[PATCH v7 7/7] mm/memory_hotplug: Enable runtime update of memmap_on_memory parameter

Michal Hocko mhocko at suse.com
Thu Aug 3 21:30:54 AEST 2023

On Thu 03-08-23 11:24:08, David Hildenbrand wrote:
> > would be readable only when the block is offline and it would reallocate
> > vmemmap on the change. Makes sense? Are there any risks? Maybe pfn
> > walkers?
> The question is: is it of any real value such that it would be worth the
> cost and risk?
> One of the primary reasons for memmap_on_memory is that *memory hotplug*
> succeeds even in low-memory situations (including, low on ZONE_NORMAL
> situations).

One usecase I would have in mind is a mix of smaller and larger memory
blocks. For larger ones you want to have memmap_on_memory in general
because they do not eat memory from outside but small(er) ones might be
more tricky because now you can add a lot of blocks that would be
internally fragmented to prevent larger allocations to form.

> So you want that behavior already when hotplugging such
> devices. While there might be value to relocate it later, I'm not sure if
> that is really worth it, and it does not solve the main use case.

Is it worth it? TBH I am not sure same as I am not sure the global
default should be writable after boot. If we want to make it more
dynamic we should however talk about the proper layer this is
implemented on.
Michal Hocko

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