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

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Date:   Fri, 30 Jun 2017 10:39:26 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Reza Arbab <arbab@...ux.vnet.ibm.com>,
        Yasuaki Ishimatsu <yasu.isimatu@...il.com>,
        Xishi Qiu <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>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm, memory_hotplug: remove zone restrictions

On Fri 30-06-17 11:09:51, Wei Yang wrote:
> On Thu, Jun 29, 2017 at 3:35 PM, Michal Hocko <mhocko@...nel.org> wrote:
> > From: Michal Hocko <mhocko@...e.com>
> >
> 
> Michal,
> 
> I love the idea very much.
> 
> > Historically we have enforced that any kernel zone (e.g ZONE_NORMAL) has
> > to precede the Movable zone in the physical memory range. The purpose of
> > the movable zone is, however, not bound to any physical memory restriction.
> > It merely defines a class of migrateable and reclaimable memory.
> >
> > There are users (e.g. CMA) who might want to reserve specific physical
> > memory ranges for their own purpose. Moreover our pfn walkers have to be
> > prepared for zones overlapping in the physical range already because we
> > do support interleaving NUMA nodes and therefore zones can interleave as
> > well. This means we can allow each memory block to be associated with a
> > different zone.
> >
> > Loosen the current onlining semantic and allow explicit onlining type on
> > any memblock. That means that online_{kernel,movable} will be allowed
> > regardless of the physical address of the memblock as long as it is
> > offline of course. This might result in moveble zone overlapping with
> > other kernel zones. Default onlining then becomes a bit tricky but still
> 
> As here mentioned, we just remove the restriction for zone_movable.
> For other zones, we still keep the restriction and the order as before.

All other zones except for ZONE_NORMAL are subject of the physical
memory restrictions.
 
> Maybe the title is a little misleading. Audience may thinks no restriction
> for all zones.

I thought the context was clear from the fact that this is a hotplug
related patch. As such we do not allow online_{dma,dma32,normal} we only
allow to online into a kernel zone. I can update the wording but do not
have a good idea how.

[...]
> As I spotted on the previous patch, after several round of online/offline,
> The output of valid_zones will differ.
> 
> For example in this case, after I offline memory37 and 41, I expect this:
> 
>  memory34/valid_zones:Normal
>  memory35/valid_zones:Normal Movable
>  memory36/valid_zones:Normal Movable
>  memory37/valid_zones:Normal Movable
>  memory38/valid_zones:Normal Movable
>  memory39/valid_zones:Normal Movable
>  memory40/valid_zones:Normal Movable
>  memory41/valid_zones:Normal Movable
> 
> While the current result would be
> 
>  memory34/valid_zones:Normal
>  memory35/valid_zones:Normal Movable
>  memory36/valid_zones:Normal Movable
>  memory37/valid_zones:Movable Normal
>  memory38/valid_zones:Movable Normal
>  memory39/valid_zones:Movable Normal
>  memory40/valid_zones:Movable Normal
>  memory41/valid_zones:Movable Normal

You haven't written your sequence of onlining but if you used the same
one as mentioned in the patch then you should get
memory34/valid_zones:Normal
memory35/valid_zones:Normal Movable
memory36/valid_zones:Normal Movable
memory37/valid_zones:Normal Movable
memory38/valid_zones:Normal Movable
memory39/valid_zones:Normal
memory40/valid_zones:Movable Normal
memory41/valid_zones:Movable Normal

Even if you kept 37 as movable and offline 38 you wouldn't get 38-41
movable by default because...

> The reason is the same, we don't adjust the zone's range when offline
> memory.

.. of this.

> This is also a known issue?

yes and to be honest I do not plan to fix it unless somebody has a real
life usecase for it. Now that we allow explicit onlininig type anywhere
it seems like a reasonable behavior and this will allow us to remove
quite some code which is always a good deal wrt longterm maintenance.

Thanks!
-- 
Michal Hocko
SUSE Labs