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
