On 19.06.19 11:04, Michal Hocko wrote: > On Wed 19-06-19 10:51:47, David Hildenbrand wrote: >> On 19.06.19 09:53, Oscar Salvador wrote: >>> On Wed, Jun 19, 2019 at 08:23:30AM +0200, Michal Hocko wrote: >>>> On Tue 18-06-19 08:55:37, Wei Yang wrote: >>>>> In case of NODE_NOT_IN_PAGE_FLAGS is set, we store section's node id in >>>>> section_to_node_table[]. While for hot-add memory, this is missed. >>>>> Without this information, page_to_nid() may not give the right node id. >>>> >>>> Which would mean that NODE_NOT_IN_PAGE_FLAGS doesn't really work with >>>> the hotpluged memory, right? Any idea why nobody has noticed this >>>> so far? Is it because NODE_NOT_IN_PAGE_FLAGS is rare and essentially >>>> unused with the hotplug? page_to_nid providing an incorrect result >>>> sounds quite serious to me. >>> >>> The thing is that for NODE_NOT_IN_PAGE_FLAGS to be enabled we need to run out of >>> space in page->flags to store zone, nid and section. >>> Currently, even with the largest values (with pagetable level 5), that is not >>> possible on x86_64. >>> It is possible though, that somewhere in the future, when the values get larger >>> (e.g: we add more zones, NODE_SHIFT grows, or we need more space to store >>> the section) we finally run out of room for the flags though. >>> >>> I am not sure about the other arches though, we probably should audit them >>> and see which ones can fall in there. >>> >> >> I'd love to see NODE_NOT_IN_PAGE_FLAGS go. > > NODE_NOT_IN_PAGE_FLAGS is an implementation detail on where the > information is stored. Yes and no. Storing it per section clearly doesn't allow storing node information on smaller granularity, like storing in page->flags does. So no, it is not only an implementation detail. I cannot say how much it is really needed now but > I can see there will be a demand for it in a longer term because > page->flags space is scarce and very interesting storage. So I do not > see it go away I am afraid. Depends on how performance-critical pfn_to_nid() is. I can't tell. -- Thanks, David / dhildenb
