, Joonsoo Kim <js1304@...il.com>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wei Yang <richard.weiyang@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm, memory_hotplug: remove zone restrictions

On Mon 10-07-17 14:12:09, Vlastimil Babka wrote:
> On 07/10/2017 01:17 PM, Michal Hocko wrote:
> > On Mon 10-07-17 13:11:29, Vlastimil Babka wrote:
> >> On 07/10/2017 08:45 AM, Michal Hocko wrote:
> >>> On Fri 07-07-17 17:02:59, Vlastimil Babka wrote:
> >>>> [+CC linux-api]
> >>>>
> >>>>
> >>>> Hm so previously, blocks 37-41 would only allow Movable at this point, right?
> >>>
> >>> yes
> >>>
> >>>> Shouldn't we still default to Movable for them? We might be breaking some
> >>>> existing userspace here.
> >>>
> >>> I do not think so. Prior to this merge window f1dd2cd13c4b ("mm,
> >>> memory_hotplug: do not associate hotadded memory to zones until online")
> >>> we allowed only the last offline or the adjacent to existing movable
> >>> memory block to be onlined movable. So the above wasn't possible.
> >>
> >> Not exactly the above, but let's say 1-34 is onlined as Normal, 35-37 is
> >> Movable. Then the only possible action before would be online 38 as
> >> Movable? Now it defaults to Normal?
> > 
> > Yes. And let me repeat you couldn't onlne 35-37 as movable before. So no
> > userspace could depend on that before the rework. Or do I still miss
> > your point?
> 
> Ah, I see. "the last offline or the adjacent to existing movable". OK then.
> 
> It would be indeed better to not change behavour twice then and merge
> this to 4.13, but it's the middle of merge window, so it's not simple...

yeah. I was thinking about about how to make the change reasonably
incremental but failed to find a way. I also didn't want to bring too
many changes at once (the code base is just too fragile already).

If there is a general consensus about the semantic we might want to push
the patch this week. I just do not want to rush it too much as this is a
users visible change and it might kick us back in future.
-- 
Michal Hocko
SUSE Labs