On Fri 26-03-21 09:52:58, David Hildenbrand wrote:
[...]

Something else to note:


We'll not call the memory notifier (e.g., MEM_ONLINE) for the vmemmap. The
result is that

1. We won't allocate extended struct pages for the range. Don't think this
is really problematic (pages are never allocated/freed, so I guess we don't
care - like ZONE_DEVICE code).

Agreed. I do not think we need them. Future might disagree but let's
handle it when we have a clear demand.

2. We won't allocate kasan shadow memory. We most probably have to do it
explicitly via kasan_add_zero_shadow()/kasan_remove_zero_shadow(), see
mm/memremap.c:pagemap_range()

I think this is similar to the above. Does kasan has to know about
memory which will never be used for anything?

Further a locking rework might be necessary. We hold the device hotplug
lock, but not the memory hotplug lock. E.g., for get_online_mems(). Might
have to move that out online_pages.

Could you be more explicit why this locking is needed? What it would
protect from for vmemmap pages?
-- 
Michal Hocko
SUSE Labs