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

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Date:   Fri, 27 Nov 2020 16:02:53 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Oscar Salvador <osalvador@...e.de>
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 Wed 25-11-20 12:20:47, Oscar Salvador wrote:
> mhp_supports_memmap_on_memory is meant to be used by the caller prior
> to hot-adding memory in order to figure out whether it can enable
> MHP_MEMMAP_ON_MEMORY or not.
> 
> Enabling MHP_MEMMAP_ON_MEMORY requires:
> 
>  - CONFIG_SPARSEMEM_VMEMMAP
>  - architecture support for altmap
>  - hot-added range spans a single memory block

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.

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.

-- 
Michal Hocko
SUSE Labs