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

[Michal Hocko]

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
SUSE Labs