linux-kernel - Re: [RFC PATCH 3/5] mm, memory_hotplug: allocate memmap from the added memory range for sparse-vmemmap

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Date:   Wed, 26 Jul 2017 14:30:41 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Heiko Carstens <heiko.carstens@...ibm.com>
Cc:     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>,
        Jerome Glisse <jglisse@...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>,
        Andi Kleen <ak@...ux.intel.com>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Dan Williams <dan.j.williams@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>
Subject: Re: [RFC PATCH 3/5] mm, memory_hotplug: allocate memmap from the
 added memory range for sparse-vmemmap

On Wed 26-07-17 13:45:39, Heiko Carstens wrote:
[...]
> In general I do like your idea, however if I understand your patches
> correctly we might have an ordering problem on s390: it is not possible to
> access hot-added memory on s390 before it is online (MEM_GOING_ONLINE
> succeeded).

Could you point me to the code please? I cannot seem to find the
notifier which implements that.

> On MEM_GOING_ONLINE we ask the hypervisor to back the potential available
> hot-added memory region with physical pages. Accessing those ranges before
> that will result in an exception.

Can we make the range which backs the memmap range available? E.g from
s390 specific __vmemmap_populate path?
 
> However with your approach the memory is still allocated when add_memory()
> is being called, correct? That wouldn't be a change to the current
> behaviour; except for the ordering problem outlined above.

Could you be more specific please? I do not change when the memmap is
allocated.

-- 
Michal Hocko
SUSE Labs