On Wed 22-04-20 10:32:32, David Hildenbrand wrote: > On 22.04.20 10:21, Michal Hocko wrote: > > On Tue 21-04-20 15:06:20, David Hildenbrand wrote: > >> On 21.04.20 14:52, Michal Hocko wrote: > >>> On Tue 21-04-20 14:35:12, David Hildenbrand wrote: > >>>> On 21.04.20 14:30, Michal Hocko wrote: > >>>>> Sorry for the late reply > >>>>> > >>>>> On Thu 16-04-20 12:47:06, David Hildenbrand wrote: > >>>>>> A hotadded node/pgdat will span no pages at all, until memory is moved to > >>>>>> the zone/node via move_pfn_range_to_zone() -> resize_pgdat_range - e.g., > >>>>>> when onlining memory blocks. We don't have to initialize the > >>>>>> node_start_pfn to the memory we are adding. > >>>>> > >>>>> You are right that the node is empty at this phase but that is already > >>>>> reflected by zero present pages (hmm, I do not see spanned pages to be > >>>>> set 0 though). What I am missing here is why this is an improvement. The > >>>>> new node is already visible here and I do not see why we hide the > >>>>> information we already know. > >>>> > >>>> "information we already know" - no, not before we online the memory. > >>> > >>> Is this really the case? All add_memory_resource users operate on a > >>> physical memory range. > >> > >> Having the first add_memory() to magically set node_start_pfn of a hotplugged > >> node isn't dangerous, I think we agree on that. It's just completely > >> unnecessary here and at least left me confused why this is needed at all- > >> because the node start/end pfn is only really touched when > >> onlining/offlining memory (when resizing the zone and the pgdat). > > > > I do not see any specific problem. It just feels odd to > > ignore the start pfn when we have that information. I am little bit > > worried that this might kick back. E.g. say we start using the memmaps > > from the hotplugged memory then the initial part of the node will never> get online and we would have memmaps outside of the node span. I do not > > That's a general issue, which I pointed out as response to Oscars last > series. This needs more thought and reworks, especially how > node_start_pfn/node_spanned_pages are glued to memory onlining/offlining > today. > > > see an immediate problem except for the feeling this is odd. > > I think it's inconsistent. E.g., start with memory-less/cpu-less node > and don't online memory from the kernel immediately. > > Hotplug CPU. PGDAT initialized with node_start_pfn=0. Hotplug memory. > -> node_start_pfn=0 until memory is actually onlined. > > Hotplug memory. PGDAT initialized with node_start_pfn=$VALUE. Hotplug CPU. > -> node_start_pfn=$VALUE > > Hotplug memory. PGDAT initialized with node_start_pfn=$VALUE. Hotplug > CPU. Hotunplug memory. > -> node_start_pfn=$VALUE, although there is no memory anymore. > > Hotplug memory 1. PGDAT initialized with node_start_pfn=$VALUE. Hotplug > memory 2. Hotunplug memory 2. > -> node_start_pfn=$VALUE1 instead of $VALUE2. > > > Again, because node_start_pfn has absolutely no meaning until memory is > actually onlined - today. > > > > > That being said I will shut up now and leave it alone. > > Is that a nack? No it's not. Nor I am going to ack this but I will not stand in the way. I would just urge to have as many assumptions you are making and as much information in the changelog as possible. -- Michal Hocko SUSE Labs
