linux-kernel - Re: [RFC PATCH] mm, hotplug: get rid of auto_online

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Date:   Mon, 27 Feb 2017 16:43:04 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Heiko Carstens <heiko.carstens@...ibm.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        David Rientjes <rientjes@...gle.com>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        linux-api@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        linux-s390@...r.kernel.org, xen-devel@...ts.xenproject.org,
        linux-acpi@...r.kernel.org
Subject: Re: [RFC PATCH] mm, hotplug: get rid of auto_online_blocks

On Mon 27-02-17 12:25:10, Heiko Carstens wrote:
> On Mon, Feb 27, 2017 at 11:02:09AM +0100, Vitaly Kuznetsov wrote:
> > A couple of other thoughts:
> > 1) Having all newly added memory online ASAP is probably what people
> > want for all virtual machines.
> 
> This is not true for s390. On s390 we have "standby" memory that a guest
> sees and potentially may use if it sets it online. Every guest that sets
> memory offline contributes to the hypervisor's standby memory pool, while
> onlining standby memory takes memory away from the standby pool.
> 
> The use-case is that a system administrator in advance knows the maximum
> size a guest will ever have and also defines how much memory should be used
> at boot time. The difference is standby memory.
> 
> Auto-onlining of standby memory is the last thing we want.
> 
> > Unfortunately, we have additional complexity with memory zones
> > (ZONE_NORMAL, ZONE_MOVABLE) and in some cases manual intervention is
> > required. Especially, when further unplug is expected.
> 
> This also is a reason why auto-onlining doesn't seem be the best way.

Can you imagine any situation when somebody actually might want to have
this knob enabled? From what I understand it doesn't seem to be the
case.

-- 
Michal Hocko
SUSE Labs