On Wed 19-06-19 11:07:30, David Hildenbrand wrote: > 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. Let me try to put it differently. NODE_NOT_IN_PAGE_FLAGS is not about storing the mapping per section. You can do what ever other data structure. NODE_NOT_IN_PAGE_FLAGS is in fact about telling that it is not in page->flags. > > 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. page_to_node is used in many important code paths. Not in the hotest ones I believe but many of them are quite hot I would say. -- Michal Hocko SUSE Labs
