linux-kernel - Re: [PATCH v4 00/13] mm: sub-section memory hotplug support

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Message-ID: <CAPcyv4hMt0s7UX=MO9KwakjXG9Uff=8XGR+Uc7YoVWoLqbKeGw@mail.gmail.com>
Date:   Thu, 16 Mar 2017 12:04:48 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Toshi Kani <toshi.kani@....com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Stephen Bates <stephen.bates@...rosemi.com>,
        Linux MM <linux-mm@...ck.org>,
        Nicolai Stange <nicstange@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v4 00/13] mm: sub-section memory hotplug support

On Thu, Mar 16, 2017 at 10:48 AM, Michal Hocko <mhocko@...nel.org> wrote:
> Hi,
> I didn't get to look through the patch series yet and I might not be
> able before LSF/MM. How urgent is this? I am primarily asking because
> the memory hotplug is really convoluted right now and putting more on
> top doesn't really sound like the thing we really want. I have tried to
> simplify the code [1] already but this is an early stage work so I do
> not want to impose any burden on you. So I am wondering whether this
> is something that needs to be merged very soon or it can wait for the
> rework and hopefully end up being much simpler in the end as well.
>
> What do you think?

In general, I think it's better to add new features after
reworks/cleanup, but it's not clear to me (yet) that the problem you
are trying to solve makes this sub-section enabling for ZONE_DEVICE
any simpler.

> [1] http://lkml.kernel.org/r/20170315091347.GA32626@dhcp22.suse.cz

ZONE_DEVICE pages are never "online". The patch says "Instead we do
page->zone association from move_pfn_range which is called from
online_pages." which means the new scheme currently doesn't comprehend
the sprinkled ZONE_DEVICE hacks in the memory hotplug code.

However, that said, I might take a look at whether the hacks belong in
the auto-online code so that we can share the delayed zone
initialization, but still skip marking the memory online per the
expectations of ZONE_DEVICE. I expect it would be confusing to have
memblock devices in sysfs for ranges that can't be marked online?

Thoughts?