On 19.06.19 11:16, Michal Hocko wrote: > 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. Okay, I get what you are saying. Storing it differently is problematic, though, if we want o minimize memory consumption and have a fast lookup. I was also looking into avoiding to store the section number in page-flags with CONFIG_SPARSEMEM. Especially, because the CONFIG_HAVE_ARCH_PFN_VALID hack is really ugly. But it's tricky :( -- Thanks, David / dhildenb
