linux-kernel - Re: [RFC PATCH 2/3] mm/memory_hotplug: Create __shrink_pages and move it to offline

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Message-ID: <20180807204834.GA6844@techadventures.net>
Date:   Tue, 7 Aug 2018 22:48:34 +0200
From:   Oscar Salvador <osalvador@...hadventures.net>
To:     David Hildenbrand <david@...hat.com>
Cc:     Jerome Glisse <jglisse@...hat.com>, akpm@...ux-foundation.org,
        mhocko@...e.com, dan.j.williams@...el.com,
        pasha.tatashin@...cle.com, yasu.isimatu@...il.com,
        logang@...tatee.com, dave.jiang@...el.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Oscar Salvador <osalvador@...e.de>
Subject: Re: [RFC PATCH 2/3] mm/memory_hotplug: Create __shrink_pages and
 move it to offline_pages

On Tue, Aug 07, 2018 at 04:54:57PM +0200, David Hildenbrand wrote:
> I wonder if we could instead forward from the callers whether we are
> dealing with ZONE_DEVICE memory (is_device ...), at least that seems
> feasible in hmm code. Not having looked at details yet.

Yes, this looks like the most straightforward way right now.
We would have to pass it from arch_remove_memory to __remove_pages though.

It is not the most elegant way, but looking at the code of devm_memremap_pages_release
and hmm_devmem_release I cannot really think of anything better.

In hmm_devmem_release is should be easy because AFAIK (unless I am missing something), hmm always works
with ZONE_DEVICE.
At least hmm_devmem_pages_create() moves the range to ZONE_DEVICE.

After looking at devm_memremap_pages(), I think it does the same:

...
move_pfn_range_to_zone(&NODE_DATA(nid)->node_zones[ZONE_DEVICE],
					align_start >> PAGE_SHIFT,
					align_size >> PAGE_SHIFT, altmap);
...

So I guess it is safe to assume that arch_remove_memory/__remove_pages are called
from those functions while zone being ZONE_DEVICE.

Is that right, Jerome?

And since we know for sure that memhotplug-code cannot call it with ZONE_DEVICE,
I think this can be done easily.

Thanks
-- 
Oscar Salvador
SUSE L3