linux-kernel - Re: [RFC PATCH v2 3/4] mm,memory_hotplug: Add mhp_supports_memmap_on

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Date:   Mon, 30 Nov 2020 09:50:22 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Michal Hocko <mhocko@...e.com>
Cc:     david@...hat.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        vbabka@...e.cz, pasha.tatashin@...een.com
Subject: Re: [RFC PATCH v2 3/4] mm,memory_hotplug: Add
 mhp_supports_memmap_on_memory

On Fri, Nov 27, 2020 at 04:02:53PM +0100, Michal Hocko wrote:
> It should also require a tunable (kernel parameter for now but maybe we
> will need a more fine grained control later) to enable this explicitly.
> Earlier discussions have pointed out that allocating vmemmap from each
> section can lead to a sparse memory unsuitable for very large pages.
> So I believe this should be an opt in.

Yeah, I already had that in mind, just did not get to implement it in this RFC
yet as I was more focused on the implementation per se.
I thought about a tunable in /sys/device/system/memory/[file], but a kernel
command line boot option would also work and for the first implementation
would be less for a hassel.

> Also is there any reason why this cannot be a preparatory patch for the
> actual implementation? It would look more natural that way to me.

I guess you are right, will re-order it in a future submission.

-- 
Oscar Salvador
SUSE L3