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

[Dan Williams]

On Thu, Mar 16, 2017 at 10:48 AM, Michal Hocko <mhocko@xxxxxxxxxx> 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@xxxxxxxxxxxxxx

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?